79 Vette Air Conditioning Troubleshooting
 

Hey Guys, This is my first thread on this forum, and may I say how awesome it is and how stoked I am to be a part of it :thumbs: I've been restoring my 79 for a little over a year, almost finished..and I can't believe I have just now discovered this forum! It would have made my life soooo much easier, had I known about it during the headaches and roadblocks that one comes across in a restoration.

So one of the last things I have to tackle and perhaps the biggest left is the AC. There are no automobile AC shops in my town so I dunno who to turn to. When I bought the car, the air blew warm, and only from the vents beneath the dash. Shortly after I decided I wanted the engine to be tip top so I had it rebuilt and after I got the car back, I noticed the moving the AC controls would cause no air to blow from any of the vents, not even the bottom ones..

The car has the entire AC system in place. I checked all the fuses, and they're good. So any ideas of what steps I should take in trying troubleshoot the AC? I'm pretty mechanically inclined but know nothing of the AC system..Thanks!!!!

first, no air means no vacuum- either the system is not connected or there's a leak. when you move the mode lever (off max a/c vent heat def) do you hear any hissing from the console area?

Does the fan run on all speeds? that's all electrical- different set of problems depending on what the fan does (or doesn't) do.

I agree with Tim, as most of the hvac dampers are operated from vacuum motors fed from engine manifold vacuum. Keep in mind, these dampers will only operate if the engine is running. A good tool to have for diagnosing this is a Mityvac hand held pump kit and aren't that expensive for the basic model. Mine's a little more deluxe metal unit with brake bleeding reservoir, and works well for that too. Also pump's pressure, so handy tool to have around.

Thanks for the reply. When I operate the controls there's no sound of hissing within the vehicle, only the hissing of suction from the carb. There are no noises at all from the ac console, and of course no air whatsoever

As far as the fan, I'm not quite sure which fan you're referring to..but if you mean the electrical fan in front of the radiator, it's been acting up on me for awhile. It started running at all times, even with the car off. I replaced the relay that's next to the wiper compartment, and it still runs all the time. Only way to get it to stop is to keep the top plug unplugged from the relay. Doing so, it runs all the time when the car is running, and stops when the car is off..

Thanks for the info, I'm gonna def look into that pump kit. So if the AC works with vacuums from the manifold, do you think the mechanic might have hooked the lines up wrong when he put the engine back in, therefore leaving the AC system non-operational?

It's very possible if the engine was changed out, some vacuum hoses may have been switched around. The mentioned fan would be the small hvac squirrel cage unit at the dash, but sound's like your's is working, since you mentioned some air movement in the lower vents. The Mityvac pump tool is nice, as can check the vacuum diaphram motor's for leak's directly, in a quiet environment

There is a vacuum feed line that goes into the transmission tunnel area. That [feed] line connects to a (circular) manifold valve located on the left side of the A/C control head. You need to determine if there is vacuum going to that line (and that it is not leaking-off somewhere else). If you find that vacuum is not getting sent to the control head, plumb it in and see what happens then. That may be all that's wrong. If not, you will have to dive into the control head area. That little manifold distributes the vacuum to the appropriate door damper actuators. It is quite a complicated arrangement. The best reference material to describe that whole system is the Chevrolet Chassis Service Manual. Electrical, vacuum, and mechanical aspects of the system are clearly defined in that manual. Good luck.

Also, my experience indicates that a vacuum gauge is a much better 'tool' (than a hand vacuum pump) for diagnostic testing on the C3 vacuum system. The engine can make vacuum very easily.

By the sounds of it I think it may very well be a vacuum issue, since I was getting some air flow in the bottom vents before the rebuild, and after the rebuild, nothing..I have a feeling once I figure out the vacuum issue, the air will start blowing and then I'll just need to charge the system. I need to look into that Mityvac tool for sure, thanks for your input

Wow sounds like I'm in for a real battle with this AC system. I'm gonna look into that manual immediately, as it sounds like the vacuum system might be the culprit. That gauge should come in handy as well, where can I get one of those?

Also is it possible that a vacuum issue could cause the air only to blow from the bottom vents, and not the top, or defrost vents?

The vacuum system (as part of the A/C) is responsible for manipulating all the diverter doors...so, yes, it is likely to be the cause of your [non-controllable] airflow dilemna. But, it could be that there is no vacuum getting to the control head. You can get a vacuum gauge at any car parts store.

Alrighty well I've got a place to start then. Find the vacuum diagrams and get a gauge to test the vacuum system. I have a feeling that that is the problem..if I can manage to get air blowing from all the vents, then charge the system and hope the freon stays. Thanks very much for your help guys!!:thumbs:

A couple thing's to add. First off, the vacuum tool I'm using, has a gauge on it, so your covered both way's there. Also a good chance some of the R-12 has leaked from the compressor shaft seal, and good to run the A/C from time to time to help keep it lubricated. Actually have a friend with a 79' L-82, and weighed a few lbs of R-12 in last summer, along with a UV dye shot. Sure enough it's a shaft seal leak, and if all your charge is gone, probably best to have the compressor repaired right away. The UV dye shot's a real good idea too.

Thanks for the tips. That vacuum tool is gonna be a must, as is a diagram of the AC vacuum system. Once I get the air blowing I'm gonna charge the system, prob. do the dye shot, hope it doesn't leak out, but if it does I'll know where from. I've heard that you can use the modern freon in this system even though it calls for R-12..any truth to that?

No, you can't just change over to R134a. 134a and R-12 are non-compatible. There are some system pieces that need to be changed for running 134a; there are conversion kits out there. But, there are many A/C shops that still service R-12 systems. If there are no real mechanical issues with your system, it would be less expensive to just keep R-12 in it. If you need to do some significant work on it anyway, that might be a good reason to change over.

7T1vette,

Could you please post a link to one of these conversion kits? I am also in the process of getting mine to work. The local air condioning guy here in town says that the R-12 is $50 a pound and the 134 is about half of that price. He said that it would probably be cheaper in the longrun to just convert it over than to keep adding the expensive R-12 and then converting at a later time.

What all is needed to convert? He said to get a dryer and start with that. We will flush the system and charge with 134 and dye. If the compressor is bad then replace at that time. I think it would be better to just replace everything right away than piece by piece????

R12 can be purchased on Ebay for in the $20 a can area. If you don't have a license, you just sign a document stating that you are turning over the R12 to someone who does such as your a/c guy. You will be better off with the R12 as it will provide cooler temps.

Sounds like you're in the right frame of mind. Since the original compressor shaft seal is likely leaking, good swapping to a rebuilt unit. Another benifit, 134a requires synthetic oil, with most residing in the compressor. A rebuild will probably already have this and save's time. As already covered, the dye shots a good idea, as easy to occasionally scan with a UV light to keep an eye on things. Recently saw a program on the Kenworth truck factory, and they put dye in all the fluids and scan with UV light before leaving the factory.

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