Microwave Oven Repair

[skorper]

I have a G.E. Profile Model# PVM1870DM3BB microwave oven.
It won't heat, but everything seems to be working normal.
If I jumper the Hood Temperature switch, the hood fan comes on and it starts cooking normal.
The Hood Temperature switch is N.O. and only closes when overheating.
I'm stumped!

[bob91343]

It's unclear what you are experiencing.  If you jumper a switch that closes in case of overheating, how can that make the unit work normally?

Read your post again and clarify.

[skorper]

This is why I am confused.
Everything seems to work normal, but won't heat.
When I jumper the 'Hood Thermal Switch', the Hood Fan turns on and the oven cooks normal.
The fan should ONLY turn on when the oven gets HOT.
I checked all of the door/interlock switches etc.
I ONLY get power to the high voltage transformer when I jumper the Hood Thermal Switch.

[bob91343]

You probably need to replace that switch.  It's probably NC, not NO.

[james_s]

Are you positive that switch is NO? Are you sure the fan that's coming on is the hood fan? Every microwave I've seen also has a fan to cool the magnetron that gets powered when the HV transformer is powered. I suspect Bob is correct.

[skorper]

I ordered and replaced that switch before I found the schematic.
It definitely show N.O. on the schematic.

[skorper]

It is definitely the hood fan. Everything is wide open and I can see everything.
The Hood Temperature Switch goes directly to the main control board.
I don't know if some other 'input' is causing this or is it just the control board.
TOO expensive to guess.
Just the Hood switch cost $50

[WattsThat]

The Hood Temperature switch is N.O. and only closes when overheating.

So prove it. Pull the the switch out of the unit and put an ohmmeter on the contacts. Does the switch operate from open to closed when you heat it with a gas match, heat gun, whatever you have?

Don’t assume the drawing is the final word, the part behavior is!

[skorper]

The schematic shows N.O.
The schematic also states that 'The contacts close at approx. 158 degrees F.'
BOTH switches meter the same.

[bob91343]

Show us the diagram.

[skorper]

Please see attachment.
It's almost 1:00 AM
I will check this post in the morning.

Thank you SO much for your efforts!!

Steve

[bob91343]

The only thermal switch I see is right at the black wire of the power cord.  And it's NC.

[james_s]

I assume the switch he's talking about is the one called "Hood T.C.O." which is down on the lower left and connects via wires. Unfortunately the schematic is not real useful since (like everything else these days) it does not show the schematic of the PCB and just treats it as a black box.

[aargee]

I wouldn't be surprised if 'Hood TCO' is in fact a bi-metallic thermal switch. When activated, disables the magnetron but keeps the fans going to cool down.

[G7PSK]

Possible that the hood temperature switch is a thermal fuse, If so it needs to be replaced as once blown it stays blown.

[perieanuo]

closing hood tco and magnetron activated is just chance, you should check for the defect maybe on pcb/relays or another switch
i think that hood tco just starts the cooler like on any other ovens, not directly related with magnetron relay(s) activation, secondary relays 1 and 2
without hood tco activated, the table is spinning? it should be related with magnetron relays

[perieanuo]

one track to follow, hood tco removes an error state, like that gas sensor triggered and stopping the magnetron relay(s), so maybe hood tco bypass some kind of error, for me this is the only logic explanation if pcb logic works normally, otherwise my previous post applies
[edit] i retract this, apparently that sensor only modulates cooking, it should not be 'touched' at least the sensor part (you can only test winding resistance), leave it alone for now, maybe i said something wrong so it needs to be analysed further if other tests give no answer