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GE Micro/Oven Combo: Oven fan on constantly

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Posted: Sun Nov 29th, 2009 08:41 pm

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tsutton

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After warming something up yesterday the microwave display turned off and would not respond. I *think* the cycle finished but my wife was using it so I'm not sure if it shut down while running or after she opened the door. The microwave light also stayed off when the door was opened. The lower oven and display worked as usual. Today I shut down the power and opened up the control panel. After checking the fuse (it is ok) and continuity/operation of the door switches/interlock switches several different times (they seemed to be working OK) I got fed up and closed it all back up and turned the power back on. Surprise - the microwave display was lit up saying "RESET". I set the time and tested it out - the microwave seems to work like usual. The problem is when I turned the power back on the oven cooling fan came on and runs constantly. The oven hadn't been used in a day so there really isn't a reason for it to be running. I did have to disconnect a few wires to get to the fuse and test the switches but I'm 99% sure I plugged everything back up correctly (the schematic is almost useless). Web searches have proven to be little help. What else should I check? Part(s) to replace? Thanks in advance for any help!

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Posted: Sun Nov 29th, 2009 08:53 pm

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appl.tech.29501
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hard to say without a diagram. does the oven fan normally come on when in normal use immediately when turned on or is it designed to turn on after a certain temperature?

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Posted: Sun Nov 29th, 2009 09:05 pm

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tsutton

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I can't say for sure because I never paid that much attention but I'm fairly certain the fan only comes on when the oven has been on. The schematic shows a "FAN THERMAL T.C.O "140/158 F)" so that kind of backs up my memory. I can scan or take a pic of the schematic if you need it...

Last edited on Sun Nov 29th, 2009 09:23 pm by tsutton

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Posted: Sun Nov 29th, 2009 10:08 pm

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RegUS_PatOff
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requested service manual link PM'd

31-9098

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Posted: Mon Nov 30th, 2009 10:07 pm

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tsutton

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So the door switches and interlock seem to check out. The fan tco out of the MW is open then closed when heated up. Fan TCO seems to be closed when power is applied but I'm not sure if I can check it that way. Not sure about the magnetron tco - seems ok since it is supposed to be normally closed. Everything works fine but the fan is still on. Need to get parts - I've pretty much narrowed it down to the Fan TCO, magnetron tco, or the MW PC board. What should I start with? Thanks...

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Posted: Tue Dec 1st, 2009 01:02 am

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RegUS_PatOff
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with M/W un-plugged ...

using OHM meter,

is Fan TCO open or closed at room temperature ?

Is that where the Fan is (now) connected to ?

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Posted: Tue Dec 1st, 2009 02:54 am

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tsutton

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At room temp the fan tco is closed - no continuity in and out of the M/W. I tested w/o power with the leads connected and disconnected when the tco was in the M/W and obviously disconnected when I removed the tco. After taking it out of the M/W I put it in a skillet over low heat to test the cutoff - after a minute or so I heard it click and I had continuity. Took it off the heat, let it cool and after blowing on it for 30 seconds or so I heard it click and I lost continuity. Seemed to work as it should. I did test continuity after the leads were reattached and power applied (both blue wires attached) - got continuity regardless of temp but I'm not sure if that is normal or not when the power is on. Based on the schematic the fan is connected to the fan relay, Fan TCO, Magnetron TCO, with a common wire also attached to the fan. With power on I did pull both fan tco wires , then both mag tco wires, and then both sets at the same time and the fan ran throughout. Not sure where that leaves me but I hope you can help. I have sinking feeling that the PCB needs replaced. Side ? - should the fan relay show continuity with the PCB removed from all power - it did? I think I probed the proper pins on the back of the PCB but I'm not quite sure.

I hate to say it but I'm ready to buy both tco's and a pcb to fix the problem - hope you can help and lead me to the right part before I spend $ I don't need to.

Thanks again...

