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Over the range stainless steel microwave oven. First released September 2013

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Why is my GE microwave bottom door switch normally open?

My microwave oven (model JVM3160DF4WW) tripped the circuit breaker when starting to heat food. I found out one thermostat was bad (open circuit). Next, I found out the middle door switch was closed whether the button was pushed or not, so it needed to be replaced. But it was odd about the bottom door switch. My impression was that it (and the middle one) was normally closed. But mine was the opposite - it was normally open and was closed when the button was pushed. Is this normal for this particular GE microwave oven model?

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Hi @james12319

Usually with the three switches with the door open the primary interlock is open, the secondary or door sense interlock is open and the monitor interlock switch is closed.

The order of the function of the switches i.e. top, middle and bottom, depends on what the manufacturer has decided to do.

Looking at the wiring diagram in the mini manual for your model oven, it shows with the door closed, the bottom switch (door sense switch) is closed, the middle switch (monitor switch) is open and the top switch (primary interlock upper switch¹) is closed.

When the door is opened i.e. button pushed, the switches would be top - open, middle - closed and bottom - open.

The monitor switch is a safety feature as it prevents the HV circuit - HV transformer, HV capacitor, HV diode and magnetron from functioning, by shorting out the HV transformer primary winding thus preventing harmful radiation from escaping from the oven when the door is open.

This is just in case there's a problem with the other interlocks if they become short circuit for whatever reason causing the oven fan, turntable motor etc to operate. This mainly occurs due to user misuse by opening the door to stop/pause the oven whilst it is in use instead of using the control panel stop or pause button. This causes arcing between the switch contacts, when the door is opened, which leads to switch contact material migration. If this is done regularly then eventually the switch contacts end up being welded together.

¹The primary interlock switch is shown closed in the diagram but it looks like the actual switch designation is in the wrong place on the diagram. The switch is shown to the left of the designation - drawing typo perhaps.

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Thanks for the answer. I called GE Appliances customer service to check the bottom door switch if it was normally open but could not get a confirmation. After I watched some youtube videos, I reached inside the microwave oven and detected the thermostat on the right was open-circuited and the middle door switch stayed closed whether the button was pushed or not. I have ordered the parts and hopefully the problem can be solved (although I have no idea why the thermostat and the middle door switch went bad).

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@james12319

The middle door switch staying closed or short circuit - same thing really but if the switch was short circuit this could also be due to the contacts being welded together.

Example could be if the door was opened whilst the oven was operating then as the monitor switch (middle switch) is the safety feature, it would short out the HV transformer primary winding. At the same time the primary and secondary interlocks would be opening cutting off the power to the HV circuit. As the power is disconnected the collapsing magnetic field in the HV transformer primary winding creates a current flow which has a path through the monitor switch - basically it would create a spark which eventually weld the contacts together if this was done often.

The only way to know is to carefully open the switch and check the state of the switch contacts.

Did you also check that the door latches and the switch actuating pins to make sure that they're not loose etc causing the interlocks not to operate correctly?

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Thanks for more insights. I opened the middle door switch. Its contacts were welded together.

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@james12319

Using the controls to stop/pause the oven rather than opening the door, definitely prolongs the life of the door interlock switches.

Here's the parts list for the oven.

Scroll down and select the "Oven cavity parts" diagram > expand diagram and find part # 127, #128 & #129.

The middle (monitor) switch is part #128. Scroll down to find the "manufacturer's part number" for the switch part and then search online for the part number to find suppliers that suit you best.

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Thanks. I found the information from the failed middle switch. It has "KW3AT-16" on it. There were door switches with "KW3A-16" that had a lower operating temperature so I did not order this type.

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James さん、ありがとうございました！
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