Year - 1991

Trying to figure out why my climate control won't switch from defrost to any other position. Trying to figure out if the damper is stuck or what.

Also, no matter what mode i push... Vent, A/C, etc... it seems that the heater is stuck in the open position... So i leave the whole thing off and just drive with the top off. But here in San Antonio... it's getting too hot!

I get my 1991 FSM in a few days and I'm sure it will be there. (Got the full set for $66 on ebay)

On the LTx cars, there is a check valve that connects the vacuum line from the HVAC system and cruise control to the intake manifold. If it disintegrates or breaks, that can cause symptoms like what you describe. I think the L98 has a similar check valve, maybe near the distributor. Worth a look, cheap fix if that's it.

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Yep L98 has it too. Mine didn't have one, along with numerous hoses and other things, and the vacuum line was broken off waaaay down in the wiring bundle. Finally got it fixed, now gotta get the A/C to work!

I'm assuming you mean where the air is blowing and not the temperature?

If it's only blowing through the defrost, the first and easiest place to look is the check valve under the rear of the plenum as Brad mentioned.

Great pic Matatk! I will check that out when the engine cools... just got back from a morning drive ... One of the new Camaro's decided to show off against me on the highway... LOL... all's fun until someone get's a ticket...

I didn't do much... Ran it up to 90 real quick and backed off... didn't want a ticket, and since I had my son in the car, didn't want to jeapordize his safety. My son (6years old) thought it was fun though.

found that check valve and I had previously replaced all the old vacume hose going to it. (didn't know what I was replacing at the time, but it got replaced

I checked for suction, and it does have suction... doesn't feel very powerful, but i don't have a guage to check it. I'll stop by an autozone and see if they have one could check it with.

i replaced the check valve... still no love... (So much for the easy fix.)

I've got the all digital climate control. I see in the FSM that there is a seperate heater/ac programmer assembly... and it looks like there are a series of vaccume ports. Those control the dampers correct?

I checked for codes on the climate control head unit and I'm just getting 00. So according to the FSM it's not detecting any faults.

Found the hvac processor and will check for vac at that point. If I find I have none at that point... I may need help finding where the source vac line enters the cabin. I found and replaced the check valve, and I did have vac there... But it may be broken further down the line.

Keep us posted.

I think I've found the issue...

I was able to trace back the vacume hose all the way to the HVAC processor and there is good suction at that point.

Also, when I disconnected the harness from the processor I heard the whoosh of air go in because it's holding suction. That's good.

However, I think the issue is that all the nipples on the Processor have collapsed at the tip. **-See picture of my processor vac harness-** Which is keeping the doors from operating properly.(I read about someone else having this issue)

I think I have 3 options:
1. clip the tip of each nipple and re-assemble the unit (see if that works.)
2. replace the harness with individual vac. hoses coming out the hole. and patch them to the proper doors.
3. replace the harness with a new one (May be hard to find.
4. Replace the entire processor. (from what I've found... Very expensive)

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  HVAC Harness.jpg (739.22 KB)

Weird, but then again I've seen things on C4s I've never seen on any other vehicle.

Oh

#5; Contact Jason @ vette2vette.com for a used part...great place, good prices.

That's interesting - I was under the impression that only the '96 models suffered from that particular affliction. In my case, I bought a used programmer from a '95 (which had hard plastic nipples) and swapped the vacuum manifold. Other folks have had success clipping the collapsed ones back, and some have also inserted a piece of smaller tube as well. Good luck!

That worked! Woo hoo! Now I can control the vents! I will post the picture of what I did when I get back in the house!

Okay I want to know exactly what you did because I'm in the same boat with my 94. I replaced the check valve and it didn't work. Where is that hvac unit located? Is it a hard fix? I HATE working under dashes but I really need to get my a/c working correctly.

If it's the programmer, it's not rocket science to fix it, but it helps to be a double-jointed midget contortionist. The box is located up above the gas pedal. To get good access, you need to remove the hush panel, and it also helps to remove the metal panel that is behind it. I may have pulled a duct out of the way, too. It's also easiest to remove the driver's seat and track so that you can lay down to work on it. The box is held on by just 1 screw, if I remember correctly. The real bitch is getting the vacuum connector off of the box - it is held on by a ridiculous little push-nut. There isn't enough slack in the lines to pull the box out from behind the dash (at least there wasn't on my car), so you have to get that push-nut off in tight quarters. I just pried the push-nut up a bit with a small blade screwdriver, then grabbed it with a needle-nose pliers and twisted it until it broke. When I put it back together, I just used a small nylock nut. When you pull the box out, you can inspect the nipples that mate to the connector - if they're collapsed, you can trim them back a bit, and/or insert a smaller rigid tube in them. As I mentioned, I understood that only the '96 had the soft ones, but apparently some earlier models do as well.

I found I had bookmarked this thread back when I was having a problem with mine:

http://forums.corvetteforum.com/c4-te ... vent-control-problem.html

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Sorry it took so long to post this... First the corvette-guru site was down, then my laptop went down. Computers... What a pain. (And that's what I do for a living. LOL!)

Basically I just removed the programmer, clipped the tip of each nipple reamed out the center with a very small drill-bit and then put everything back together. I'm attaching a picture of the final product.

As to the location and removal of the processor

(This is for informational purposes only. I can not be held liable for incorrect information, breakage or injury by anyone following these instructions. Do so at your own risk.)

Now that the legal stuff is out of the way

1. Disconnect the negative battery cable
2.remove the hush panel from the driver's side
3. While sitting up-side down in the drivers seat, throw your feet up out out of the open top of the Car while putting your head all the way down at the brake pedal looking up. (This is the fun part... and believe me I felt my age the other day doing this)
4. look up. Just above the gas pedal is a black box. with one bolt on a tab toward the bottom.
5. Up toward the top of the box should be a manifold of colorful vac. hoses.
6. Pull the whole manifold out. away from the box. (there should be a slip nut on the center bolt that should come off with the manifold.
6. If you were getting good suction here, you will hear the whoosh of air from the vac. as you pull the manifold off.
7. Once the manifold is off you can unbolt the processor from the firewall. (the previously mentioned bolt at the bottom of the box.)
8. Remove the processor buy pushing up on the box until it clears the bottom bracket, then tilt the box slightly out so it once again can clear the bracket as you pull down. (There are two tabs on top that go into slots holding it in place. Be careful not to tilt or twist while they are in the slots because they will break off)
9. wiggle and turn the processor box until it's free from under the dash.
10. Disconnect the wire harness.
11. Inspect the nipples. (No giggles please)
12. Do your repair
13. Use electronic cleaner to clean the pins on the connector and the female connector (Since you're already here)
14. Replace the processor in reverse order.

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