Tons of snow and defrost doesnt work????

[GM Man]

This morning at 5am I'm driving to work and my defrost isnt working, making it very hard to see out of the windshield. The selector seems to work cause I can turn it off, but its stuck with pushing air out on the floor. No other position works. I'm thinking a vacuum line might have gone T.U. Its a 78 Jimmy with A/C. Any ideas??
Its a bitch to drive in the snow when you cant see!

 

[HarryH3]

Many years ago, I bought a '77 with the same problem. Turned out that a BIC lighter had fallen down though one of the defroster vents and found its way into the door that rotates to select various air outlet options. It coudn't move because the lighter was jamming it. The door is right where the main plenum screws to the housing under the dashboard. Remove the 4 or so screws and see if some junk has found its way down into there.

 

[Derf00]

Harry,

There is a door in the main duct coming from the heater core that directs the flow of air from the floor to the defroster ducts. The door has a plastic piece that a control arm hooks to, they break quite often on our older vehicles. My experience is with my '90, but I'll bet that GM hasn't come up with a much better design.

 

[Can Can]

I had a really good post about this repair. Unfortunately, it looks like the system has deleted it somehow- the link takes me to an "Oooops" page.

 

[mouse]

That detailed fix was a great write-up. I took the easy route. As the PO had removed the AC, I "hard wired" the ducts to defrost only. Its works fine for heat and defrosting. I don't need it for anything else. Just some wire to hold the door in the "defrost" position.

 

[spearchucker]

Is this the Oooops page you were looking for?


.

 

[Can Can]

Roger that. I'm not sure why it keeps telling me it's not in the database anymore....

 

[spearchucker]

Were you trying to get to it from a link to the old database?

 

[GM Man]

Harry, glad to see your still around! Thanks guys for the info, now that my driveway is free of snow, I'll get on it tomorrow.

 

[GM Man]

Fixed it today! I had to run my Edelbrock carb while I was waiting for my Holley Truck Avenger, I had unplugged the vacuum canisiter on the passenger side firewall. Once I plugged that in today everything worked like it should have. I never thought that canister would have affected the HVAC! Thanks everyone!

 

[HarryH3]

Glad to hear ya got it fixed easy! BTW, that canister stores vacuum to keep the a/c controls in the correct position during times when there isn't any manifold vacuum available (think WOT while climbing a long grade).