My HVAC in my 94 only blows on the windshield or floor. I read somewhere at one time that possibly removing the electronic HVAC control unit and cleaning the push button contacts might correct the problem. I don't have any FSMs so I need some help on how to remove and dismantle the HVAC control unit. I thank you gurus in advance for your invaluable assistance and knowledge.

I would start with the vacuum input.

On the right side of the engine, under the fuel rail cover, there are a couple of vacuum connectors, one of which has a tendency to break...

See page two of this tech tip for details:

Installing Injectors, LT1 & LT4

Thanks bogus, next question, I have never removed the fuel rail covers. What is the proper procedure to follow to remove them? Thanks, as it would be I had looked at the tech tips earlier today but had no idea those vacuum lines could effect the HVAC control unit. I am learning!

many climate control systems use vacuum to open the various ducts. This isn't just a Corvette issue.

As for removing the covers, yank. It's a snap fit. The pins are about 3" in from each corner or something like that. Start at the top, easier to reach, then pop up accordingly.

It def sounds like a vac problem but as Bogus said, check the source first.

This issue is pretty common on 96 and a few 95s. I've seen it a handfull of times on earlier models like yours. I've fixed about a half a dozen myself including my own 96. Every one of them so far has been the vac source line coming into the controller gets soft and gooey and collapses. This prevents any vaccum from getting to the controller and it goes to the default setting which is, defrost/floor. This sounds like something we can easily walk you through. I've got a few pics but no way of hosting them. If you need them I'll email em.

dr evil that would be of great help. As the old adage goes: a picture is worth a 1000 words! My e-mail is: fcandrews@charter.net and thanks in advance. ps would you know the part# of the vacuum switch? Thanks.

Since I posted last I went to Advanced Auto and bought 2 of the vacuum switches. The one on the vette was most likely the original one. It had a cracked nipple (Big one connected to the intake), so I replaced with a new one and the controls still only blow on the windshield and floor, NO DASH!
So, would the next step be to remove the HVAC control unit and do the contact cleaning routine? If so this is where I need some experienced assistance in how to remove the control unit and how to dismantle the unit to clean the contacts.

Is there any help out there?

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Is your AC head unit changing modes, but the air still blows out of the same places?

Or does the head unit itself refuse to change modes, refuse to turn on or off, refuse to change fan speeds, etc?

If the latter, clean the contacts.

If the former, it's vacuum and/or controller problems, not head unit contacts.

Below is an image of one of the vaccum check valves that is under the right fuel rail cover. It is the cause of many vaccum leaks. I had a similiar problem with both the AC ducts and the Cruise control not working. $8 fixed it. This is the old one before replacement. you can see how it looks like Swiss Cheese. I even tried to glue it to see if it would work but it fell apart!

yea, that about sums it up... looks like it's been soaking in battery acid.

Multiple pictures and a lengthy email have been sent


C4DERF, try posting the entire email I sent if you can post the pictures along with it to see if anyone has anything to add. I have no way of hosting them.
<--- too lazy to sign up on photobucket

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I replaced that one , but it didn't cure my HVAC problems. Thanks though as that is good info!

I hope this works as photobucket only allows certain picture codecs and file extensions. I am posting this info from DrEvil in the C4 Tech thread. Follow this link: http://www.c4guru.com/modules.php?nam ... ewtopic&p=10795#10795

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I totally missed your post.

My head unit will change fan speeds and temp settings and reflects the selection of different discharge settings but the air flow always comes out the defroster and floor locations.

I have already changed the vacuum check valve under the right fuel rail cover but the symptoms remain the same as above.

DrEvil sent me some guidance on the removal and repair of the HVAC controller and I managed to convert it to a ,jpg file and posted it here:
http://www.c4guru.com/modules.php?nam ... ewtopic&p=10795#10795

Your problem may lie with the rest of the vacuum hose. It may be dry and brittle and cracked somewhere along the route. That would be my next step.

The fact that your control unit changes settings leads right back to a vacuum issue. OR the gate is simply stuck in one place.

Do you hear the "hiss" when you punch up different settings on the control unit?

here is a good test...

start the engine... then turn off the engine, leaving the key on. be sure to have the windows open, the radio off and the fan at 0.

The goal? to listen to the duct work. If the system is holding vacuum, it will work for 3-4 cycles before loosing enough held pressure to open the dashpots.

If you don't have ANY such action, there is a serious vacuum problem.

Back in the early 80s a friend had commented that her car wasn't running right... It was a 78/79 vintage Oldmobile Cutlass, with a small V8. I started the car, and it was a bit funky... I tried to work the climate controls and they were frozen on defrost. I pulled off the air filter element and started looking for a vacuum leak... I found a small hard vacuum line that had fallen off the carb base. I found a piece of soft hose to replace the bad nipple, attached and the car ran better... and amazingly, the climate controls worked, too...

She later told me that the local GM dealer charged her some $50.00 and couldn't find it.

jhammons01

No I haven't heard the hissing sound at all.

bogus

I'll try your suggestion tomorrow after church and try to make a determination based on your description.

Thanks folks and I'll post my findings soon.