Gas furnace wont turn on but with a gentle tap

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  #1  
Old 01-21-18, 07:03 PM
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Gas furnace wont turn on but with a gentle tap

Hi,
A for a few week now, my furnace doesnt want to start unless I mess with it in some way.

the small induction fan is operating fine but thats it... no lights on the controle module, no spark, no flame, no blower.... I supected the module (S8670K) to be faulty, since I always managed to turn the furnace on by tapping more or less intensively on it... since I had a replacement module (S8610U) I installed it and tough that was the problem since the furnace started just fine.... but the day after it started to do the same problem again... tested the pressure switch (very sensitive at .20) works just fine.... and I can hear the click as I connect or disconnect from the fan.

now I'm there...

From messing around testing things, voltage and different stuff, I realise today that a VERY gentle tap on the lower part of the furnace, where the transformer and control circuits are, the control module instantly lights up and the furnace do it's cycle and starts.....

I'm lost....

Is it a bad ground.... and when I touch the furnace it creates a short or something..... ???

please help... ;-)
 
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Old 01-21-18, 07:34 PM
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Welcome to the forums.

That doesn't sound like a faulty module. On a call for heat you should get a fast flash on the status LED. You need to check for 24vac from 24V to 24V(gnd) on the module. If you aren't getting the 24vac you may have a defective limit switch.
 
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Old 01-21-18, 07:56 PM
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when I tested the 24v to 24v ground directly on the module... I was getting close to nothing... like some mVolts.
tested the 24v at the pressure switch since it circles trough it first and nothing there either...
I just manage to find that the furnace starts right up by taping on the bracket holding the control board and transformer while the door is off and the safety switch taped.

maybe just a bad cable connection.... I took out every one of them and put them back on... that seemed all pretty solid to me.... it is running right now.... will see if it starts again on the next call for heat.
 
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Old 01-21-18, 08:00 PM
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The thermostat via W starts the 24vac path. Then it travels thru the pressure switch and limit switches. The actual path/wiring diagram should be on the blower door. You can post the model of the furnace too.
 
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