1991 Chev C1500 no defroster air flow - AC Auto Repair Help Forum


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1991 Chev C1500 no defroster air flow

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MANelso
New User

Oct 16, 2007, 9:08 AM

Post #1 of 4 (2920 views)

1991 Chev C1500 no defroster air flow	Sign In

I have a 1991 5.0L chev 1500 ext cab. The defrost option is not working but floor and AC vents produce strong, heated air. When i switch to defrost, it sounds like the air flow becomes baffled -like it's forced against a closed door and the floor heat door closes.
In defrost mode the air still comes out the midrange vents. I'm a little nervous about tearing into the dash.
The other posts indicate that a default defrost occurs when vacuum is missing. Does that mean this can't be vacuum under the hood?
What are the logical steps to isolate the cause? If it is a vacuum leak in the driver compartment, where doe the vacuum line for the defrost diverter connect?
I pulled the glove box and there are three relays which make clicking sounds when I move through the three flow choices. Could one of these be the cause?
Thanks very much in advance. Nice forum.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Oct 16, 2007, 3:41 PM

Post #2 of 4 (2917 views)

Re: 1991 Chev C1500 no defroster air flow	Sign In

Must be Halloween already! The default is defrost in the absence of vacuum. If it didn't get any vacuum from leaking at the distribution box the other controls shouldn't work either. Quite honestly I've never heard of this failing this way so see what others might say on this. If you don't hear pretty soon hit back and follow me to site full of techs mostly from AZ that see a lot of kooky stuff.

You might try to see if you can move flaps by hand getting in there thru glove box as you have tried,

T

way2old
Veteran / Moderator
way2old profile image

Oct 16, 2007, 4:55 PM

Post #3 of 4 (2915 views)

Re: 1991 Chev C1500 no defroster air flow	Sign In

Hey Tom, our A/C guru, is there a possibility the heat/defrost door is broken? We have run into a couple over the years and it was on that era truck.

Being way2old is why I need help from younger minds

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Oct 16, 2007, 5:08 PM

Post #4 of 4 (2914 views)

Re: 1991 Chev C1500 no defroster air flow	Sign In

It's been a long time but they can break. The pivots can wear or fall out of place and the rubber ends can jam up but it's been ages. It's possible that this is not vacuum controlled but I don't think so. One thing is for sure is that the dash will look like road kill after you get at just what is wrong!

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