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Microwave Ovens

increased in the mid-1970s. Over the years, the cost to purchase a microwave oven

has decreased. Today, you can purchase a microwave oven for under $50, as

compared to the 1940s price of around $3000.

Microwave ovens produce electromagnetic non-ionizing radiation to cook the food,

operating at approximately 3000 volts. They operate in the microwave region in the

electromagnetic spectrum at a frequency of 2450 MHz. For domestic use, the microwave

oven (see Figures 1-6, 24-1, 24-2, 24-3, and 24-4) is manufactured in different sizes and styles,

countertop, hanging, and built-in models with electromechanical or electronic controls.

Microwave energy can travel at the speed of light in straight lines; it is produced in the

oven by a device called a magnetron tube and reflected by metal channels to enter the oven

cavity. Microwaves can pass through non-metallic objects and be absorbed into the food.

This chapter will provide the basics needed to diagnose and repair the components in a

microwave oven. Figure 24-5 identifies where various components are typically located

within the microwave oven. However, this illustration is used as an example only. The actual

construction and features might vary, depending on the brand and model you are servicing.

Safety First

install, maintain, or repair any microwave oven. Any improper installation, preventive

maintenance, or repairs could create a risk of death, personal injury, death to others, or

property damage.

If you do not fully understand the installation, preventive maintenance, or repair procedures

in this chapter, or if you doubt your ability to complete the task on the microwave, do not

attempt to make the repairs. Call your service manager instead.

with the door open.

845

CHAPTER

completed.

oven, you must discharge the high-voltage capacitors and inverter first.

oven is operating.

running. It is unnecessary and unsafe to do so.

FIGURE 24-1

A countertop

microwave oven with

electronic controls.

FIGURE 24-2

A countertop

microwave oven with

electromechanical

controls.

PART VI

needs to be absorbed into food or a liquid; otherwise, the energy will bounce around,

making its way to the high-voltage system and causing damage.

Before continuing, take a moment to refresh your memory of the safety procedures in

Chapter 2.

Principles of Operation

The microwave oven circuitry consists of two primary circuits divided by the high-voltage

transformer. The high-voltage circuit consists of the secondary winding of the high-voltage

FIGURE 24-3

Over-the-range

microwave ovens are

available in various

colors and with

various features.

FIGURE 24-4

An under-the-counter

microwave oven.

transformer (or inverter on some models), magnetron, high-voltage capacitor, and the highvoltage

diode. The low-voltage circuit consists of the primary side of the high-voltage

transformer, fans, lights, switches, timer, motors, fuses, thermostats, and a low-voltage

transformer that controls the electronic components in some models.

When the microwave oven is turned on and running, the low-voltage alternating

current (120 VAC) is stepped up to a high-voltage alternating current (1800 to 2000 VAC)

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

T S

U

V

W

X

Y

Z

R

Q

AA

BB

A. Oven cavity

thermostat

B. Humidity sensor

C. Exhaust hood fan

motor assembly

D. Cavity thermostat

E. Fuse

F. Fuse

G. Motor capacitor

H. Cavity thermostat

I. Fuse holder

J. Resistor

K. Magnetron

L. Magnetron

thermostat

M. High-voltage

transformer

N. Cooling fan motor

O. High-voltage

capacitor

P. High-voltage

diode

Q. Lamp holder

R. Cooktop lamp

S. Exhaust fan

thermostat

T. Fuse

U. Electronic control

V. Touch panel

W. Interlock switch

(secondary)

X. Interlock switch

(monitor)

Y. Interlock switch

(primary)

Z. Turntable motor

AA. Cavity lamp

BB. Stirrer motor

locations.

PART VI

and converted into an even higher negative direct current (–3000 to –4500 VDC, depending

on the model).

The direct current travels to the magnetron. The output of the magnetron produces

microwave energy out of the antenna (2450 MHz) into the waveguide. From the waveguide,

the non-ionizing energy will then be channeled to the stirrer blade (on some models). The

stirrer blade will begin to spin around, dispersing the microwaves into the oven cavity

evenly. Then the microwaves will reflect off the interior walls, ceiling, and floor of the oven

cavity, penetrating the food. On some models, a glass tray is used that will rotate within the

oven cavity so that the food will cook evenly.

Cooking the food is achieved by three methods: conduction, convection, and radiation.

The microwave energy will penetrate the food approximately ¾ inch to two inches, depending

on the density of the food and how much water it contains. Microwaves cook the food from

the outside toward the center. The molecules within the food will begin to resonate violently,

producing heat to cook the food. When the cooking cycle is completed, the oven will turn off

and the magnetron will stop producing non-ionizing energy.

Microwave High-Voltage Circuit

Figures 24-6, 24-7, 24-8, 24-9, and 24-10 depict how the high-voltage circuitry operates in a

microwave oven. Figures 24-11 and 24-12 are the inverter board that replaces the high-voltage

transformer, high-voltage capacitor, and high-voltage diode in some models.

section is a stepped-up winding of 2000 VAC used in the voltage doubler circuit; the second section is

a stepped-down winding to 3.8 VAC. The stepped-down winding is used for the ilament circuit for the

magnetron ilament. As the magnetron ilament heats up, it causes an increase in molecular activity

within the magnetron tube.

Filament

circuit

Rectiier

(Diode)

10 MΩ

resistor

+

–

Capacitor FA

Magnetron

Filament

3.8 VAC

Power transformer

primary

H.V.

2,000

VAC

F

FIGURE 24-7

The voltage doubler

circuit consists of a

high-voltage capacitor

and diode. As shown

in the wiring diagram,

when the high-voltage

secondary winding is

on, the voltage is

being stored in the

capacitor. The diode

will only allow voltage

to pass through it in

one direction as

indicated in the

diagram.

FIGURE 24-8

AC current sine wave

for the high-voltage

transformer. The

positive 2000 volts

will be stored in the

capacitor in the

voltage doubler circuit.

The negative 2000

volts will be added to

the positive 2000

volts to start the

magnetron tube to

produce microwave

energy.

FIGURE 24-9

When the AC current

changes to the

negative voltage, the

negative 2000 volts

will be added to the

already stored positive

2000 volts in the

capacitor.

(+) Half

voltage

doubler

circuit

Rectiier

(Diode)

10 MΩ

Resistor

+

–

Capacitor FA

Magnetron

Filament

3.8 VAC

Power transformer

primary

H.V.

