Furnace Blower Question

[turbojimmy]

I apologize if I've asked this before. I searched my old threads and didn't see it. I'm looking to fix this thing this week if possible.

It's a 1984-era Suburban 35k BTU furnace. From a cold start, it will turn on as it should and get the RV up to temp. When it comes time to turn back on, however, the thermostat will click and the fan will run for about half a second and quit. That's it. If I turn the thermostat on and off a few times it will eventually come on and stay on. Then functions as it should until it gets up to temp and does the same thing again.

I noticed that the fan behavior on this one is different than the rear, 18k BTU furnace. That one waits about 15-30 seconds before the fan starts. This front furnace fan comes on immediately. I think the change in fan strategy is because I can see the circuit board in the front one has been replaced - the whole unit has obviously been removed before so there must have been trouble with it.

So - I don't want to start throwing parts at it without some direction, but maybe that's the only course of action. Circuit board first? Or blower fan? Are there diagnostics (blinking lights) on the circuit board in a unit this old that would tell me anything?

[turbojimmy]

Well, it was indeed the thermostat. I had tried holding the 2 wires together and *thought* that I still had the issue but I guess not. In a fit of genius, I moved the front thermostat to the rear furnace and the rear thermostat to the front furnace. Suddenly, the front furnace worked but the rear didn't. Go figure.

They're old-school Suburban thermostats with 2 wires. I used an emery board and rubbing alcohol to clean the contacts on the faulty thermostat. Works great now.

[dougrainer]

turbojimmy wrote:
Thanks for the replies.

So would a faulty sail switch kill the fan right away? Or just prevent ignition? Whatever it is, it seems to be getting worse. It's getting harder and harder for the fan to start again. It seems that shutting off the TStat and turning it back on works better than moving the temp setting, if that's a clue.

When the fan doesn't run, I can still hear the clicking at the furnace when I move the TStat. So I'm now leaning toward blower or sail switch (if it prevents the fan from running).

1. 30 year old furnace ignition module and wiring will NOT shut the fan blower OFF if no burner lighting and staying lt. UNLESS, someone has replaced the Ignition module with a current that has the Fan switch ON the Ignition module. YOUR old ignition module just had the 4 wire connector and the Spark plug wire to the spark plug. The current that has the built in fan switch will have 2 extra wires connected on the Ignition module with standard wire spade connections. No 2 extra wires and you do not have the current ignition module. You year had a "Time/Delay" relay which is the relay that supplies 12 volts to the fan motor after a 15 to 30 second delay(hense its name) after the 2 wall tstat wires are closed together. The sail switch does NOT prevent your fan from running on your old model furnace. The sail switch comes into action only after the motor is running and stays running. From your description, either the Time delay is bad or the fan motor is bad. Doug

[dougrainer]

Grandpere wrote:

turbojimmy wrote:

Teacher's Pet wrote:
Circuit board was at the root of our furnace problems. We had no "clicking" of the ignition system after thermostat activated, when listening down by the furnace.

I get a loud click at the furnace when the thermostat calls for heat. Fan runs for a second or less and shuts off with the same loud click. Might be the board or maybe the fan motor is bad and drawing too much. The latter wouldn't explain why it stays running on a cold start.

The circuit board wasnt installed properly. It's hanging by the wires. Maybe I'll goof around with it to see if there's a loose connection.

The circuit board is suppose to be grounded to the chassis of the furnace, since it is hanging by the wires that could very well be the source of the problem. I am surprised that the furnace was working at all.

The furnace circuit/ignition module is NOT grounded to the chassis. It can "hang" and will still function correctly unless it is defective. Doug

[turbojimmy]

Thanks for the replies.

So would a faulty sail switch kill the fan right away? Or just prevent ignition? Whatever it is, it seems to be getting worse. It's getting harder and harder for the fan to start again. It seems that shutting off the TStat and turning it back on works better than moving the temp setting, if that's a clue.

When the fan doesn't run, I can still hear the clicking at the furnace when I move the TStat. So I'm now leaning toward blower or sail switch (if it prevents the fan from running).

[deandec]

Had similar symptom. Fan on, no ignitor click, no ignition.

I removed the 20 yr old furnace, blew everything out, including the burner cavity.

Disconnected cleaned, re-installed all connections and tested for voltage.

Bench tested and reinstalled.

It all now works as the instructions say it should.

I have no clue what I did to fix it....

[steveh27]

Sounds similar to my furnace problem last year. It was a dirty sail switch that was the culprit. Some lint & debris was interfering with it. The rv shop diagnosed and cleaned it. All good. I wish I had known it was the problem & knew how to access it for cleaning.

[2oldman]

turbojimmy wrote:
I apologize if I've asked this before. I searched my old threads and didn't see it.

Looks like you asked something similar back in October.

[turbojimmy]

Unfortunately the thermostat isn't the problem. I removed it and put the 2 wires together. It does the same thing. I'll try to get the circuit board mounted properly.

[Grandpere]

turbojimmy wrote:

Teacher's Pet wrote:
Circuit board was at the root of our furnace problems. We had no "clicking" of the ignition system after thermostat activated, when listening down by the furnace.

I get a loud click at the furnace when the thermostat calls for heat. Fan runs for a second or less and shuts off with the same loud click. Might be the board or maybe the fan motor is bad and drawing too much. The latter wouldn't explain why it stays running on a cold start.

The circuit board wasnt installed properly. It's hanging by the wires. Maybe I'll goof around with it to see if there's a loose connection.

The circuit board is suppose to be grounded to the chassis of the furnace, since it is hanging by the wires that could very well be the source of the problem. I am surprised that the furnace was working at all.

[Grandpere]

The blower motor is suppose to run to activate the sail switch which tells the circuit board that it is ok to open the gas valve and activate the igniter. I am betting that your problem is the thermostat, it is the cheapest part to replace. If that doesn't fix it then you need to decide how to spend $4-500. My wife and I decided to go with a new new 40K Suburban in place of our 30K Suburban. Our repair place gets $90 an hour for labor plus parts so we opted for new as ours had been repaired at least once by the PO. If you decide to go with a new furnace, we got ours at PPL.

[turbojimmy]

Teacher's Pet wrote:
Circuit board was at the root of our furnace problems. We had no "clicking" of the ignition system after thermostat activated, when listening down by the furnace.

I get a loud click at the furnace when the thermostat calls for heat. Fan runs for a second or less and shuts off with the same loud click. Might be the board or maybe the fan motor is bad and drawing too much. The latter wouldn't explain why it stays running on a cold start.

The circuit board wasnt installed properly. It's hanging by the wires. Maybe I'll goof around with it to see if there's a loose connection.

[Teacher_s_Pet]

Circuit board was at the root of our furnace problems. We had no "clicking" of the ignition system after thermostat activated, when listening down by the furnace.