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SmokinVette.com Forums : Corvette Forums : C3 Corvette Forums : C3 Corvette General Discussion : A/C problems 1978 Vette
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Old 05-02-2011, 10:01 AM   #1
classic68
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Unhappy A/C problems 1978 Vette

I have a 1978 Vette with factory A/C that is blowing warm/hot air out of the vents when the car is driven. This happens even when the blower and A/C unit are turned off. My guess is there is some door that is not closing properly,but as you know you can not see a damn thing under the dash. I was thinking that if I could find where the air enters the A/C unit I could detach the vent tube and plug up the outside air and then allow the A/C to pull air from inside the car.
Please if you have any suggestions let me know as I can not afford to take it to an A/C repair shop now. Thanks
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Old 05-02-2011, 12:50 PM   #2
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Quote:
Originally Posted by classic68 View Post
I have a 1978 Vette with factory A/C that is blowing warm/hot air out of the vents when the car is driven. This happens even when the blower and A/C unit are turned off. My guess is there is some door that is not closing properly,but as you know you can not see a damn thing under the dash. I was thinking that if I could find where the air enters the A/C unit I could detach the vent tube and plug up the outside air and then allow the A/C to pull air from inside the car.
Please if you have any suggestions let me know as I can not afford to take it to an A/C repair shop now. Thanks

First of all make sure your "heat/cold" selector (the bottom one) is slid over to the left side as that selector opens and closes the air blend door. Next, check your heater control valve to see if it's working properly. The valve is normally open and allows hot water to go thru the heater core when your selector is in any of the non-A/C positions. Then when you select either MAX or NORM vacuum closes the valve and shuts off the hot water going thru the core.
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Old 05-02-2011, 04:53 PM   #3
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I agree, check that heater control valve is shutting off hot water flow, or you will never get cold air
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Old 05-02-2011, 06:59 PM   #4
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Default Hot Air Blows Constantly Thru My Vents

Like I said, hot water flows through the heater control valve (and through the heater core) in all the selector positions except MAX and NORM. In those two positions vacuum is applied to the control valve and shuts it off. Then the HOT/COLD slide switch must be moved over to the left (COLD) to close the air blend door. If you forget to close the air blend door warm air will continually flow out of the vents because air pressure at the base of the windshield will keep the vents slightly pressurized.
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Old 05-05-2011, 06:22 AM   #5
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Quote:
Originally Posted by classic68 View Post
I have a 1978 Vette with factory A/C that is blowing warm/hot air out of the vents when the car is driven. This happens even when the blower and A/C unit are turned off. My guess is there is some door that is not closing properly,but as you know you can not see a damn thing under the dash. I was thinking that if I could find where the air enters the A/C unit I could detach the vent tube and plug up the outside air and then allow the A/C to pull air from inside the car.
Please if you have any suggestions let me know as I can not afford to take it to an A/C repair shop now. Thanks
Sounds like you have a vacuum problem. Make sure you have vacuum going to the control head (A/C and heater controls). Then check the vacuum actuators to make sure they hold vacuum when vacuum is applied, they operate the different modes, like heat ,vent, defrost, and A/C. Then check the temperature door lever and cable, this adjusts the temperature of the cabin air. The lever should have some drag on it. After that check to see that the compresser turns on. Also like the OP say check the Heater control valve too. But generally IF the temp lever is moved all the way to cool you should get some cool air, even if the heater control valve is broke. Let us know what you find.
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Old 05-05-2011, 07:03 PM   #6
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The bottom lever operates the air blend door by way of a push/pull cable. When it's moved all the way to the left the door is completely shut which prevents hot air from coming from your heater core. As the air blend door operates independently from your A/C you can add a bit of heated air to your otherwise cold A/C air.
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Old 08-04-2013, 12:47 PM   #7
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Default No A/C to cab -1978 C3 L82

I have a '78 Corvette that I stored at my Dad's place and just trailered it up to my place. Been cleaning it up, replacing the belts, other things. Runs good but big irritation since I started working on it is the A/C. I can turn on the A/C and the compressor kicks in but not air coming into the cab whether you have A/C or Heat turned on. I suspect a vacuum problem but locating it, not successful so far. A friend of my that has a 78 Vette said that he had that problem and there's a clear vacuum hose that goes thru the firewall up under the dash that his was broke. I have no clear vacuum hoses. Bought the complete kit for the heating and A/C vacuum hoses and going to replace. Anyone have any experienced advise? Tks, Don
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Old 08-06-2013, 07:11 AM   #8
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I have a '78 Corvette that I stored at my Dad's place and just trailered it up to my place. Been cleaning it up, replacing the belts, other things. Runs good but big irritation since I started working on it is the A/C. I can turn on the A/C and the compressor kicks in but not air coming into the cab whether you have A/C or Heat turned on. I suspect a vacuum problem but locating it, not successful so far. A friend of my that has a 78 Vette said that he had that problem and there's a clear vacuum hose that goes thru the firewall up under the dash that his was broke. I have no clear vacuum hoses. Bought the complete kit for the heating and A/C vacuum hoses and going to replace. Anyone have any experienced advise? Tks, Don
Here's a diagram of the vacuum lines. http://willcoxcorvette.com/instructi...82_secured.pdf Get a gauge like this to test each of the vacuum lines and components. http://www.amazon.com/Mityvac-MV8000...rds=mighty+vac

Remove the blower motor resister located in the top of the hvac box under the hood, with a flashlight check to make sure that the temperature control door is sealing completely when you move the control on the console all the way to cold, and then all the way to hot. If you need to make adjustments there is a plastic turnbuckle along that cable from the control back behind the console. Also, remove the glove box and behind it on the HVAC box is the other end of the temperature select cable, that when slid all the way to cold should make contact with a vacuum switch (that switch may need to be adjusted also) which controls the heater core shut-off valve on the right inner wheelwell.

If you find that the vacuum select switch is leaking vacuum http://willcoxcorvette.com/product_i...ducts_id=12686, take a small screwdriver and pry it slightly apart, then lube the rubber disk in between with silicone grease and work it in by turning the switch.

Start off by making sure the cable/ temperature door are adjusted properly, and then testing everything with the vacuum gauge to find any issues. Isolate each section while testing it. Good luck.
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Old 08-07-2013, 04:09 PM   #9
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Mike,
Thanks. I hope that I get to check this out this weekend but my gal has a lot of chores to do first, Ha! Anyway, I'll get back with you and let you know how I do.
Don
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Old 08-08-2013, 05:51 AM   #10
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Mike,
Thanks. I hope that I get to check this out this weekend but my gal has a lot of chores to do first, Ha! Anyway, I'll get back with you and let you know how I do.
Don
No problem. Also, if you want to help your blower motor work better, replace the small gauge ground wire going to it in the engine compartment with a 10 gauge ground wire grounded to the frame.
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