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How to test a microwave's door switch:

Testing a door switch

NOTE: Before you begin to test a door interlock switch, first make sure your microwave oven is unplugged, and that your capacitor is discharged.

A door switch is a simple on/off mechanism that prohibits the microwave from operating when the door is open. Microwave door switches are only an inch long. Most are black in color and all have metal prongs, called terminals, extending out from the body of the switch. Microwave door switches are normally mounted to a bracket near the door latch. Also, microwaves usually have three or four door switches.

There is a latch (sometimes referred to as a hook) attached to the inside of your microwave door that comes in contact with and activates the door switch when the door is closed. Examine this prong first. You may not be experiencing any problems with your door switch. Perhaps the latch on the inside of your microwave door is missing or damaged, leaving it unable to activate the door switch.

Some door switches have only two metal prongs extending from the body, while others have three. Those with three terminals will have a common (COM) terminal, a normally closed (N.C.) terminal, and a normally open (N.O.) terminal. Those switches with only two terminals will have either a common terminal and a normally open terminal, or a common terminal and a normally closed terminal. We are providing directions for testing a door switch with three terminals. If you are dealing with a door switch that has only two terminals, ignore the part of this test that does not apply to you.

Using caution, remove all wiring harness leads from the switch's terminals. Be aware that some door switches may have a locking clip keeping the harness from coming loose. In this case, there is a protruding lever which must be depressed while the harness is gently pulled away from the terminal.

Use your ohmmeter to test your switch for continuity. Begin by setting your ohmmeter to measure resistance at a scale of Rx1. If you are using an analog meter, touch the metal tips of the test leads together and zero your ohmmeter by adjusting the thumbwheel in the front of the meter until the needle reads '0' on the scale.

Touch one meter lead to the COM terminal and the other lead to the N.O. terminal. Do not push in on the actuator. Your meter should give a reading of infinity, meaning the circuit is open, and there is no continuity. Without moving the meter's leads, press down on the actuator until you hear a 'click'. With the 'click' of the actuator, the meter should produce a resistance reading of zero ohms. This means the circuit is closed and continuity is present.

Hint:
The presence of a distinct 'clicking' noise is evidence that your switch is working as it should. Be that as it may, switches with only two terminals do not 'click' when actuated.

Keep the meter lead that is touching the COM terminal in place, but move the other meter lead from the N.O. terminal to the N.C. terminal. You should receive a reading of infinity when you push in on the switch's actuator. When the actuator is released, you should receive a resistance reading of zero ohms.

Now, set your ohmmeter to its highest resistance scale and touch one meter lead to the N.O. terminal and the other meter lead to the N.C. terminal. The resistance reading between these two leads should be infinite.

Finally, take a resistance reading from both the N.C. terminal and the N.O. terminal to any metal mounting hardware that is a part of the switch assembly. You should receive a normal reading of infinity.

Any readings that differ from the ones presented here are indicative of a defective door switch that will need to be replaced.


CAUTION: Your microwave oven is capable of giving you a serious electrical shock, even when it is unplugged. We strongly suggest you seek the assistance of an appliance repair technician when conducting any microwave oven repair.