Chosen Solution
Hi Everyone, Will someone please give me advice or help me find info on how to diagnose a problem with my Whirlpool Microwave m/n: MH7140XFQ-1? (Please ignore the ‘Choose a Device’ microwave model above!) Here’s the problem, It keeps burning up the thermal sensor/fuse that sits in the middle and on top of the cabinet above the little fan (it’s inaccessible unless you remove the outer sheetmetal body). I believe it’s supposed to sense the temperature inside the cabin. But I’m not sure how the rest of the system works in conjunction w/that fuse/sensor. I bought a new sensor/fuse and installed it but that one burned out too. But before I bought that I replaced the control panel b/c the LED clock quit working, and eventually the power shut off. After having tested everything based on the little paper booklet/manual that was stuffed in a pocket behind the control panel, I decided the problem was a bad control board. After replacing the control board (with a used one), the microwave worked again. I opened up some of the main relays on my old control b board and found the contact badly burnt, which tells me that was the problem. Anyway, after installing the new control board, the thermal sensor/fuse burnt out and the power cut off on it again. One of the wires connecting to the thermal sensor/fuse was burnt too, including about 1” of insulation being melted. So that’s my story, and any help is appreciated! Cheers…Chris Update (02/21/2022) Wiring diagram…
Hi @cadman777 Thanks for the wiring diagram. If it gets too hot in the oven the cavity thermostat will operate and prevent the oven from being used until it is cool enough by disconnecting the “neutral” wire (N) from the circuit When the oven has cooled sufficiently the cavity thermostat releases at the preset cooler temp and reconnects the “neutral” wire (N) connection to the circuit. In electrical circuits current flows from the active wire (L1) through the circuitry to operate the components and then to the neutral wire (N) which in this case is connected to the thermostat, to complete the circuit Which wire connected to the cavity thermostat has melted, the one coming from the control board (term P12 - the wire is actually a brown wire on P12 and changes to a white to connect to the thermostat - must be a join somewhere), the one from the primary door interlock switch or the black (blue?) neutral wire? I’m assuming that there are 3 wires connected to the thermostat, 2 white wires on one terminal and 1 black (or blue?) wire on the other or perhaps just 2 wires, 1 white and 1 black(blue?)? Whichever, it seems that it was carrying too much current for the wire and the thermostat and also the relay contact. Also I’m wondering why the fuse didn’t blow if this was the case. Since both white wires are fed through a different relay, as you know which relay had the problem as well you can test that part of the circuit to hopefully find out what the cause is. If it was the “door relay” that connects to P12 you can test the resistance of the three motors in the circuit i.e. stirrer, cooling fan and turntable motor as their circuits all pass through the door relay (and also the TTM relay in the case of the turntable). I’m assuming that the oven light is OK. Too low a resistance in any component causes more current to flow than what the circuit was designed for. If it was the “microwave relay” on the control board which connects to control board terminal 1 it is harder because you would have to test the resistance value of each of the components in the “high voltage system” comprising the HV diode, HV transformer (image only to show what it looks like - the part # is 4393809), HV capacitor and magnetron. Here’s a link that may help with this. Be safety aware when working in microwave ovens. The HV capacitor can store >6000V DC for months even if the power has been disconnected for this length of time. This amount of voltage can seriously injure you. The HV capacitor needs to be correctly discharged as soon as it can be accessed after the oven’s cover has been removed and before commencing any further work inside the oven. Hopefully I haven’t confused you too much with all this. I find it’s a bit difficult explaining circuit diagnosis to someone else. I’ve been more used to just thinking it out for myself when trying to find a problem. ;-) Cheers