2,000

VAC

F

+ 2,000 Volts

– 2,000 Volts

–

+

0 Volts

(–) Half

voltage

doubler

circuit Rectiier

(Diode)

10 MΩ

resistor

+

–

Capacitor FA

Magnetron

Filament

3.8 VAC

Power transformer

primary

H.V.

2,000

VAC

F

PART VI

Location and Installation of Microwave Ovens

Locate the microwave oven where it will be well lighted and have access to proper ventilation.

The oven must be level for proper baking and cooking results. The microwave oven must also

have the proper electrical and grounding requirements, as described in the manufacturer’s

installation instructions.

FIGURE 24-10

When the voltage is at

negative 4000 volts,

the magnetron will

start producing

microwave energy.

This wiring diagram

illustrates that the

high-voltage circuit is

operating and

producing microwave

energy.

FIGURE 24-11

Inverter board (front

view) used in some

microwave models.

The inverter board

replaces the highvoltage

transformer,

high-voltage capacitor,

and high-voltage

diode. Consult the

technical data sheet

for the model you are

servicing to locate the

high-voltage capacitor

discharge test points.

Capacitor

charge/

discharge

circuit

Rectiier

(Diode)

4,000 volts

from

capacitor

10 MΩ

resistor

+

–

Capacitor FA

Magnetron

Filament

3.8 VAC

Power transformer

primary

H.V.

2,000

VAC

F

The proper installation instructions for your model are included with your purchase.

These instructions will assist you with the installation requirements (dimensions, cutout

dimensions, venting, etc.) needed to complete the installation according to the manufacturer’s

specifications.

Microwave Oven Cookware, Utensils, and Coverings

The following “do not use” list for microwave cooking should be followed for safety reasons:

Any of these items can cause arcing, fire, and damage to the microwave oven. If you are

uncertain if a dish is safe for microwaving, place the dish in the microwave on high for 30

seconds. If the dish is hot, it is unsafe for microwaving and should not be used.

The following items are safe to use in microwave ovens:

FIGURE 24-12

The inverter board,

back view.

PART VI

If you are not sure if the item can be used in the microwave oven, check your use and

care instructions or go to the manufacturer’s website.

Can aluminum foil be used in the microwave? Yes, it can. Small, flat pieces of aluminum

foil can be placed on the food in areas that cook too quickly to prevent the food from

overcooking. When using aluminum foil, you must make sure to keep it from contacting

any part of the oven’s interior surfaces (about one inch clearance to prevent arcing). For best

cooking results, always follow the manufacturer’s recommendations in the use and care guide.

Microwave Ovens in General

Much of the troubleshooting information in this chapter covers microwave ovens in general,

rather than specific models, in order to present a broad overview of service techniques. The

illustrations that are used in this chapter are for demonstration purposes only to clarify the

description of how to service these appliances. They in no way reflect on a particular

brand’s reliability.

Step-by-Step Troubleshooting by Symptom Diagnosis

When servicing an appliance, don’t overlook the simple things that might be causing the

problem. Step-by-step troubleshooting by symptom diagnosis is based on diagnosing

malfunctions, with their possible causes arranged into categories relating to the operation

of the microwave oven. This section is intended only to serve as a checklist to aid you in

diagnosing a problem. Look at the symptom that best describes the problem that you are

experiencing with the oven, and then proceed to correct the problem.

Microwave Oven Dead in the “Off” Position

Microwave Oven Will Not Operate After the Controls Are Set

interlock switches.

Unable to Program the Touchpad Controls

control?

Microwave Oven Works, but There Is No Display

Microwave Oven Still Runs with the Door Open

Microwave Oven Starts Running as Soon as the Door Is Closed

PART VI

Microwave Oven Is Running but Will Not Cook

Microwave Oven Is Running, but It Takes Too Long to Cook the Food

Microwave Oven Is Running, but the Operation Is Erratic

Microwave Oven Keeps Blowing Fuses

Microwave Oven Is Running, but Will Not Cook and Makes a Loud Humming Noise

Microwave Oven Has a Burning Smell

Microwave Oven Is Arcing

Microwave Oven Is Very Noisy

PART VI

Microwave Oven Door Will Not Close Properly

Electromechanical Timer Will Not Advance in the Cooking Cycle

Microwave Oven Performs the Wrong Functions

oven.

reprogram the controls. Does it operate okay?

Touchpad Does Not Function or the Wrong Commands Appear

and try to reprogram the electronic controls.

electronic control board.

Microwave Oven Turntable Is Not Working

Microwave Oven Runs Okay, but Randomly Shuts Off During the Cooking Cycle

Stirrer Blade Will Not Rotate

Microwave Oven Is in Cook Mode and Running, but the Light Is Out in the Oven

Microwave Oven Only Heats on High Power

PART VI

Microwave Oven Maintenance

The maintenance on a microwave oven is simple: keep the interior and exterior clean. Make

sure that the air vents are clear of debris for proper ventilation. The interior can be cleaned

with a damp cloth and a mild detergent. If food particles are allowed to collect in the oven

cavity, on the stirrer blade or cover, or waveguide, the food will become carbonized, causing

arcing, improper heating, and possible damage to the components. When cleaning the

control panel, use a damp cloth only, and do not use a spray cleaner or harsh chemical on

the touch panel. This will prevent the touch panel from shorting out or preventing some of

the keypad functions from working properly. Inspect the service cord and outlet; check the

prongs of the plug for a tight fit. A loose connection can cause the service cord to heat up

and short out. This is especially important on built-in models, given the amount of heat

that some of these ovens can generate. It is also recommended that the consumer keep the

area around the microwave oven clean.

After cooking in the microwave oven, the smell of food lingers for quite a while. There

have been cases where cockroaches have penetrated into the microwave oven, traveling to

the electronics area, and eating the wires and the metal traces off the printed circuit boards,

causing expensive repairs. This can be prevented by placing a fine mesh over the intake and

discharge vent areas. You might have to clean the mesh more often, however, because dust

and dirt will build up on the mesh much faster.

Recipe for Microwave Oven Odor Removal

To remove cooking odors from the microwave, combine one cup of water, grated lemon

peels, lemon juice, and several slices of lemon. Stir and place in a microwave-approved

glass container. Cook on high power, and let the mixture boil for several minutes. When the

oven shuts off, let the mixture cool, remove the container from the oven, and wipe the

interior with a damp cloth.

