Model #PLMV169DCD, this over the range microwave was newly installed in the home and had yet to work. The display, lights and fans seemed to work as expected, but when a heating cycle was started, everything would go dark. This occurred several times, each time the circuit break had to be reset, but the last time the customer tried it, the oven remained dark.
Generally speaking if one of house circuit breakers trips, it’s going to be due to a short circuit or some other form of excessive current draw. Each breaker has a rating that is its maximum before it will trip and open the circuit. This is a safety measure to prevent fires and electrocution due to a short or even when someone tries to put to much of a load on one one outlet. So if a breaker trips, reset it. If it trips again, it is time to go looking for the failure.
The breaker in this home would reset and the last time the microwave didn’t come back to life. Given the description from the customer, I had a good idea of where the problem was going to be found. As I said above, the breaker will trip when a short is created in a circuit and in a microwave, there is one created on purpose for the sake of safety.
Each microwave oven will use 3 or 4 interlock switches mounted together to make up the door latching system. These switches and their operation are designed to prevent the microwave oven from operating with the door open. One of them, called the monitor, is designed to create a short circuit inside the microwave when the door is closed. This short poses no problem as long as the other switches are closed and the start button is not pressed. But if someone were to bypass the other switches and attempt to start the oven, the monitor switch would short the circuit and trip the breaker and/or blow the ovens internal line fuse.
Given the customers description, I headed straight for the monitor switch, figuring this was the problem and found it was loose in its mount. This allowed the switch to move out of position when the door was closed resulting in the short circuit never opening properly. When the start button was pressed, the short quickly caused the circuit breaker to trip. Because this was a new microwave, the internal fuse was still in good shape at least for a few times, but the last time the breaker was recent was also the last time for the fuse to pass current.
I reinstalled the monitor switch into the mount and installed a new line fuse onto the filter board. With everything clear, I started a heating cycle and everything worked just the way it should.
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