My AC/Heater fan speed was stuck on one setting. I eventually found out that the fan speed switch was burned up. You can see pictures of the offending switch here. https://www.thirdgen.org/forums/tech...tml?highlight=

I pulled a unit from a Junk Yard and replaced the switch. Everything worked after that (actually my AC isn't cold and the compressor cycles on and off every few seconds, but I don't think that's related). But I guess I never found out WHY the switch burned up in the first place.

I usually have the control unit set to the AC settings or the Vent setting. Well the other day the weather was just right and I turned off the unit. After a few minutes I started to smell something burning. After a few more minutes I saw smoke coming from the control unit. I turned the lever back to the vent setting and the smoke subsided. I turned the unit back off and I started to see smoke again.

Any ideas why this is happening?

 

Because you replaced a used one for
a used one you didn’t gain anything.
Also, even in the “OFF” position
there is current flowing thru the switch
to maintain a very slow blower speed.
Before you install a new switch do a
routine check for shorts in the blower circuit.

 

I just thought it was interesting how the issue only arises when the unit is set to off. I'll dig up the schematic and inspect.