Microwave Leakage Testing

On every microwave oven service call, you must perform a radio frequency (RF) leakage

test (Figure 24-13), before and after the repairs, to determine that the energy emission level

second (approx. 1 in/sec). Make sure that you use an approved microwave leak detector for

testing.

the center of the floor in the oven cavity. Set the microwave power level to high, set the

timer for three minutes, and press the start button. Take the RF leakage detector probe, and

scan the following areas at a rate of one inch per second:

Microwave Oven Magnetron Power Output Test

The magnetron’s performance can be tested for proper operation by performing a power

output test in the following manner:

1. The oven cavity must be cool and clean. Inspect the waveguide and stirrer blade.

3. Place an 8-ounce glass of water heated to about 75 degrees Fahrenheit in the middle

of the cavity floor (remove the metal rack).

4. Take a thermometer and stir the water for 30 seconds. Record the water

temperature.

5. Set the oven controls to 100 percent power and the timer to one minute, and press

the start button.

6. When the oven turns off, remove the glass, and stir the water with the thermometer

for 30 seconds. Record the water temperature.

7. Subtract the two recorded water temperatures to determine the temperature rise.

The temperature rise should be between 19 and 26 degrees Fahrenheit at 120 volts

FIGURE 24-13

Performing a

microwave leakage

test. Move the wand

at 1 inch/second

around the door, front

and rear of the

cabinet, intake and

discharge vents, oven

feet (countertop

models), and the

bottom of the

microwave oven (overthe-

range models).

PART VI

Repair Procedures

Each of the following repair procedures is a complete inspection and repair process for a

single microwave oven component, containing the information you need to test a component

that might be faulty and to replace it, if necessary.

any microwave ovens. Any improper installation, preventative maintenance, or repairs will

create a risk of personal injury, as well as property damage. Call the service manager if

installation, preventative maintenance, or the repair procedure is not fully understood.

Electronic Control Board, Touchpad Panel, or Display Board

The electronic control board, touchpad, or display board operate the microwave oven

functions. The electronic control board also displays the cooking or defrost settings, the time

of day, timer, preprogrammed functions, and any error or fault codes. The electronic control

board, touchpad, or display board are located in the control panel.

The typical complaints associated with the electronic control board, touchpad panel, or

display board are:

error codes.

To prevent electrostatic discharge (ESD) from damaging expensive electronic components,

follow the steps in Chapter 11.

To handle these problems, perform the following steps:

electronic models, turn off the electricity to the appliance and wait for two minutes

before turning it back on. If a fault code appears, look up the code. If the microwave

oven will not power up, locate the technical data sheet behind the control panel for

diagnostics information. On some models you will need the actual service manual

for the model you are working on to properly diagnose the microwave oven. The

service manual will assist you in properly placing the microwave oven in the

service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? The voltage at the

receptacle is between 108 volts and 132 volts during a load on the circuit. Do you

have the correct polarity? (See Chapter 6.)

electricity. This can be done by pulling the plug out of the wall receptacle. Or disconnect

the electricity at the fuse panel or circuit breaker panel. Turn off the electricity.

to the electrical components, touchpad panel, or electronic control board (some

models have multiple circuit boards) by removing the access panel or the outer

cabinet on a microwave oven or combination oven. If you are repairing a wall

oven or an eye-level combination range, the control panel can be removed (see

Figures 11-1 and 22-12) by opening the door and removing the screws that secure

the panel. These might be underneath the front of the exhaust hood or just below

the control panel. Some control panels are hinged; just tilt the control panel toward

you for servicing.

run the microwave oven’s diagnostic test mode, check the different functions of the

oven. Use the technical data sheet for the model you are servicing to locate the test

points on the wiring schematic. Check all wiring connections and wiring. Using the

technical data sheet, test the electronic control and display board, input voltages, and

output voltages. On some models, fuses are soldered to the printed circuit board (PCB).

These fuses must be tested before condemning the component. The touchpad panel

can also be tested for proper operation by checking the continuity for each keypad

function.

screws that secure the boards to the control panel or oven frame. Disconnect the

connectors from the electronic control board, touchpad membrane, or display board.

procedure, and reassemble. Reinstall all panels or the console panel, and restore the

electricity to the oven. Test the microwave oven operation. Perform the microwave

performance and leakage test. Make sure to take the microwave oven out of the

service test mode when the repair is completed.

Magnetron

The magnetron produces the microwave energy that will cook the food. The magnetron is

attached to the outer oven cavity (Figure 24-14).

The typical complaints associated with the magnetron are:

PART VI

To handle these problems, perform the following steps:

glass of water and the microwave test light bar centered on the oven floor (Figures 24-15

and 24-16). The lights in the light bar will light up if the magnetron and high-voltage

system are working correctly. At the same time, place an ammeter on one wire of

the service cord—the meter should indicate approximately 15 amps at 100 percent

power. It may be necessary to turn off the electricity to the appliance and wait for two

minutes before turning it back on. If a fault code appears, look up the code. If the

microwave oven will not power up, locate the technical data sheet behind the panel or

cabinet for diagnostics information. On some models you will need the actual service

manual for the model you are working on to properly diagnose the microwave

oven. The service manual will assist you in properly placing the microwave oven in

the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? With over-the-range models,

is the vent system blocked? The voltage at the receptacle is between 108 volts

and 132 volts during a load on the circuit. Do you have the correct polarity? (See

Chapter 6.)

Magnetron

High-voltage

capacitor

High-voltage

diode

High-voltage

transformer

Filament

wires

electricity. This can be done by pulling the plug out of the wall receptacle. Or disconnect

the electricity at the fuse panel or circuit breaker panel. Turn off the electricity.

Microwave

test light bar

FIGURE 24-15

Place a glass of water

in the microwave

cavity along with

the microwave test

light bar.

FIGURE 24-16

The microwave oven

door is closed, the

controls are set, and

the oven is on. This

illustration shows that

the microwave test

lights are on,

indicating that the

magnetron and the

high-voltage system

are functioning

correctly.

