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1988 Chevrolet 1500 - Defrost disabled

heater won’t move to defrost. vent and floor work fine

A couple of possibilities. Something is NOT working fine or a furry four legged critter built a nest in there or someone’s pet ball python took up residence in there.

My 96 GMC had a level fall off of the blend door duct work. It was directly next to the gas pedal under the dash. It would blow on face and floor, but nothing on the windshield. I was lucky and was able to just put it back on the shaft for the doors. It was keyed.

Another possibility is the door actuator is out of calibration and not moving the door into the correct place. These can be purchased at most local parts stores.

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The HVAC system in your vehicle is operated with engine vacuum.

And this engine vacuum is directed thru the vent controller.

The three vacuum nipples you see pointing up control the mode motors in the vent system.

So all the other modes work except defrost, its got to be the defrost vacuum motor.


Tester, pretty sure the blend door motors are electronic on that model. They were in my 1994 Silverado. I believe 88 and 94 were the same body style, interior, etc. I think 1987 was the last year for the version in your pic. Not 100% certain, though.

You may be right.

It’s a blur when they went from vacuum to electronic control.

After all, it’s a GM.


Strangely enough, my 98 Dodge Ram was vacuum operated.

Those 88-95 Silverado trucks are one of my all time favorites. The weird dash included

My 89 Buick Riviera was vacuum operated. I was waiting for a train and the thing went to full defrost and nothing else. Turned out the vac hose dropped down and got a hole melted in it. Cheap fix.