Model #KCMS185JSS2, this counter top microwave was operating just fine for the customer, but the light that normally illuminates the ovens interior was no longer working. It didn't matter if the door was opened, or when a cooking cycle was started, there was no light. A minor frustration for many of us, but this customer wanted her appliances to be 100% functional, and with a microwave part or two, I was going to make that happen.
The interior lights of most microwave ovens are simple 120vac incandescent bulbs that are connected to a lamp circuit which provides power when the door is opened, and again once the start button is pressed. This circuit often times is controlled by one of the interlock switches which are actuated by the door latch when the door is closed. This switch will either provide an electrical current path to the lamp, or will simply provide door position information to the control board. In that case, the control board will provide power to the lamp directly.
Although a dark lamp is usually a sign of a burnt out bulb, sometimes depending on the type of circuit, the bulb may be just fine and it's actually the interlock switch that is causing the problem. A simple way to determine which problem you are dealing with is to test the unit by opening the door, then closing the door and starting a cooking cycle. If the bulb remains dark in both cases, most likely you have a bad bulb, where as if the bulb works in one case but not the other, then the interlock switch is more likely the problem.
Many of these lamps are easily replaceable on most of the microwave hood combination units as I outlined in my post Replacing Microwave Cavity Lights, while others including many of the counter top models require removal of the units outer wrapper to get to the bulb. Some of the higher end built in units use halogen bulbs that seem easy to get to, but actually require some dis assembly to get to the bulb itself. Because this light was dark and had been dark for some time, I was quite confident a new light bulb was going to be needed, and this being a small counter top model, if wasn't going to take to long.
After unplugging the microwave, I removed the outer cabinet from the chassis being careful not to touch any part of the high voltage section. The light bulb is mounted just to the left of another microwave part call a magnetron and held in place by a metal clip and tab. Using my meter, I did a resistance check of the bulb and found an open circuit which just confirmed my earlier suspicion. I installed the new bulb in place of the old after reconnecting the wiring harness. Inserting the power cord back into the outlet and opening the microwaves door confirmed success when the light turned on. Quick replacement of the cabinet and this unit was again ready for use.
Microwave Service Disclaimer
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