Furnace and fan won't stop running

[Coreyfatboy]

Hey everyone, I have a 01 cardinal 5th wheel and the furnace fan started running with the tstat turned off. The only way I could get it to stop was to pull fuse. I have since replaced limit switch, trd, and circuit board. Still running. Today I went out and the fan on the ceiling unit was running. Tstat was off the only way I could get it to stop was to unplug unit. Does anyone have an idea what the hell is going on. Thanks

[cadman99]

There is a time delay relay on the heater the runs the fan after the burner stops, until it cools a bit.

Here's a note from the service manual - it mentions time delay relay

When the thermostat has reached its set point and the demand for heat ends, the gas valve will be de-energized and the flame will go out. The post purge period of 90 seconds begins. When it times out, the blower motor output is removed, and the blower stops.

Here's a link for the service manual.
http://rv52.com/wp-content/uploads/2...n-Furnaces.pdf

[Coreyfatboy]

Thanks for reply. I have replaced limit switch, time delay relay and circuit board. Still runs as soon as I put fuse back in and now my a/c can is running as soon as I plug in and tstat is off

[cadman99]

How about disconnect the thermostat?

[Iggy]

Originally Posted by Coreyfatboy

Thanks for reply. I have replaced limit switch, time delay relay and circuit board. Still runs as soon as I put fuse back in and now my a/c can is running as soon as I plug in and tstat is off

On your wall thermostat change the fan speed from HI or LO to Auto and all will work correctly.
And keep the new parts for spares. or sell them here.

[Coreyfatboy]

I have pulled all the wires out of the thermostat when I was working on furnace. Furnace still ran. I have not tried it since the ceiling unit has been running

[Coreyfatboy]

Fan control is only on and auto. Has another switch for high and low

[cadman99]

Interesting - the only thing that can run the fan is the circuit board.

I have seen (on a motorhome) a secondary relay on the air conditioning circuit board that controlled the heater. The two control wires from the thermostat went to this board then to the heater. (Long shot but).

Must be a bad circuit board. Unfortunately there are no real test for the circuit board - You have to have the circuit board tester. That's a trip to the dealer which will cost more than a new circuit board.

[Coreyfatboy]

Is this other circuit board in the ceiling ac unit

[cadman99]

Yes - I discovered this while working on a 2000 Trek motorhome. Don't know if yours might be the same - as you know wiring diagrams are nonexistent.

[Coreyfatboy]

Is there any certain thing I should look for in trying to find where this circuit board is.

[azjerry]

In my new to me Surveyor, I noticed that the furnace would run when it was cold enough in the trailer even though the thermostat was set to off. I found that there is a tab on the temperature slide that is supposed to separate two contacts when in the off position. The separation wasn't happening. I added a small blob of superglue to the tab which was enough to separate the tabs.

[Coreyfatboy]

It runs hot,cold, thermostat unhooked, no matter what. The only way it will stop is if I pull fuse

[cadman99]

The board I found was in the cavity inside under the air conditioning cover - this was a nonducted unit.

Based on the every I wiring diagram I see the +12VDC to the fan motor passes through the time delay relay which is controlled by the circuit board. The circuit board triggers the time delay to purge the furnace before ignition. This time delay circuit is the [COLOR="black"]only[/COLOR] way the blower gets power.

I realized you have replaced all of this but it has to be a defective circuit board or a stuck time delay. This would be true even if you have a second relay in the air conditioner. It would only trigger the furnace circuit board and the time delay.

If you can get to the furnace look for the two small wires in the loom with the 12DVC wires (usually blue) to and from the thermostat. If possible I would disconnect them, at the furnace, which will totally eliminate the the thermostat and the wires to and from. If somewhere these two wires are touching they are completing the circuit just as the thermostat would.

[Coreyfatboy]

Ok. I'll give that a try when I get a chance. Thanks for the help

[YoungKopernik]

Never had an issue like this but……
Your problem could be a short: have you put any screws through the wall(hanging a picture)…or could wires have gotten pitched together from a slide. An easy check would be to disconnect the wires from thermostat and see if the signal wire has constant power(with fuse for furnace installed). Or a better check would be at the circuit board to see if it is constantly getting a signal(power)to run. If there is no short you should be able to eliminate that part of system.
Boards can come bad and it can be a headache wanting to believe it can’t be bad.
My mom would say, “it’s a ground problem”. Over the years she has seen my dad and I chase so many electrical issues that turned out to be a ground issue. Mom is right more times than not.
For me, when trying to chase electrical issues, I slow down and think of the system. I get as much paperwork as I can(schematic, wire diagram, trouble shouting from manual, warranty, and manufactures phone number for technical issues…..) and then start chasing.
This forum is a great source of personal experience but might go straight to calling the manufactures technical support. These guys are usually very nice and can give a direction to look.
Good luck
John