The test

lights are on

PART VI

the magnetron by removing the access panel or the outer cabinet on a microwave

oven or combination oven. If you are repairing a wall oven or an eye-level

combination range, the control panel can be removed (see Figures 11-1 and 22-12)

by opening the door and removing the screws that secure the panel. These might be

underneath the front of the exhaust hood or just below the control panel. Some

control panels are hinged; just tilt the control panel toward you for servicing. On

some combination models, you might have to uninstall the appliance to gain access.

Carefully remove the wires from the magnetron filament terminals (Figure 24-17).

Inspect the terminal seal for any damage. Take your ohmmeter probes, and place

one probe on each filament terminal. The ohmmeter should read less than 1 ohm.

Next, leave one probe on the filament terminal, and place the other probe on the

chassis. The meter should read infinite ohms. Do the same procedure for the other

filament terminal. The meter should read infinite ohms also. If the testing results

differ, replace the magnetron.

FIGURE 24-17

The magnetron.

Filament terminal

Terminal seal

Cooling fin

Antenna

Embossed ring

screws that secure the magnetron to the oven frame. Carefully remove the magnetron

from the oven so as not to damage any other components. Inspect the magnetron

tube antenna and embossed ring; replace the magnetron if there are signs of arcing,

burn marks, or a broken antenna. Also, inspect the wave guide for burn marks.

If there are burn marks, replace the microwave oven.

procedure, and reassemble. Reinstall all panels or the console panel, and restore the

electricity to the oven. Test the microwave oven operation. Perform the microwave

performance and leakage test. Make sure to take the microwave oven out of the

service test mode when the repair is completed.

High-Voltage Transformer

The high-voltage transformer will step up the voltage to 2000 VAC on one part of the

secondary coil. On the other secondary coil, the voltage will be stepped down to 3 to 4 VAC.

The primary winding is connected to 120 VAC. The high-voltage transformer is located

behind the control panel, mounted to the frame of the microwave oven (Figure 24-14).

The typical complaints associated with the high-voltage transformer are:

is closed.

To handle these problems, perform the following steps:

a glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. It may be necessary to turn off the electricity to the appliance

and wait for two minutes before turning it back on. If a fault code appears, look up

the code. If the microwave oven will not power up, locate the technical data sheet

behind the panel or cabinet for diagnostics information. On some models you will

need the actual service manual for the model you are working on to properly diagnose

the microwave oven. The service manual will assist you in properly placing the

microwave oven in the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? The voltage at the receptacle

is between 108 volts and 132 volts during a load on the circuit. Do you have the

correct polarity? (See Chapter 6.)

PART VI

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

the high-voltage transformer by removing the access panel or the outer cabinet on

a microwave oven or combination oven. If you are repairing a wall oven or an

eye-level combination range, the control panel can be removed (see Figures 11-1 and

22-12) by opening the door and removing the screws that secure the panel. These

might be underneath the front of the exhaust hood or just below the control panel.

Some control panels are hinged; just tilt the control panel toward you for servicing.

On some combination models, you might have to uninstall the appliance to gain

access.

disconnect the wires from the primary winding terminals on the transformer (label

primary winding terminals on the transformer. The ohmmeter scale should indicate

resistance in the winding of less than 1 ohm. Now remove the wires from the

secondary winding terminals on the transformer (label them as well), and measure

the resistance of the winding. The ohmmeter scale should indicate resistance in

the secondary winding from 50 to 130 ohms, depending on the type of transformer

used. Next, test the resistance of the filament winding—the ohmmeter should

indicate zero to 1 ohm. Finally, take the ohmmeter probes and test each winding to

ground: you should have an infinite reading for each winding. If the readings differ,

replace the transformer.

remove the screws that secure it to the oven frame. Be careful: the high-voltage

transformer is heavy.

just reverse the disassembly procedure, and reassemble. Reinstall all panels or the

console panel, and restore the electricity to the oven. Test the microwave oven

operation. Perform the microwave performance and leakage test. On electronic

models, make sure to take the microwave oven out of the service test mode when

the repair is completed.

Inverter Board

On some models the inverter board replaces the high-voltage transformer, high-voltage

capacitor, and high-voltage diode (Figures 24-11, 24-12, and 24-19). The inverter board is

located behind the control panel, mounted to the microwave oven frame.

The typical complaints associated with the inverter are:

is closed.

Filament

winding

terminals

Primary

winding

terminals

Oven chassis

High-voltage

secondary

winding

terminals

FIGURE 24-18

The high-voltage

transformer.

Inverter

120 volts input Ground

Discharge

point

I/O signal to

electronic control

board Voltage output to

magnetron

FIGURE 24-19

The high-voltage

inverter board.

PART VI

To handle these problems, perform the following steps:

a glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. It may be necessary to turn off the electricity to the appliance

and wait for two minutes before turning it back on. If a fault code appears, look up

the code. If the microwave oven will not power up, locate the technical data sheet

behind the panel or cabinet for diagnostics information. On some models you will

need the actual service manual for the model you are working on to properly diagnose

the microwave oven. The service manual will assist you in properly placing the

microwave oven in the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? The voltage at the receptacle

is between 108 volts and 132 volts during a load on the circuit. Do you have the

correct polarity? (See Chapter 6.)

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

the inverter by removing the access panel or the outer cabinet on a microwave oven

or combination oven. If you are repairing a wall oven or an eye-level combination

range, the control panel can be removed (see Figures 11-1 and 22-12) by opening the

door and removing the screws that secure the panel. These might be underneath the

front of the exhaust hood or just below the control panel. Some control panels are

hinged; just tilt the control panel toward you for servicing. On some combination

models, you might have to uninstall the appliance to gain access.

capacitor, and high-voltage diode. To test the inverter (Figure 24-19), locate the service

cord wires, and connect an ammeter to the white wire. Then place a glass of water in

the oven cavity, turn on the electricity, and run the microwave oven at 100 percent

power. If the reading on the ammeter is above 0.5 amps, check the magnetron and the

wiring. If the reading is below 0.5 amps, there is no 120-volt input voltage to the

inverter. In this case, you will have to check the cooking relay, electronic control

board, or power relay board. Under normal operating conditions the high-voltage

circuit can be measured with an ammeter and it will read 12–15 amps.

