retaf

Is there a troubleshooting guide/diagram describing how vacuum operates the HVAC vents?
My fan now blows air in all switch positions thanks to help from forum members and I'm unfamiliar with the vacuum system on these cars as well. Only air blowing is at driver's feet and can't seem to get it to reroute anywhere else. Now I did find behind the glove box a control unit that appears to route air via three hoses yet the first hose, the one closest to passenger area is missing.

retaf

OK, so can someone tell me where the main vacuum feed line comes in? Could the missing line I mentioned in the previous post be the main feed line to HVAC? Now another dumb question, does the engine need to be running for the vacuum to operate the ventilation controls?

7T1vette

The best reference material on the C3 HVAC system is in the Chevrolet Chassis Service Manual in the "Air Conditioning" section. The main vacuum feed comes in through the firewall on the driver's side from the "T" fitting in the main vacuum supply line...just after the little filter and check valve. The vacuum gets distributed to the various door servos via a circular manifold mounted to the left side of the heater/A/C control head in the transmission console.

Peterbuilt

http://willcoxcorvette.com/popup_image.php?pID=26715
From the Willcox help section, and yes the engine must be on.

78IndyPace

Here's a diagram for a '77 Vette that might help:

Ganey

I wrote this up awhile back for this problem.
On at least later C3 the problem is that heater is the default position (no vac. or vac. leak) due to being designed in the north. Vent should be default. Very safe bet the problem is 1 OR 2:
1) The vac. line from engine may be damaged from heat - a small plastic tube that goes in pass. side of dist. below the hood alarm switch into wiper valley. Replacing most of engine side w/ hose will normally fix. OR
2) The vac. switch itself is not switching due to vac. leak as the usually plastic rivet is not holding the 2 main switch parts together well. Besides replacing, it is also possible to tighten this &/or disassemble to lube w/ a thin layer of white lithium grease to fix. Try pressing the 2 halves of the switch together to test in A/C or VENT w/ engine running.
3) If you have a heater, the small vac. tube to heater hose valve under hood may be damaged from heat & need replacing. Block w/o heater.

PS The design problem comment is not intended to be offensive to any of our northern friends.

Easy Mike

The AIM has a couple of pages of HVAC settings/diagnostics.

CA-Legal-Vette

This is a pretty simple diagram. I like it!

retaf

Is this the same for a 79'?

retaf

So this storage tank I've read about is that only for the headlights operation while motor is off? Where could I hook up a small vacuum cleaner as I've read elsewhere in lieu of engine running?

retaf

I see now in this diagram on the bypass valve vacuum switch where I thought a hose was missing it is labeled vent and has nothing connected there.....

7T1vette

The '79 car HVAC design may be different from the earlier C3 cars. But, the Chassis Service Manual for 1979 would still be the best reference manual for the poster. And, I believe that the vacuum feed to the interior for the '79 is in the same relative location as for earlier C3's. The control head and 'plumbing' is different, of course.

dgood

It looks like the same for a 79. The vacuum switch only has 2 hoses going to it. nothing on that third one (on a 79). I had to pull the center console, glove box and gauges out to access everything. Need to drop the ducting also. WILCOX sells new hoses for all this and it comes with a diagram.

retaf

So this is better than a corvette specific service manual?

dgood

Yes and get an AIM also. For now, just go to the Willcox website and they have drawings and diagrams for almost everything.

0Willcox Corvette

77 - 82 charts are the same.

retaf

I've reviewed all diagrams, read all applicable posts, purchased the shop service and chassis manuals CD as suggested, bought the mighty vac and I'm still clueless! I'm just not grasping how to go about testing the vacuum system to determine why air only comes out at driver's foot well. I'm troubleshooting with the engine off and using the Mighty Vac. Here's what I do know:
1. heater not connected/no heater hoses yet being in Florida I don't need heat.
2. AC I do need and it does work with some cool air coming out driver's side foot well when engine is on. I would like it to come out the dash vents as well.
3. a small green hose and yellowish hose in engine bay coming thru firewall passenger side are both disconnected and plugged. I hooked up might vac to each of these plugged lines and they wouldn't hold vacuum in any control switch positions. Should they?
4. on driver's side a black line coming off tee going thru firewall doesn't hold a vacuum in any control position except for OFF but bleeds off.
Should it hold vacuum in any control position?
Your help is certainly appreciated...

Rick_V

I am working through the same problems with my 79. I made major strides today I finally worked through all the vacuum lines and found some that were leaking and some that need rerouted. I got my air to come out of the dash vents but not at idle only at a stable RPM. I started with the smoke test. Get a cigar or cigarette and a helper. Blow smoke through the lines and the leaks will show themselves. I checked all the vacuum lines in the engine compartment and found the leaks and repaired the bigger ones. You will need to take the gauge bezel out to see the smoke for the AC circuit. I also replaced the AC Control Vacuum Valve as smoke was pouring out of it. The only thing I still need to do is put in a dedicated vacuum tank with a check valve for the AC for when I am idling or under heavy acceleration when my vacuum drops. Any recommendations on that would be greatly appreciated.

Mike Ward

It should hold vacuum in all positions,. Start troubleshooting here and work your way downstream.

retaf

Hey good for you! I'm glad someone is making progress on this system. Now that's a darn good idea with the smoke! But tell me, how do you know if a particular hose is supposed to hold a vacuum or not or are they all suppose to hold a vacuum and if you see the leak via smoke then there's the defective hose or connection? That's part of what's throwing me a curve. I'm assuming and could be wrong on this that in certain positions of the switch a hose would hold vacuum and in some other position would not. Please comment and thanks.