The blower on my Goodman gas furnace came on all by itself tonight? Every 5-10 minutes the blower turns on for approx 1 minute then turns off by itself. Then the cycle repeats.

I verified that the thermostat is in the off position (no heat/no cool) and that the fan switch was set to auto.

I have checked wiring behind t-stat and disconnected t-stat completely. Blower still appears to be haunted.

Nothing new has been installed or repaired recently.

What might cause the blower to auto cycle like this?

Defective fan/blower control is a possibility. Depending upon the unit it could also be a circuit board. Depending upon the thermostat also. Some have an intermittent blower setting that could have been turned on.

Thanks for your suggestions hvac1000.

I'd like to try and make the repairs myself. My furnace is a Goodman model 1026. It is about 10 years old.

I think I've been able to rule out any problems with the thermostat, but I'm not sure where to start to troubleshoot the board or the motor control

Any help would be greatly appreciated!

The only other help I can give you is to tell you that you should or will have to catch the problem when it is happening the figure out why it is happening. I know this is not much information but you have a very difficult problem.

The number you posted for a model is no listed in any of my books. The usual Goodman model number usually start with 3 letters then numbers. Example GUI 125-3. Please post there real model number.

Some Goodman furnaces have limit switches that won't allow the blower to shut off until the plenum has cooled down. You may want to see if your furnace has one. These are usually adjustable and it could be out of adjustment or calibration causing the blower to cycle on and off.

Sorry about that, correct model number for gas furnace is GMP 075-4

(Fortunately) the problem occurs somewhat consistently. Blower motor is OFF for approx 5-10 minutes, then the blower turns ON by itself and runs for about 1 minute, then turns OFF again for another 5-10 minutes.

Trouble light glows steady when blower is OFF and blinks rapidly when blower is ON. But the rapid blinking appears to stutter, sort of two short blinks together, like there is an issue with power to the LED/board.

Strange problem?

By the way, I don't see an adjustable limit switch.

My model number is GMP 075-4.

That model has no adjustable fan and limit control. You need to find out when the fan comes on if there is any low voltage present at the thermostat wire connections in the furnace, red will always have power, green, yellow and white should not. If there is no voltage pressent when the blower comes on I would guess it to be a bad PC board.

I had the same symptoms on a GMP075-3 furnace. Completely turn off or even disconnect the thermostat wiring. If it continues to cycle the blower, it's the circuit board. The one minute on time is really the shut-off delay that the blower operates after the burner shuts off during a normal cycle.

I replaced the circuit board and it was fine all winter. Do some searching online for GMP circuit board or GMP controller board. You should find two models. A Goodman brand replacement (18099-13) and a 3rd party replacement. I found a new Goodman brand one on eBay for about $60. Most places charge $125-$175 Comes with installation instructions. Took about 30 minutes for the swapout.

Keep careful track of the wires. The new Goodman board is not an identical replacement of the original. It is a newer design so some of the wire orientations are different. Not by much, but a little. The markings on the board are the same, though. So just match them 1 for 1. Unmount the old board without disconnecting the wires. Attach the new board to the furnace mount. Then transfer wires 1 by 1 from the old board to the new board. Also doesn't hurt to take some digital photos of the old board while it is still in place so you can go back and recheck where each color wire was plugged originally. Good luck.

Thank you for offering me your great advice everyone.

NorthernHeat, I will make the voltage measurements to verify that the board is bad.

Thank you bkspero. The shut-off delay makes sense - I'll track down another board and report the results once it's up and running correctly again (hopefully). :)

NorthernHeat,

I measured low voltage at thermostat. I left the system in the OFF/AUTO mode and the blower is still automatically cycling on and off.

When the blower is OFF:
Common+red=27V
Common+green=0V
Common+yellow=0V
Common+white=2.7V

When blower comes ON:
Common+red=26.4V
Common+green=0V
Common+yellow=0V
Common+white=2.6V

Does the voltage measured at the white wire mean there could be a short on the control board?

Do the voltage measuremnts above mean anything to anyone?

If you remove the red and green wire from the thermostat screws, will the blower still cycle off and on. If so you have some other problem not related to the thermostat.

I replaced the control board yesterday and the unit is working correctly again.

Thank you bkspero! Your advice was perfect. I picked up a new board on eBay and swapped it for the old one just as you said.

When the blower is OFF:
Common+red=27V
Common+green=0V
Common+yellow=0V
Common+white=0V

When the blower is ON:
Common+red=26.4V
Common+green=26.4V
Common+yellow=0V
Common+white=0V

There are no areas on the old circuit board that look suspect, so my guess is there's an open solder joint (or an internal failure of a component) somewhere on the board?

Many thanks to everyone for your help!

Quote:

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Originally Posted by h-man

I replaced the control board yesterday and the unit is working correctly again.

Thank you bkspero! Your advice was perfect. I picked up a new board on eBay and swapped it for the old one just as you said.


There are no areas on the old circuit board that look suspect, so my guess is there's an open solder joint (or an internal failure of a component) somewhere on the board?

Many thanks to everyone for your help!

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Glad to have been able to help.