79 - Air through heater vents when lights are on
 

I recently refreshed the A/C systems and it appears to work just fine (during daylight hours). Tonight it was the first time I drove the Vette in the night with the A/C on. Airflow was coming through the heater vents only. Once I turned the lights off, the air-flow was restored through the center and side vents.

Is this unusual or a known issue?

Years ago I restored a 1979 that was fire damaged... (never again will I do a fire car).... But I had a similar issue.... The problem was the ground at the number 1 body mount. The fire caused the ground to corrode and not make good contact. My issue was when I'd turn the headlamps on the gauges would stop working in the center console. So I'd suspect that this might be a similar issue for you.

You might check the main wire ground at the A pillar (see number 3 in the picture below) and/or run a temporary ground from the number 1 mount to the frame and see if this fixes your issue....

Willcox

http://repairs.willcoxcorvette.com/w...sh-grounds.jpg

I need to revive this thread - my issue is not an electrical issue but vacuum - When I turn on the lights (vacuum operated) air comes out through the heater vents (bottom) only(!) no matter where the A/C control is set up. You turn off the lights and the air flow is restored through the center vents. I need some clues before tearing up through the dash. The Vette was on the lift for the last 4 weeks and was excited to have it back on the road trying to show the wife working A/C!!!! But it was dusk and as soon as I turned the lights on, air through center vents was gone. One more trip to the restaurant with the windows down in the Texas scorching heat. I had forgotten all about this - I have the cycles now.... I have to fix it.

Floor is the default outlet when the HVAC control doesn't get vacuum. A severe vacuum leak in the headlamp system vacuum or other defective component(s) will cause the vacuum to drop so low that the HVAC control can't operate and air comes out the floor.

The following photo is from a '79 so you should see very similar. The only difference should be if you have cruise control.

The picture is taken at the left, rear of the engine compartment between the distributor and the brake booster.

The red arrow points to the filter. The other end of its hose connects to a fitting at the rear of the intake manifold. Remove it paying attention to the orientation. Suck on the system (non-manifold) side. It should be like sucking through a straw--no effort. If not nice and clear it needs to be replaced. Pay attention to reinstall in the same direction lest and trapped crud be sucked into the engine.

The yellow arrow points to the one-way valve. It has three connections (one on one side, two on the other). The tube on the side with one connection leads to the filter. The two on the other side lead to: 1) the control portion of the headlamp doors and 2) the operation portion of the headlamp doors + HVAC. The purpose of the one-way valve is to prevent vacuum in the accessory systems from dropping during periods of low engine vacuum. A hand vacuum pump is best to use for testing but you can do it by mouth. Pump or suck on one of the connections on the "two side" with the other capped. It should hold vacuum indefinitely (impossible to suck).

The green arrow points to a vacuum tee. The tiny branch leads to the HVAC. The big branch leads to the operation system of the headlamp doors. Obviously the tee should be in good shape.

https://cimg0.ibsrv.net/gimg/www.cor...4897bc8205.jpg

If everything is OK up to this point, you have a BIG vacuum leak in the some part of the headlamp door system. To determine the general location of the problem, pinch off the large hose of the tee identified by the green arrow. Do the hose NOT leading back to the one-way valve. Vise grips work reasonably to do this. With the engine and A/C running of course, turn on the headlamps. If the air continues to blow from the dash vents (more likely) the problem is in the operating part of the system; if not the problem is in the control portion.

Please report back to say if the problem is found or if you need further instructions.

One question for you. Do you know if you have a stock or a "racing" camshaft?

Thanks Mike for your informative reply! I just checked the aforementioned vacuum valve and it works as you described.

The vacuum routing on my engine bay is a bit different from yours - see picture below. Al emission apparatus has been removed from the engine bay by PO; also no cruise control on this vehicle.

The camshaft is not the original camshaft - it is, what has been described to me, as a "mild" spec cam - I get about 14in of vacuum at idle

I do not have that skinny hose off of that tee as shown in your picture. There is a different tee close by which may be the equivalent one??

https://cimg5.ibsrv.net/gimg/www.cor...21203478bf.jpg
https://cimg6.ibsrv.net/gimg/www.cor...f3c89af42a.jpg

Shoot, what do I have to lose? I looked through my vacuum tee fittings drawer and did some simple re plumbing that looked like your pic Mike that took less than a minute and voila! I can have lights and A/C blowing cold air through the center vents at the same time! Woo Hoo! Not convinced I fixed the real problem but I will roll with it until something else related pops-up.

Thank you Mike!!

https://cimg5.ibsrv.net/gimg/www.cor...ad458030d9.jpg
https://cimg6.ibsrv.net/gimg/www.cor...2ef5211a38.jpg

always good to find a (or hopefully find) a solution!

GREAT! :hurray:

Hard to believe that it's that simple but stranger things have happened.

Not a bad idea to get one of the vacuum line maps and trace it all out when you feel like a good winter project for the two weeks in January that we don't use the A/C here in Texas. It is likely that a repair was made at some point and that branch was just plumbed wrong. I'll be starting mine back from scratch as I begin with reassembly, so I have no choice but to go back to square one. It has been too many years since I tore it all out to remember how it all came apart and I was foolish enough to not label in the process.

After looking at the proper "T" I might know why everything works. Not only is the HVAC connection directly across from the vacuum inlet but the large interior diameter extends past the large side branch. Vacuum will thus favor that little HVAC line. Your other tee not only feed the HVAC from the side branch but its connection to the body was very small--vacuum greatly favored the straight-through path.

While things are working I suspect you have multiple leaks in the headlamp door system. If it were reasonably tight then the air should have returned to the dash pretty quickly after diverting to the floor. I can almost guarantee that one or both of the boot seals on the shaft of the headlamp door vacuum motors are leaking. Fortunately you can buy new ones (and the little metal retainer) to fix that problem. There are a number of good threads and other references for troubleshooting the vacuum headlamps. The vacuum reservoir in the '79 is actually a very thick walled framing member so that's one component extremely unlikely to have a leak :)

The hot water cutoff system is a VERY common HVAC problem in '79s. To test for operation have the engine hot, set the HVAC to "VENT" and temperature all the way left (cold). The air coming out the vents should be ambient temperature. If it feels heated then there is a problem and your newly working A/C will ALWAYS be fighting to keep the interior cool.

Just looked at your photos more carefully and you still don't have the tee hooked up properly. Put the vacuum source (coming from the one-way valve and filter) opposite the HVAC (small connection). The side branch feeds the headlamp door operation. I'm certain that the hose arrangement and general design of that "T" are done to avoid the exact problem you had even when the headlamp door portion has leaks.