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Posted: Wed Dec 2nd, 2009 09:32 pm

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tsutton

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Here is the schematic and wiring diagram scanned from the TDS.

http://mysite.verizon.net/reszv36o/s...es/image-3.pdf - you'll definately need to zoom but it is readable.

I did pull the board today and checked continuity on the fan relay (RY-3) (two pins on the back side of the PCB that are by side that are slightly offset) - the switch was open. I put the board back in and powered up - the switch was then closed with the door open - it should be open based on the schematic. I suppose the circuit could have been completed through the wires that led to each side of the relay - I hate electronics. Also, there is a thermostat/TCO (FAN THERMAL TCO) that turns on the magnetron fan when the MW isn't running - the switch is normally open but closes and turns on the fan when the sensor detects a temp above 157 degrees caused by the lower oven being on. It is supposed to prevent damage to the MW electronics when the oven is/was on.

Did I mention I hate electronics? Grrrrr.

Can anyone help? Please!

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Posted: Wed Dec 2nd, 2009 10:10 pm

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RegUS_PatOff
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The Fan can either be powered by the Fan Relay and/or the Fan T.C.O.

Disconnect the Fan T.C.O. or un-plug that 4-pin Connector and see if the Fan still runs.

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Posted: Thu Dec 3rd, 2009 04:04 am

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tsutton

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The fan runs regardless if one or both fan tco blue wires are connected or disconnected. The fan stops along with MW display and interior light when CONN2 (4 pin 3 wire white/orange/blue) connector is detached. I was lazy - I disconnected the wires when the MW had power and the fan was on. Side note - I could hear and feel a click in the fan relay on the PCB - if that matters. I also pulled the power relay plug - same result as pulling the CONN2 connector - everything MW related shut off - I suppose that would be normal given that it is a power relay. Dunno..I feel pretty dumb.

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Posted: Thu Dec 3rd, 2009 04:10 am

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RegUS_PatOff
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sounds like the Relay Contacts are welded together ...

it could still make the click sound ..

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Posted: Tue Dec 15th, 2009 01:07 am

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tsutton

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So I gave up and bought a new PCB board. I plugged it in and the fan stopped running - awesome! However, the MW display was blank and toggling the display button made no difference. I called the appliance shop and told them the part was bad - my guess is someone had a bad display, bought a new board and returned this one. Anyway they shipped a new part. In the mean time I took the power and fan relays from the new board and soldered them to the old board - I figure they were good since the fan wasn't running - maybe just a bad display on the new PCB. I installed the old PCB with the new relays - no difference - the fan ran just like before. I got the new PCB today. I powered off, plugged everything in, and everything was back to OEM condition. Finally I can hear the TV!

My question - what was bad on the old board if it wasn't the power or fan relay? I have two semi bad PCBs and if the problem happens again it would be nice to know what was the real problem with the original PCB.

Thanks - any insight is much appreciated!

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Posted: Tue Dec 15th, 2009 02:18 am

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RegUS_PatOff
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tsutton wrote: .. My question - what was bad on the old board if it wasn't the power or fan relay? ..

could be a bad controller chip or some driver transistor, etc ...

may have been caused by lightning, voltage spikes, etc ..

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Posted: Mon Nov 15th, 2010 04:04 pm

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14^th Post

tsutton

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The microwave fan has decided it doesn't want to run when the microwave is on. It will still heat food and the light and turntable work as expected.

I opened up the control panel and jumped the fan TCO (normally open) and the fan kicks on. That leads me to believe the fan motor is working fine. Is there something other than the fan relay that could be causing the problem? I really don't want to buy another new PC board. Schematic and wiring diagram is attached.

Thanks.

Attachment: Schem.pdf (Downloaded 1 time)

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Posted: Mon Nov 15th, 2010 04:22 pm

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RegUS_PatOff
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tsutton wrote: ... Is there something other than the fan relay that could be causing the problem? ...
looks like either a bad Relay (or wire connections to the Fan) or a bad PCB Board ..

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