mobilehomerepair.com

Have a programmable thermostat that went haywire - furnace was functioning normally, then quit, and would not restart. Digital display, thermometer, etc. were not functioning properly, so replaced thermostat. Still wouldn't start (furnace or air conditioner).

The fuse was blown on the circuit board, so we replaced with a new fuse, which blew immediately even with new thermostat unhooked. We actually blew two fuses, one with the thermostat hooked up, then another with the circuit breaker off. Once we turned the breaker back on, it also blew the fuse immediately. The original fuse was blown prior to any work being done with replacing the thermostat.

Any thoughts on what might be causing this issue? Any help is appreciated!

I just put a new board in it still does same. Could the transformer be causing this? If I unplug the white line wire it wont blow but as soon as I touch that wire back to the transformer it blows. The voltage is 120 line and 26.9 load side. Guess it's space heaters for the rest of the weekend,at least its not the dead of winter yet!

Hi,

Trace the white wire to see if you find a short or bare spot where it may be touching a hot wire.

Check the line voltage wiring to the board.


Thanks,
Robert

Which wire do you check the line voltage at? I dont have wiring diagram. The white wire has 120 at board as well as the other 2 white wires right beside them but they are all common right? this is with no fuse in the board.

Hi,

You check line voltage at L1. Are you saying you read 120vac at the white wire from transformer at the board ?

If so, that is why the fuse blows when you connect it to transformer.

That wire and the other two are neutrals and should not read any voltage.

Somewhere in the circuit, a hot wire has contacted a neutral, creating a short.


Thanks,
Robert

I found it last night the wires running to the secondary limiter switch were grounding out. I can't believe they run those wires down through the back of the furnace with no grommets. They pass through bare metal in two places behind and under the blower motor.

Hi,

Yep, I've wondered that for 29 years now. Glad you found it.


Take care and best wishes,
Robert