Heater function switch changes by itself

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Sompin2do

New Member

Nov 30, 2004

59

0

0

TX

Jan 31, 2006

#1

• Jan 31, 2006

• #1

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

Member

Dec 12, 2005

131

0

16

Tonganoxie, KS

Jan 31, 2006

#2

• Jan 31, 2006

• #2

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.

 

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Sompin2do

New Member

Nov 30, 2004

59

0

0

TX

Jan 31, 2006

#3

• Jan 31, 2006

• #3

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

 

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jaredgoodwin

New Member

Sep 29, 2004

62

0

0

Jan 31, 2006

#4

• Jan 31, 2006

• #4

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!