Some models use a smaller inverter (about 40 watts) to provide 12 volts DC to the

hood lamp and the cooling fan motor. To test this type of inverter, set your voltmeter

to the AC voltage scale, and test for 120 volts AC to the input connector on the

inverter. Then set your voltmeter to the DC voltage scale, and check for 12 volts DC

from the hood lamp and cooling fan motor connectors on the inverter.

use a 20,000-ohm 2-watt resistor to discharge the inverter. Hold the resistor body

with a pair of insulated pliers (see Figure 3-8f), and discharge the inverter at the

discharge points, as shown in Figure 24-19. To remove the inverter, remove the

inverter cover, wire connectors, grounding wire, and screws; then slide the inverter

out of its holding container. On some models, you might have to remove other

components to gain access to the inverter.

procedure, and reassemble. Reinstall all panels or the console panel, and restore the

electricity to the oven. Test the microwave oven operation. Perform the microwave

performance and leakage test. On electronic models, make sure to take the

microwave oven out of the service test mode when the repair is completed.

High-Voltage Diode

The high-voltage diode is an electrical device that allows current to flow in one direction

only, and it is part of the voltage doubler circuit. The high-voltage diode is located behind

the control panel attached to the high-voltage capacitor and to a ground screw (Figures 24-14

and 24-20).

High-voltage

diode

Cathode

Anode

FIGURE 24-20

The high-voltage diode

is attached to the

high-voltage capacitor

and to ground.

PART VI

The typical complaints associated with the high-voltage diode are:

To handle this problem, perform the following steps:

glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. It may be necessary to turn off the electricity to the appliance

and wait for two minutes before turning it back on. If a fault code appears, look up

the code. If the microwave oven will not power up, locate the technical data sheet

behind the panel or cabinet for diagnostics information. On some models you will

need the actual service manual for the model you are working on to properly diagnose

the microwave oven. The service manual will assist you in properly placing the

microwave oven in the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? The voltage at the receptacle

is between 108 volts and 132 volts during a load on the circuit. Do you have the correct

polarity? (See Chapter 6.)

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

the high-voltage diode by removing the access panel or the outer cabinet on a

microwave oven or combination oven. If you are repairing a wall oven or an eye-level

combination range, the control panel can be removed (see Figures 11-1 and 22-12) by

opening the door and removing the screws that secure the panel. These might be

underneath the front of the exhaust hood or just below the control panel. Some

control panels are hinged; just tilt the control panel toward you for servicing. On

some combination models, you might have to uninstall the appliance to gain access.

and connect the leads to the diode. The meter may or may not read continuity. Then

reverse the leads on the diode and check for continuity. You should read continuity

in one direction only. If continuity is not indicated in either direction, the diode is

open (see Figure 7-20a). If continuity is indicated in both directions, the diode is

shorted (see Figure 7-20b). Some high-voltage diodes may have to be tested with a

9-volt battery. Place the battery in series with the diode and measure the DC voltage.

Then, reverse the diode on the battery and take another meter reading. You should

read voltage in one direction only. If not, the diode is defective.

the wires and remove.

the disassembly procedure, and reassemble. Reinstall all panels or the console panel,

and restore the electricity to the oven. Test the microwave oven operation. Perform

the microwave performance and leakage test. Make sure to take the microwave

oven out of the service test mode when the repair is completed.

High-Voltage Capacitor

The high-voltage capacitor stores electricity, up to 2100 VAC, and it is part of the voltage

doubler circuit. The high-voltage capacitor is located behind the control panel attached to

the oven frame (Figure 24-14).

The typical complaints associated with the high-voltage capacitor are:

To handle this problem, perform the following steps:

a glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. It may be necessary to turn off the electricity to the appliance

and wait for two minutes before turning it back on. If a fault code appears, look up

the code. If the microwave oven will not power up, locate the technical data sheet

behind the panel or cabinet for diagnostics information. On some models you will

need the actual service manual for the model you are working on to properly diagnose

the microwave oven. The service manual will assist you in properly placing the

microwave oven in the service test mode for testing the microwave oven functions.

PART VI

the appliance. Is there electricity to the microwave oven? The voltage at the receptacle

is between 108 volts and 132 volts during a load on the circuit. Do you have the

correct polarity? (See Chapter 6.)

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

the high-voltage capacitor by removing the access panel or the outer cabinet on a

microwave oven or combination oven. If you are repairing a wall oven or an eyelevel

combination range, the control panel can be removed (see Figures 11-1 and

22-12) by opening the door and removing the screws that secure the panel. These

might be underneath the front of the exhaust hood or just below the control panel.

Some control panels are hinged; just tilt the control panel toward you for servicing.

On some combination models, you might have to uninstall the appliance to gain

access.

discharged.

Do not remove the wires from the capacitor yet. Use two screwdrivers with

insulated handles to discharge the capacitor by placing one screwdriver blade on

one capacitor terminal and the other screwdriver blade on the other capacitor

terminal. Now cross both screwdrivers and touch them together to discharge the

capacitor. Next, place one screwdriver on one capacitor terminal and the other

screwdriver to chassis ground. Now cross the screwdrivers together to short out the

capacitor. Next, repeat this procedure for the other capacitor terminal. If you are

not sure that you discharged the capacitor, repeat the procedures until you feel

comfortable with removing the wires from the capacitor. Remove the wire leads

(label them), one at a time, with needle nose pliers. Set the ohmmeter on the highest

scale, and place one probe on one terminal and the other probe on the other

terminal (see Figure 24-21). Observe the meter action. While the capacitor is

charging, the ohmmeter will read nearly zero ohms for a short period of time. Then

the ohmmeter reading will slowly return toward infinity. If the ohmmeter reading

deflects to zero and does not return to infinity, the capacitor is shorted and should

be replaced with a duplicate of the original. If the ohmmeter reading remains at

infinity and does not dip toward zero, the capacitor is open and should be replaced

with a duplicate of the original.

from the capacitor terminals. Then remove the screws from the capacitor-mounting

bracket.

reverse the disassembly procedure, and reassemble. Reinstall all panels or the console

panel, and restore the electricity to the oven. Test the microwave oven operation.

Perform the microwave performance and leakage test. On electronic models, make

sure to take the microwave oven out of the service test mode when the repair is

completed.

Microwave Oven Thermostats

Microwave oven thermostats operate a switch within the body of the thermostat. The switch

is actuated by a change in temperature. There are between three and five thermostats (each

model may vary) located within the microwave oven (oven cavity thermostats, magnetron

thermostat, and exhaust fan thermostat) (see Figure 24-5 part # A, D, H, L, and S).

