Platinumpanther

Hi Everyone,

I am having trouble with the dash vents and defrost vents. Here is the situation (not Working):
• Vent does not work; no air (hot or cold) comes through the top vents above the center instrument panel. No air (hot or cold) comes through the square eye ball vents (left or right) in the dash.
• Defrost does not work; when switched to defrost the air does not come through the vent in the dash aimed at the windshield.

Here is what is working:
• Heater and AC are working but only delivering air to my feet at the lower vents on either on either side of the consul under the dash. Heat appears to deliver varing degrees of heat when switch is slid to each position. AC delivers cool air when in AC mode.
• Everything else controlled by the consul switch works
o Rear Window defrost works
o Fan speed switch is working
o AC control head switch from cold to hot is working
o Switch AC to Heat works

I suspect vacuum line to damper door or failed actuator or some other vacuum line issue. I have the Corvette Shop manual and the vacuum diagrams; however they are not explicit.

The the problem maybe a vacuum line repair or modification made by a previuos owner. The line going to the water shut off valve does not agree with the vacuum diagram in the manual. The white striped vacuum line coming through the fire wall grommet is not connected to anything. This is the white striped vacuum line that passes through fire wall along with the black and green striped vacuum lines.

Instead there is a black rubber vacuum line running to the water shut off valve from a tee to the black line that goes into the cabin through that grommet in the fire wall. ( I suspect that this is a bypass that someone put in over the years) and if my suspicion is correct I believe that it keeping the water shut off valve open.

1) Would this line, if a jury rigged bypass, keep the hot water flowing through the heater core at all times?
2) Should I remove this rubber bypass vacuum line and hook up the white stripped line to the water shut off valve?
3) How can I test the vacuum on the lines and what should they measure?
4) How should I approach trouble shooting these problems?

I appreciate anything you can tell me. I searched the forum for previous posts that addressed the water shut off valve and vacuum line photos as they are routed in the engine compartment without luck.

Thanks

0Willcox Corvette

First I would check the vacuum select switch to make sure it is operating properly and make sure the rivet in the middle isn't broke. If the rivet is good you may try to add a few drops of oil into the valve. Sometimes doing this will seal it back up. You can tell from my diagram below that the black hose in the number 1 position is the supply vacuum hose to the system...



But your issue could be as simple as one item or it could be multiple items.. Vacuum pods do go bad over years, the center deflector door has a plastic retainer on it that breaks (show below) and hoses could be spliced or cut or even running to something entirely different.



If the select switch is working fine then I would go to the heater control and pull off the vacuum connector to the select switch.

Using the chart below I would follow and test each hose and diaphragm attached to those hoses with a migty vac. If the hose operates the item pump up the vac and see if it holds vacuum. If not replace that item or repair the hose.

texasbaehr

I worked on my 81 HVAC t try t get it to work properly. I was able to get the fan to work but the vac. pots are all seized up, some of the ducting under the dash is missing and the wiring for the A/C was repaired by Bubba at some point. A A/C shop said it would be about $2000 to get everything working. I gave up and decided the rip the entire factory system out put a blank plate to replace the blower and evap box and install a Vintage Air mini system with eletronic controls. No more vacuum controls. I'll spend about $1500 to do this but thats less than a A/C shop wanted to repair the factory system.
My 81 is modified anyway so I don't care about originality.

LastC3AZ

did you check the vent cable that attaches to the vent door ?? pull the glove compartment out and check that ... I had simular issues and found the cable broken where it connects to the door lever ... ???