Cratecruncher

I was working on an unrelated problem that required removal of the upper dash pad. Before putting everything back together I thought it would be fun to cycle through the different HvAC settings and watch the doors do their thing - NOTHIN! To my shock the actuator doors aren't moving at any setting. Even when I assist the linkages physically they don't stay in the correct position. The temperature door is working fine between hot/cold and the AC is coming on and blowing cold at the correct settings.

So I crawled under the car to look at the vacuum reservoir. The car was recently in a minor accident that broke the reservoir so I immediately suspected that. Turns out I have vacuum at the reservoir. Then I checked the connection between the black and purple line in the passenger footwell and had vacuum there too but a bit weaker.

So how much vacuum is enough? My idle seems ok. I had an acid spill ages ago in the battery area. anybody have their line disintegrate over time? I suspect I have a leak somewhere. What is "good" vacuum?

BigGun

The actuators are operated electronically and not controlled by the vacuum lines. If air is not coming out of all the vents then you have enough vacuum. If the actuators fail it will set a code. Use the dic and see if you have hvac codes.

73Corvette

This is from Bill Curlee's post:


The vacuum actuators have nothing to do with temperature. The vacuum dampers just control air flow thru various vents:

recirc/vent, floor vents/upper vents/defrost

The HEAT & COOLING are controlled by the ACTUATORS.

Try this:

When you Turn the ignition switch ON, Turn the AC System "OFF"

Clear the HVAC DTCs. When you Clear them, see if you can hear the ACTUATORS CYCLE thru their full ranges. If they do, see if the DTCs stay OFF. Do this several times so that they can calibrate them selves.

Turn the HVAC to ON / AC and see if the DTCs remain OFF. If not, CLEAR the DTCs with the system ON and see if you can hear the actuators cycle.

I did this procedure several times and it fixed my HVAC issues

If this fails to resolve the issue, use a meter and read the fuses for the HVAC head unit to ground and see what voltages you get. The voltage should match actual battery voltage AT the BATTERY. If not, you have a Voltage issue and most likely its being caused by the ignition switch.

The "HOT IN ON" voltage supplied by the ignition switch is very often LOW due to damaged contacts in the switch. Read the voltage AT THE FUSE in the below schematic (Instrument Panel Fuse center, HVAC, Mini Fuse18 )

On the top of each fuse are two small slots. Those are test points. Read that test point to Chassis Ground. It should read the same as voltage take directly off the battery terminals

The ignition switch MUST be ON for this test

Cratecruncher

Thanks for the help guys.

I suspected my vacuum line from the engine manifold might be the culprit. So I hooked up my MightyVac to the purple input hose at the head unit and all my vacuum actuated vent doors worked properly again. Then I put a gauge on the black vacuum line coming through the firewall from the engine side and it didn't even move the needle.

Next step is to continue moving back to the source until I find the leak. I'll check the line from the reservoir to the purple line first, then move upstream. Somewhere there is a hole or a crack in that line. I just hope I don't have to take the intake manifold off to fix this thing. I'll post when I find out more.

73Corvette

This is a good thread with lots of pics...might help



https://www.corvetteforum.com/forums...nge-vents.html

dadaroo

Quite typical to have an acid leak to damage the vacuum line in that area.

Cratecruncher

Thanks for the input everyone. The link to that thread with the photos was quite helpful 73. Bill's pic of the back of the intake manifold was VERY helpful in visualizing what I'm trying to do back there fumbling around with my fingertips.

These vacuum lines remind me of a headlight problem I had with my '68 35 years ago! Vettes are fun. I was in the dark on that one, literally.

92GA

The electric actuators should still work even without vacuum. Make sure they are working right before you put it back together.

Cratecruncher

I found the leak! There was a 2mm hole on the vinyl line going through the firewall about 12" from the reservoir canister where battery acid had penetrated through. That line does NOT hold up to battery acid! I had just enough good vinyl sticking out of the harness bundle to splice in a new length of tubing. Tested 40# vacuum held for 10 minutes, and done!

92GA, I mentioned my "temperature door" was working in an earlier post. I guess I should have said the electric actuator. I only have one zone on my '99.

All the HVAC doors are working great now and it only cost $2.00 worth of vacuum line. I shudder when I think what a dealer would have charged.

73Corvette

Thank you for posting your results have COOL summer!