I have an old Sears Power Miser 80 Furnace and the fan blower won't turn on when the thermostat is set to "Fan Auto". The furnace ignites, the Fan Limit Control spins around while heating up and when it hits the fan notch, nothing happens. Eventually the furnace will reach the shut off notch and turns off. If the thermostat is set to "Fan ON", the furnace works fine (but of course the fan never will shut off).

A little history on the furnace. We just had it inspected this spring by Centerpoint and the tech said it looks OK and it passed all checks. Later in the summer we had an issue with the Central Air and the tech had a difficult time troubleshooting it - was messing with wires inside the furnace and thermostat. Eventually he found the wall thermostat was bad, so he replaced it. The Central Air has been working great since then. But the first time I fired up the furnace today, I have this problem.

First Question: Could the Central Air tech have messed up something with the wiring that would cause this furnace fan issue?

I've tested the Fan Limit Control with an ohm meter - spun it by hand and when it hits the fan notch, my meter shorts. So looking over things and based on some research I believe the next cause could be the fan Relay? Any ideas ?? Thanks

Furnace Info: Sears Kenmore Power Miser 80, model 867.768020

 

Welcome to the forums.

Typically the fan relay is used for the FAN ON mode and cool. The fan/limit is the sole control for the heat speed of the blower. You need to check for 120vAC between the motor neutral and the heat speed wire. You may have lost a single winding in your motor.

 

Thanks for your reply. So the Fan Relay is only used for "Fan On" mode from the thermostat in heat mode? I guess that's not the problem for a "Fan Auto" failure then. I'll check for voltage on the Fan Limit Control when the dial rotates to the fan kick in spot. So checking for continuity shows it closes when it hits fan on, but I understand checking the voltage is more accurate.

You mentioned that I might have lost a "single winding" on my motor. Does that mean this is a 2 speed motor and one of the speeds may have gone out? The fact that my Air Conditioning is working properly (fan and all), would that help determine whether the fan windings are OK or not?

 

Typically the FAN ON/ A/C fan is the high speed (black wire) winding.
The furnace would use a red or blue wire for slower speed operation.

 

OK, I can't believe I fixed it. In my original post I mentioned that an AC tech was messing around with the wiring inside the furnace - I could tell he was getting frustrated, he was at my home a long time. Well he was this cause of the issue.

I opened the box below the fan relay to trace the yellow wire from the Fan Limit Control switch, and I noticed the yellow wire went into the relay, and another yellow wire came out of the relay but it didn't look like it was fully pushed it. So I grabbed the wire and it just fell out of the hole. I reseated the yellow wire, and tested it. Sure enough the fan works now.

Nice to have heat again - it was cold last night in Minneapolis!