Hey guys! I know this is a truck forum, but you guys are all DIYers so hopefully someone here can point me in the right direction to fix my furnace. I've got an old dinosaur RUUD from 1991 or so that turns on the blower every time the unit is plugged in, regardless of temperature setting or cold/heat setting. It basically ignores the thermostat as far as the blower's concerned. If the unit is plugged in to the wall, it turns on the blower. Heater (gas) burner kicks on and off normally, as does the A/C, but neither turns OFF the blower when they're finished. It just keeps on blowing.

here's what I've done so far:

Replaced the fan limit switch - old one was working fine, but this was my first guess.
Replaced the relay
Checked for a bad thermostat by disconnecting it altogether, blower still runs.
Checked for shorted green wire by disconnecting from control board, blower still runs

I do have a sensor that sits on the outside of the blower (pictured) that is a little old and crumbly. Does anyone know wha that might be (and could it be my problem?)






Thanks

 

I don't really recommend getting too deep into furnace repair -- I've seen houses blown up by furnaces.

But, there's probably a circuit board in there that controls the operation of the heat, A/C and the blower. It may be bad. There may also be a relay inside of the blower that gets its signal from the control board. It could be shorted which would be a lot cheaper to fix. .

Also, you probably have a switch on our thermostat that can have the fan run constantly or not. Since it runs withou the stat it could be a shorted wire in the cable to the furnace.

If you are comfortable with a meter, you can check some of these things out. Possibly you can search for your furnace online and find a schematic of the electrical hookups. Without a schematic you are really guessing unless you're familiar with how things are normally hooked up.

If you don't know how to check out electrical circuits, I'd recommend a service guy. There's 110 v on the loose inside there. If you do think you can manage that, please turn off the fuel before working on it. FTE wants all of its members

hj