blower motor goes on and off by itself

[rob77c]

Goodman 100-3 furnace, 1993. Blower motor (and exhaust fan) comes on and off randomly, even when the thermostat is set to off. Removed the thermostat and separated the - no change. Any suggestions appreciated.

[heatseeker]

This is tough over internet you may have a short or a failing circuit board but you should call pro for this one.

[rob77c]

No way you know of to test the circuit *

[heatseeker]

Yes look in the yellow pages and call a company otherwise you can guess, you see troubleshooting is just an educated guess based on facts, if you think you have the experience to troubleshoot a circuit board go ahead but you will probably save time and money paying for an expert.

[Freon]

Turn off the furnace for 20 minutes, turn it back on and see if a control board reset changes the behavior. When the inducer starts randomly, does the flame also *

It seems like the control board is possessed by a poltergeist. If it's easy to remove (TURN OFF POWER). Check the board for any cold solder joints... you may get lucky. If a component (IC) is fried, you'll need a new board. Any bad storms recently or power issue due to the power company doing *

[rob77c]

Thanks. First thing to note is that when the thermostat is on, whether it be heating or cooling, everything works fine. Cycles run as normal, with no problems. Then, out of nowhere, at no particular time of day, when the thermostat is off, or even disconnected, I'll hear the fan kick in. The inducer motor and fan motor will both be running (no gas or ignition). Then, after awhile, the inducer motor will go off, and the blower soon after. Sometimes, the inducer motor will go on and off a couple of times while the blower motor is still running. I'm usually pretty handy, but this has me stumped. I was hoping someone a lot smarter than me would nail this! I'll inspect the board today, but if nothing is obvious, I'll have to bite the bullet and call someone.

[rob77c]

More to the saga. Checked the board, and nothing unusual there. As soon as I heard the fan kick in, I took the door off to watch. The inducer fan and blower would both come on together. The inducer would then start and stop once or twice, all the while with the blower running. Once the inducer stopped for good, the blower motor would run for another minute or two then shut down. This time, however, I noticed that the circuit board light was blinking 5 times-"False flame sensed", with the recommendation, "check sticking gas valve". There's nothing wrong with the gas valve, it always works when the thermostat calls for heat. My next thought was to check the flame sensor. I disconnected the sensor, and the blower and inducer still came on a few minutes later. I would think there's no way that the flame se nsor could be faulty if even when disconnected, the fans come on. So I'm back to thinking it's a faulty board.

[Freon]

You have done a great job. You either have some faulty control wiring between the thermostat and the furnace control board or you have a bad board. I think it's the board but check the wiring especially if there are any connections with wire nuts from the thermostat to the furnace control board.

If the inducer was coming on because of a call for heat (and we know there is no call for heat at the thermostat) then you'd expect the flame sensor fault since there's no flame produced. But you get the main blower also running and that is probably from the timer which may start after the inducer starts. But a flame sensor fault will shut everything down so the main blower should not even run at all. It seems like a control board problem and that's why your next test is to disconnect all thermostat control wires from the control board at the furnace. MARK/LABEL the wires for easy reconnect. This test will eliminate the control wiring as a possible cause.

If you still get this behavior I suggest a new control board.

[rob77c]

That's a good suggestion. Even though I previously disconnected the thermostat, you never know if there could be a pinched wire on the way to the furnace. I'll try that tonight. Thanks for all of the help. Oh, one more thing. Today I pulled the green blower wire off the board. The inducer and blower still came on anyway.

[heatseeker]

freon you are very knowledgeable and explain your diagnostics very clearly. You do have patience and I am envious, however I can learn from you and will try harder. Merry X-mas.