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Hi my name isXXXXX have been an appliance repair technician since 1992. The unit will have a cool down period after you set it, but if it is coming on without setting it, you should have a damper micro switch stuck. It is located under the damper motor on the right side of the cabinet. Here is a diagram of the unit :http://www.searspartsdirect.com/partsdirect/part-model/GE-Parts/Microwave-Parts/Model-JVM1790SK01/0432/0123370/G0903227/00005?blt=06&prst=0&shdMod=JVM1790SK01And the item is #1445 and part# WB24X10112Also make sure it is the convection fan you are hearing not the cooling fan. This would be another issue all together. Also there are two thermostats mounted on the bottom in the right side of the cabinet , if one opens up it will cause the fan to stay on.I hope this helps you determine the issue. Let me know, before rating please, i will continue to help until you are happy.
Thanks Rick. I'll take a look tonight when I get home and will get back to you.
No problem, let me know.
I took the control panel and outer cover (underneath) out to see if I can reach the switch. The compartment houses the fan motor and transformer (behind the control panel) and is pretty tight so I couldn't locate the switch. I would have to more of it apart. I ordered the part anyway; it will be in late next week.
I can't see if I would have another/better access if I removed the unit from the cabinet. In your opinion, would I get better access by removing the unit from the wall cabinet?
At long last, my microwave/convection oven is fixed. You were on the right track. The problem was that the damper (part 4003 on parts diagram) that that triggers the switch in the closed position for convection, was somehow obstructed. In the closed position, the damper makes contact with the switch and turns the convection fan on. In the open position, the damper move away from the switch, opening the circuit (removing power) to the fan. Once the damper was freed from whatever prevented it from re-opening, normal operation resumed. No parts had to be replaced.
The location of the switch is a bit unclear from the parts diagram. It is actually at the top of bracket (part# XXXXX) at the front of microwave. Locating it made all the difference. Once I understood the mechanical aspect, I was able to eliminate the "electronics" as a root cause.
Thanks for your help.