The typical complaints associated with the thermostats are:

FIGURE 24-21

Placing ohmmeter test

leads on the capacitor

terminals.

PART VI

To handle these problems, perform the following steps:

a glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. Check the operation of the cooling fans and stirrer blade. It may

be necessary to turn off the electricity to the appliance and wait for two minutes

before turning it back on. If a fault code appears, look up the code. If the microwave

oven will not power up, locate the technical data sheet behind the panel or cabinet

for diagnostics information. On some models you will need the actual service

manual for the model you are working on to properly diagnose the microwave

oven. The service manual will assist you in properly placing the microwave oven in

the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? Is the vent system blocked?

The voltage at the receptacle is between 108 volts and 132 volts during a load on the

circuit. Do you have the correct polarity? (See Chapter 6.)

the electricity. This can be done by pulling the plug out of the wall receptacle.

Or disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

the thermostats by removing the access panel or the outer cabinet on a microwave

oven or combination oven. If you are repairing a wall oven or an eye-level

combination range, the control panel can be removed (see Figures 11-1 and 24-12)

by opening the door and removing the screws that secure the panel. These might be

underneath the front of the exhaust hood or just below the control panel. Some

control panels are hinged; just tilt the control panel toward you for servicing.

On some combination models, you might have to uninstall the appliance to

gain access.

Figure 24-5) in the oven that operate as a safety switch to turn the oven off if the

oven temperature exceeds its limits. The cavity thermostats monitor the airflow

temperatures in the oven. The magnetron thermostat monitors the operating

temperature of the magnetron. Some models have an exhaust fan thermostat that

will monitor the temperature from beneath the oven and the airflow temperature

(this is common in over-the-range models).

The cavity and magnetron thermostats are normally closed—when the temperature

exceeds its rated limits, it will open the circuit. When the temperature drops below

its rating, it will reset and close the circuit. The exhaust fan thermostat is normally

open; it closes on temperature rise, turning on the exhaust fan. When the temperature

drops below the rated temperature, the thermostat will reset to open the circuit.

thermostat terminals, and test for continuity. Most wire connectors are a lockingtype

connector. See Figure 24-22 for instructions on how to remove them without

breaking the connectors or the components. The lock connector will prevent the

connector and wire from coming off the component terminal connector, avoiding

circuit faults.

from the terminals, and remove the screws that secure the thermostat to the cavity

or magnetron.

procedure, and reassemble. Reinstall all panels or the console panel, and restore the

electricity to the oven. Test the microwave oven operation. Perform the microwave

performance and leakage test. On electronic models, make sure to take the microwave

oven out of the service test mode when the repair is completed.

Lock

connector

Lever

Terminal

Push

1

Pull down

2

FIGURE 24-22

To remove the lock

connector: (1) push

on the lever and (2)

pull down to remove

the lock connector

from the terminal.

When installing the

lock connector, always

have the lever face

you for easy removal

the next time you

service the

microwave.

PART VI

Microwave Oven Door Switches

The oven door switches are electro mechanical devices used for directing and controlling the

flow of current in a circuit. Simply put, the door switches are used for turning a component on

or off (Figures 24-23 and 24-24). The monitor switch is intended to render the microwave

oven inoperative by means of blowing the fuse when the contacts of the primary switch or

secondary switch fail to open when the door is opened.

The typical complaints associated with the door switches are:

To handle these problems, perform the following steps:

glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. It may be necessary to turn off the electricity to the appliance

and wait for two minutes before turning it back on. If a fault code appears, look up

the code. If the microwave oven will not power up, locate the technical data sheet

behind the panel or cabinet for diagnostics information. On some models you will

need the actual service manual for the model you are working on to properly

diagnose the microwave oven. The service manual will assist you in properly

placing the microwave oven in the service test mode for testing the microwave

oven functions.

Actuator

NC

NO

Silver switch

contacts

Terminals

Mounting hole

FIGURE 24-23

An exploded view of a

microwave oven door

switch.

the appliance. Is there electricity to the microwave oven? The voltage at the

receptacle is between 108 volts and 132 volts during a load on the circuit. Do you

have the correct polarity? (See Chapter 6.)

electricity. This can be done by pulling the plug out of the wall receptacle. Or disconnect

the electricity at the fuse panel or circuit breaker panel. Turn off the electricity.

the door switches by removing the access panel or the outer cabinet on a microwave

oven or combination oven. If you are repairing a wall oven or an eye-level

combination range, the control panel can be removed (see Figures 11-1 and 22-12)

by opening the door and removing the screws that secure the panel. These might be

underneath the front of the exhaust hood or just below the control panel. Some

control panels are hinged; just tilt the control panel toward you for servicing. On

some combination models, you might have to uninstall the appliance to gain access.

Door

Secondary

interlock

switch

Monitor

switch

Monitor

fuse

Primary

interlock

switch

Stop

switch

Latch

hook

Latch

heads

and open. (b) A typical view of one oven model with the door interlock switches. The latch heads and the door

switch assembly can be adjusted.

PART VI

secondary interlock switches (see Figures 24-5 and 24-24). Identify each switch

using the technical data sheet for the model you are servicing. The switch housing

(Figure 24-23) is usually marked with the terminal identification numbers that

correspond to the wiring diagram. It identifies the contacts by number: normally

open (NO) contacts, normally closed (NC) contacts, or common (COM). Internally,

the switch can house many contact points for controlling more than one circuit.

In Figure 24-24a, the wiring diagram illustrates the door switches in the closed

and open positions. Figure 24-24b illustrates the location of the door switches in a

primary interlock switch terminals, and place your meter probes on the terminals.

With the door closed, the ohmmeter will indicate continuity. When the door is

opened, the ohmmeter reads no continuity. Next, test the monitor switch. Remove

the wires from the switch terminals (Figure 24-22), and place your meter probes

on the terminals. With the door closed, the ohmmeter will indicate no continuity.

When the door is opened, the ohmmeter reads continuity. Finally, test the secondary

interlock switch. Remove the wires from the switch, and place the meter probes on

the terminals. With the door closed, the ohmmeter will indicate continuity. When

the door is opened, the ohmmeter reads no continuity. If any of the readings differ

from what is stated here, replace all of the switches in the microwave.

them. Depending on the model you are servicing, remove the door switch assembly

as a complete unit or remove each switch separately.

procedure, and reassemble. Check the in-and-out play in the door: it should be less

than 0.5 mm when the door is closed. Reinstall all panels or the console panel, and

restore the electricity to the oven. Test the microwave oven operation. Perform the

microwave performance and leakage test. On electronic models, make sure to take

the microwave oven out of the service test mode when the repair is completed.

