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G3GM 1973 to 1977 A-bodys. Chevelles, Monte Carlos, El Caminos, Laguna, 442, Cutlass, Malibu, GTO, LeMans, Century, Regal, Grand Am, Grand Prix, Sprint, Gran Sport, Can Am, Hurst-olds, and other 73-77 A-bodies


inoperative defroster

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inoperative defroster Empty inoperative defroster

Post by dhw3971 Mon Mar 16, 2015 10:27 am

So I've discovered that the defroster (and vent) setting does not work--air only blows from the lower vent right above the trans tunnel. I had this problem before and learned it was due to a disconnected vacuum line, but now it doesn't seem like any lines are disconnected. Can anyone tell me what magic is happening inside of the dashboard that makes the different settings work? I would like to get the vent and defrost working correctly again.
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Post by Mcarlo77 Mon Mar 16, 2015 3:11 pm

The vacuum to the air flow doors is achieved through the Vacuum Control Switch on the back of the Temp Control unit in your dash. Perhaps it's defective. Or, the blend doors are not reacting to the dedicated vacuum...either because the vacuum pods are shot (doubtful) or the doors are gummed up and not moving freely. This is assuming you have vacuum to the switch to begin with. WARNING: Be careful if you're considering removing the Temp Control unit as it's easy to break off one of the plastic vacuum hose nipples at the plug due to brittleness of the plastic over the years.
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Post by dhw3971 Mon Mar 16, 2015 5:00 pm

I'm not considering removing anything at the moment; I figured since the vacuum lines were recently all disconnected due to the engine work that was done that there must be something that didn't get hooked up again. So maybe the switch doesn't have vacuum? Where should I start tracking down that connection?

A few years ago I had the same problem but found out there was a line disconnected that leads to that round ball on the firewall...what is that?
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Post by Mcarlo77 Mon Mar 16, 2015 8:53 pm

On this older thread is a diagram I posted showing the vacuum hose routing for a '76/'77. Pretty sure '75 would be the same.

https://www.g3gm.com/t7015-climate-control-vacuum-lines#75575
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Post by dhw3971 Tue Mar 17, 2015 11:42 am

ah, great! I see others have had the same question. I'll get under the hood and see what's wrong.
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Post by dhw3971 Thu Mar 19, 2015 9:57 am

I discovered that there was no vacuum line connecting the engine to the HVAC system. Once I traced the disconnected hose and hooked it up my defroster and vent were restored.
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Post by thatfnthing Thu Mar 19, 2015 10:27 am

Excellent!
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