Heat /Defrosters don't switch

ih8bills

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1991 Jimmy FULL size 5.7L V8
The heat won't switch to defrost or vice-versa-- stays at floor level all the time. I believe this is vacuum actuated-- can you give me a likely place to start trouble-shooting this ? Obviously I don't want to rip the dash apart if unnecessary. :( The truck runs great -- it's the stupid s__t that drives you up a wall... lol.
 
I have just dealt with the same issue. My problem ended up being a real easy fix. Remove the glove box. Remove the piece of the duct work that is on the left side of the glove box hole. Behind this you will see a small plastic part which is attached to the metal door that closes off the air flow to the vents. The plastic piece is attached to the door with one machine screw. Where this attaches will break and not allow the door to be pulled shut to shut off the air flow. My fix was simple, remove the screw, find a washer that is large enough to cover the edge of the plastic, tighten the whole thing back down carefull not to crush the plastic. Make sure if you use a different screw that it is not too long or it will not allow the door to open all the way and shut off the air flow to the other direction. It sounds a little confusing, but when you see in there it is very easy to see the problem and the fix too.

I hope this is your problem, if it is, fix it soon or it will lead to the main door inside the blower housing to break, that job requires removing the entire blower/heater core assembly to fix. That is where I had to go! Keep us posted on your fix.
 


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