Temperature Probe

The temperature probe is a thermistor device that is used to penetrate the food to monitor

the cooking temperatures. One end of the probe is plugged into the oven receptacle which

sends signals to the electronic control board.

The typical complaints associated with the temperature probe are:

The temperature probe is a separate component that plugs into a receptacle located in

the oven cavity (on some models). The consumer inserts the probe into the food product

being cooked, programs the controls, and starts the oven. The temperature probe will sense

the temperature of the food cooking and sends a

signal back to the electronic control to turn off the

microwave oven when the food is fully cooked.

To test the temperature probe, set your ohmmeter

end that goes into the receptacle (Figure 24-25).

With some models, at room temperature the

ohmmeter scale will read between 19,000 and

75,000 ohms. Next, with the ohmmeter probes still

attached to the male jack end, insert the probe tip

in a glass of hot water, and measure the resistance.

The resistance reading should decrease. If the reading does not decrease, replace the

temperature probe.

Humidity Sensor

The humidity sensor (Figure 24-26) is attached to the outer microwave oven cavity in the

airflow to detect how much humidity (vapor) comes off the food being cooked. The humidity

sensor will then send a signal back to the electronic control board that the food is cooked

properly and to stop the sensor cooking process.

The typical complaints associated with the humidity sensor are:

To handle these problems, perform the following steps:

a glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-11 and 22-12). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

Temperature

probe

temperature probe.

Sensing part Sensing part

Sensing part

Sensing part

Thermistor

element

Thermistor

element

Sensor

case

Ventilation

openings

the outside of the oven cavity.

PART VI

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. Check the operation of the cooling fans and stirrer blade. It may

be necessary to turn off the electricity to the appliance and wait for two minutes

before turning it back on. If a fault code appears, look up the code. If the microwave

oven will not power up, locate the technical data sheet behind the panel or cabinet

for diagnostics information. On some models you will need the actual service

manual for the model you are working on to properly diagnose the microwave

oven. The service manual will assist you in properly placing the microwave oven in

the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? Is the vent system blocked?

On countertop models, is there enough open space surrounding the microwave oven

for proper ventilation? The voltage at the receptacle is between 108 volts and 132 volts

during a load on the circuit. Do you have the correct polarity? (See Chapter 6.)

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

humidity sensor (see Figure 24-5) by removing the access panel or the outer cabinet on a

microwave oven or combination oven. If you are repairing a wall oven or an eye-level

combination range, the control panel can be removed (see Figures 11-1 and 22-12) by

opening the door and removing the screws that secure the panel. These might be

underneath the front of the exhaust hood or just below the control panel. Some

control panels are hinged; just tilt the control panel toward you for servicing. On

some combination models, you might have to uninstall the appliance to gain access.

control board behind the control panel. Locate the sensor wires, trace them to the

electronic control board, and disconnect the three-wire connector from the board.

(check the technical data sheet for the pin designations), located on the humidity

sensor wire connector. The ohmmeter scale should read approximately 2800 ohms

pins 2 and 3—the ohmmeter scale should read approximately 2800 ohms at 77 degrees

electronic control board and trace it back to the sensor. Remove the screws that

secure the sensor to the oven cavity.

disassembly procedure, and reassemble. Make sure that you place the wires in the

same route that you removed them and do not pinch the wires. Reinstall all panels

or the console panel, and restore the electricity to the oven. Test the microwave oven

operation. Perform the microwave performance and leakage test. On electron models,

make sure to take the microwave oven out of the service test mode when the repair

is completed.

Stirrer Motor

The stirrer motor is located under the access panel (Figure 24-5 part # BB). The stirrer motor

turns the stirrer blade in the microwave oven.

The typical complaints associated with the stirrer motor are:

To handle these problems, perform the following steps:

glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. Check the operation of the cooling fans and stirrer blade. It may

be necessary to turn off the electricity to the appliance and wait for two minutes

before turning it back on. If a fault code appears, look up the code. If the microwave

oven will not power up, locate the technical data sheet behind the panel or cabinet

for diagnostics information. On some models you will need the actual service

manual for the model you are working on to properly diagnose the microwave

oven. The service manual will assist you in properly placing the microwave oven in

the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? Is the vent system blocked?

On countertop models, is there enough open space surrounding the microwave oven

for proper ventilation? The voltage at the receptacle is between 108 volts and 132 volts

during a load on the circuit. Do you have the correct polarity? (See Chapter 6.)

PART VI

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

stirrer motor (see Figure 24-5) by removing the access panel or the outer cabinet on a

microwave oven or combination oven. From inside the oven cavity area, remove the

stirrer cover and stirrer blade first in order to remove the stirrer motor if necessary.

disconnect the wires, and attach your ohmmeter leads to the motor wires. Set your

ohms. If the reading is different from this, replace the stirrer motor. On some older

models, the stirrer blade is belt-driven—check for a broken belt. On other models,

the stirrer blade turns with the airflow from the cooling fan motor that circulates the

microwave energy. Make sure the stirrer blade spins freely.

have to be removed first. The stirrer motor is mounted to the oven cavity with

screws—remove the screws.

disassembly procedure, and reassemble. Reinstall all panels and restore the electricity

to the oven. Test the microwave oven operation. Perform the microwave performance

and leakage test. On electronic models, make sure to take the microwave oven out of

the service test mode when the repair is completed.

Turntable Motor

The turntable motor is located on the bottom of the microwave oven (Figure 24-5 part # Z).

This motor turns the glass tray so that the food will cook evenly.

The typical complaints associated with the turntable motor are:

To handle these problems, perform the following steps:

glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. Check the operation of the cooling fans and stirrer blade. It may

be necessary to turn off the electricity to the appliance and wait for two minutes

before turning it back on. If a fault code appears, look up the code. If the microwave

oven will not power up, locate the technical data sheet behind the panel or cabinet

for diagnostics information. On some models you will need the actual service

manual for the model you are working on to properly diagnose the microwave

oven. The service manual will assist you in properly placing the microwave oven in

the service test mode for testing the microwave oven functions.

the appliance. Is there electricity to the microwave oven? Is the vent system blocked?

