Question: No heat from front defrost on "81

[ladams]

Hi,
I was wondering if someone could tell me in very simple steps, how I can get heat to come out the top vents just for defrosting purposes. I have no need for any other functionality and do not wish to restore the entire system for anything other than defrosting. I can control the temperature but the air only blows on the floor. I am simply looking for a quick fix and nothing else for my 1981.

Thanks,
Lee

 

[6880 Mike]

You need vacuum to the flapper door to move it from floor to dash defrost. Do you have the AIM? The AIM has a diagram of the various vacuum lines from the AC controls to their various locations and what they do. Good for troubleshooting.

 

[damoroso]

A quick fix? Maybe. If it's as simple as the vacuum line off the actuator. These systems are vacuum operated and usually it takes more that a quick fix, but you might get lucky. I suspect you have one of two issues, vacuum or a failed part. You could have a disconnected or broken vacuum line. If it's a part, it's either the HVAC control or defroster actuator. Most likely the problem is at the actuator, if the heat/vent/AC control is working in all other positions. The vacuum actuator on the defroster door likely isn't working for one of three reasons. First, the actuator isn't getting the vacuum it needs becuase the line is off/cracked/damaged. Second, the actuator isn't working any more (rusted, cracked diaphragm) or third, the door is frozen/jammed.

This actuator is located on the drivers side of the guage cluster. You'll have to get under the dash, pull the duct to the driver side vent and you'll be able to see it and get to it. Pull the console side off too, it's only three screws, and then you'll be able to see the controll unit.

To test the actuator, you really should have a hand vacuum pump. They're inexpensive and easy to find, plus with C3 corvettes, there's alot of vacuum actuated stuff so it's a good investment if you're a do it yourself type. Once you locate the actuator, connect the hand pump to it and pump it a couple of times. If the actuator doesn't move, disconnect it from the ducting and try it again. If it still doesn't move, the actuator is bad. If it does move, try and move the door manually. If you can't, the door is frozen.

If the actuator and the door work, connect the line that goes to the actuator to the hand pump, start the car and move the control lever to the defrost position. See if you get vacuum on the pump guage. If you don't your problem is the control unit. There is a vacuum "manifold" plug on the back of the control that could be the problem, you'll have to pull the console to see it and determine if it's that or the control itself.

It's really not as bad as it sounds, and it doesn't take that long to do. Hope this helps, let us know what you find!

 

[73shark]

Usually GM systems default to the defrost position as sort of a failsafe so at least you can still see to drive.

 

[Yoda]

Lee,

Looks like you got some really good trouble shooting advice, so I'll provide you with a "Bubba Fix"

(When all else fails , there is always a "Bubba Fix" )​

Bud

 

[73shark]

He even used duct tape. That should be worth extra credit.

 

[6880 Mike]

...He even used duct tape...

Duct tape makes his warm air warmer.