, this counter top microwave operated and cooked as it should, but the light inside the oven cavity would stay on when the door was either open or closed. Not a big deal, but the customer didn't want to waste electricity having the light on all the time and risk needing an early bulb replacement.
Cavity lights in microwave ovens will generally illuminate the cavity when the door is opened, and also when the cooking cycle has begun to allow you to watch over your food. To do this, the lights will be powered directly by one of the door interlock switches, or indirectly by the control board monitoring one of the interlock switches. This will be done through the use of a number of interlock devices used to ensure the oven door is properly closed prior to activating the magnetron tube. These interlock switches are aligned and arranged to prevent someone from tampering with the door and attempting to start a cooking cycle while the door is open. The result of any purposeful interlock manipulation will be a dead microwave needing a service call.
Because of the symptom, I figured one of the interlock switches was either out of position, had a loose wire, or had simply failed. With the cover removed, I was able to do a resistance check on each switch and found one of the four was not changing state when actuated. This fourth switch directly powers the cavity lamp circuit when the door is in the open or closed position. It is also the only SPDT (single pole, double throw) switch of the four meaning it can power one circuit when open and another when closed.
With the failure identified, it was a simple repair requiring removal and replacement of the lamp control interlock switch, while paying close attention to the wires colors. Microwaves use locking terminal connectors which take a bit more accessibility to remove, but once completed, the light in this oven was only on when it needed to be.
Microwave Servicing Disclaimer