Heater function switch changes by itself

[Sompin2do]

I've searched and can't seem to find this answer, but since I'll be changing the heater & evaporator on my 90 GT, I thought I would see if anyone can tell me the cause of my heater function switch seeming to change directions back and forth from heat to defrost at will.
Would be my luck to get the cores replaced and find out this is something behind the dash also.

Thanks
Mike

 

[87_stang]

It is probably a vaccum leak on the back of the knob on your dash. If it is getting good vaccum it will run on where you have the knob set and when the vaccum starts leaking it will fall back to the defrost. Im guessing that would be the problem. I am having the same problem on my Blazer and need to get a new switch (knob) setup to fix mine. Anyways, that would be my guess and good luck.

 

[Sompin2do]

Just to clarify. The knob doesn't actually move, just the heat. From heater to defrost back and forth. Sorry if I confused anyone.

 

[jaredgoodwin]

The flaps in the heater ducts are controlled by vacuum. With no vacuum applied they default to defrost. The knob doesn't move by vacuum. You are probably losing your vacuum signal to the heater controls, or as 87_Stang said, you have a vacuum leak!