On countertop models, is there enough open space surrounding the microwave oven

for proper ventilation? The voltage at the receptacle is between 108 volts and 132 volts

during a load on the circuit. Do you have the correct polarity? (See Chapter 6.)

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

oven on its back, and remove the bottom panel. On older models, the turntable

motor might be belt-driven—inspect the belt.

disconnect the wires, and attach your ohmmeter leads to the motor wires. Set

to 40,000 ohms. If the reading differs from this, replace the turntable motor.

that secure the motor to the cavity. On some older models, you might have to

remove the gearbox before removing the motor.

PART VI

disassembly procedure, and reassemble. Reinstall all panels and restore the electricity

to the oven. Test the microwave oven operation. Perform the microwave performance

and leakage test. On electronic models, make sure to take the microwave oven out of

the service test mode when the repair is completed.

Cooling Fan Motor

The cooling fan motor serves two purposes in some models. The fan motor cools the

magnetron and high-voltage transformer and circulates the air through the waveguide to

the stirrer blade and out the discharge vent (Figure 24-27). Over-the-range models use two

fan motors to cool the microwave oven. Other models with stirrer motors circulate the air

only in the high-voltage section of the microwave oven.

The typical complaints associated with the cooling fan motor are:

Cooling

fan motor

magnetron, waveguide, stirrer duct, and stirrer blade; through the oven cavity; and then out through the

discharge vent.

glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. It may be necessary to turn off the electricity to the appliance

and wait for two minutes before turning it back on. If a fault code appears, look up

the code. If the microwave oven will not power up, locate the technical data sheet

behind the panel or cabinet for diagnostics information. On some models you will

need the actual service manual for the model you are working on to properly

diagnose the microwave oven. The service manual will assist you in properly

placing the microwave oven in the service test mode for testing the microwave oven

functions.

the appliance. Is there electricity to the microwave oven? The voltage at the receptacle

is between 108 volts and 132 volts during a load on the circuit. Do you have the

correct polarity? (See Chapter 6.) Is the vent system blocked? On countertop models,

is there enough open space surrounding the microwave oven for proper ventilation?

electricity. This can be done by pulling the plug out of the wall receptacle. Or disconnect

the electricity at the fuse panel or circuit breaker panel. Turn off the electricity.

cooling fan motor by removing the access panel or the outer cabinet on a microwave

oven or combination oven. If you are repairing a wall oven or an eye-level combination

range, the control panel can be removed (see Figures 11-1 and 22-12) by opening the

door and removing the screws that secure the panel. These might be underneath the

front of the exhaust hood or just below the control panel. Some control panels are

hinged; just tilt the control panel toward you for servicing. On some combination

models, you might have to uninstall the appliance to gain access.

sheet for the model you are servicing, and look up the motor’s operating voltage.

Microwave oven manufacturers use a 120-volt AC or 12-volt DC motor. Check and

see if the fan blade spins freely. Then disconnect the wires from the motor, isolating it

from the rest of the circuit, and place your ohmmeter probes on the motor wire leads.

PART VI

scale should indicate 15 to 30 ohms. For alternating current (AC) motors, the

ohmmeter scale should indicate 80 to 140 ohms. If the readings are different from

this, replace the cooling fan motor.

wiring, and remove the screws that secure the motor assembly to the oven frame.

Depending on the model you are servicing, you may have to remove other

components to access the motor.

disassembly procedure, and reassemble. Reinstall all panels and restore the electricity

to the oven. Test the microwave oven operation. Perform the microwave performance

and leakage test. On electronic models, make sure to take the microwave oven out of

the service test mode when the repair is completed.

Exhaust Fan Motor

The exhaust fan motor is located on the top of microwave oven (over-the-range model). See

Figure 24-28. The exhaust fan will remove the heat that accumulates under the microwave

oven when the consumer is cooking on the range.

The typical complaints associated with the exhaust fan motor are:

To handle these problems, perform the following steps:

glass of water and the microwave test light bar centered on the oven floor (see

Figures 24-15 and 24-16). The lights in the light bar will light up if the magnetron

and high-voltage system are working correctly. At the same time, place an ammeter

on one wire of the service cord; the meter should indicate approximately 15 amps at

100 percent power. It may be necessary to turn off the electricity to the appliance

and wait for two minutes before turning it back on. If a fault code appears, look up

the code. If the microwave oven will not power up, locate the technical data sheet

behind the panel or cabinet for diagnostics information. On some models you will

need the actual service manual for the model you are working on to properly

diagnose the microwave oven. The service manual will assist you in properly

placing the microwave oven in the service test mode for testing the microwave

oven functions.

the appliance. Is there electricity to the microwave oven? The voltage at the receptacle

is between 108 volts and 132 volts during a load on the circuit. Do you have the

correct polarity? (See Chapter 6.) Is the vent system blocked? On countertop models,

is there enough open space surrounding the microwave oven for proper ventilation?

electricity. This can be done by pulling the plug out of the wall receptacle. Or

disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the

electricity.

the exhaust fan motor by removing the access panel or the outer cabinet on a

microwave oven. If you are repairing a wall oven or an eye-level combination

range, the control panel can be removed (see Figures 11-1 and 22-12) by opening the

door and removing the screws that secure the panel. These might be underneath the

front of the exhaust hood or just below the control panel. Some control panels are

hinged; just tilt the control panel toward you for servicing. On some combination

models or over-the-range models, you might have to uninstall the appliance to gain

access (see Figure 24-28).

Screws

Screw

Exhaust

vent

cover

Exhaust

fan

motor

FIGURE 24-28

To gain access to the

exhaust motor in this

model, you will have

to uninstall the

microwave oven.

PART VI

harness. Check the motor windings for continuity. To check for a grounded winding

in the motor, take the ohmmeter probes, and check from each motor wire lead

terminal to the motor housing. The ohmmeter will indicate continuity if the

windings are grounded.

oven for easy accessibility. This is accomplished by removing the screws that secure

the exhaust vent cover (Figure 24-28).

the disassembly procedure, and reassemble. Reinstall all panels or reinstall the oven,

and restore the electricity to the oven. Test the microwave oven operation. Perform

the microwave performance and leakage test. On electronic models, make sure to

take the microwave oven out of the service test mode when the repair