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Hi all, we have an original 1990 Intertherm MGH-055ABWN-07 gas furnace. The fan switch has options FAN, OFF and ON. With the thermostat/heat off, switching it to FAN (which I'm assuming is the same thing as "auto") kicks the fan on immediately and blows cool air. Same thing when switched to ON.

I am suspecting a bad limit switch but I'm not 100% sure. Our fan/limit control board is identical to this one:



How should I go about diagnosing the limit switch before buying this part?

Thank you.

Check it with an ohm meter, the Low switch should be open until the chamber warms up and closes the switch to turn the fan on. The High switch should be closed until the chamber heats too much and it turns the burner off.

Greg

Assuming the Low switch is the white one labeled Fan and the High switch is the red one labeled Limit, then the resistance values were as you said:

High switch - 2.5 Ω
Low switch - Open

Interestingly, I had turned the pilot off to take out the switch assembly. I put the assembly back in and reconnected it, pilot still off, and now the fan turns on when set to FAN, but simply hums when set to ON.

Is it possible the pilot light was too large/hot and was causing the fan to switch on? Or is it a result of a worn out switch that comes on at too low of a temperature?

Bump. Anyone know what I'm dealing with here? The fan and limit switches appear to have checked out OK with the ohmmeter.

Here are some more images of my fan control and furnace in case it helps.

The FAN setting is an override and will run the fan 24/7 continuously. When in the ON position the fan will only come on as needed (when the thermodisc switch activates it due to heat in the heat exchanger). YES there are times the pilot can be too large and cause the switch to come on in a non heat cycle. I would lean towards the switch being weak though. That part is sold as a complete unit, fan and limit assembly. The hum you hear is the transformer, they are loud on some of the older models.