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Newbie needs vacuum line help Pages: 12

Dave5701 on Fri October 01, 2010 9:55 AM User is offlineView users profile

Year: 79
Make: corvette
Engine Size: 350
Refrigerant Type: ??
Country of Origin: United States

I AM NOT and AC guy, but understand the principles. I just got this car. Appears all the components are there. Was told from the start the AC did not work, but lead to believe (HA HA) that it was just a leak. The system has been messed with. Nothing works. So I figure I need to start from the beginning with just getting things to act like they work. Of course this being a 79 GM product, it has the standard sliding type controls. For the AC side, Max, Normal, Vent, Heat... On the Temp Blue slider to Red. I finally got the blower fan to run. Wires disconnected. When I try to move the AC control from off to AC or Vent, it still just blows out the foot vents. Ok, I believe these systems worked on vacuum control. There are a lot of vacuum lines under the hood. So can someone hold my hand a little and tell me how to find the one that comes into the sliding control, what the sliding control should do or things I need to check for. This is just a first step. Compressor will not come on. So is it shot, low freon, other????? Just want to get the basics working one step at a time. Then take it to someone who can deal with the compressor issue. I am sure if you decide to help, you need more info from me. Ok, Im stupid, but I can and will get this working - just need someone to guide me and tell me what I need to do and look for. Thanks.

NickD on Fri October 01, 2010 11:05 AM User is offline

Your climate control sounds very similar to the one I found in my 82 P-30 motorhome, with a terrible minor exception, mine had all spade lugs on it with some odd 10 wires and 4 vacuum lines. Light truck manuals were worthless, all had vehicle connectors on them with an external diagram, you have the advantage of getting shop manual to make it the way it was.

I started off by pulling the climate control out and making my own circuit diagram, that mode switch operates both an electrical and a vacuum switch. Being that old, vacuum switched leaked like crazy, but fortunately was of the O'Ring type, could clean and replace all those with silicone grease, so it would hold a vacuum again. Used plastic caps to seal off the feeds after figuring out where the vacuum inlet was to learn which one corresponded to the mode switch. Used a hand held vacuum pump for that. Did the same for those spade terminals. Only difference between Max and normal AC was in the vacuum switch that closed the recir door, but from the 12V in, had to find that, only one terminal showed continuity, that told me that was the compressor clutch coil terminal. And so forth for the rest of the terminals.

Then finding which vacuum line was what, each had a vacuum actuator, finding the recir and defrost door were the easy ones, but the bi-level didn't work at all that determines whether the air comes out on the floor or the dash vents. On that thing, had to pull the entire dash to repair that. And talk about finding 24 years at the time of debris, mice, and wasp nests. Lots of soap water and installed screens on all the air intakes. Would rather brush off a screen than go through that again. Bi-level door was never installed correctly from day one. Using a hand held vacuum pump on each actuator not only checks for leaks, but for proper operation, but have to be able to see it.

Would start off by finding a manual and some R-12, the latter if you want any kind of cooling.

Temperature control was just a speedometer type cable to a heater valve, but was rusted shut. Always manage to find a lot of debris in the AC/Heater system, but mostly on the face of either the evaporator or heater core, drains are normally plugged. More of a janitor than anything else.

Dave5701 on Fri October 01, 2010 11:33 AM User is offlineView users profile

This is great stuff. So if you can, be patient with me and I want to tell you what I think I should do. Pull the dash panel to get to AC controls. Use the vac pump, connect to lines under hood to find the line that feeds the control? Make sure that line is good. Using the vac pump to feed the control, move controls to see if doors in the "air handler"under the dash work. If not, trace those lines to see what is the prob.

Tell me more about the 12V feeding the compressor. I take it that in the AC position, like you say, it sends voltage to the compressor to tell it to turn on, then also vacuum to direct air out the vents??? If that is right, what is the realtionship between the hot/cold slider control and the AC/vent control or is there one. Does the AC/vent just bring in air from the outside/ac coils and the hot/cold slider adjust for letting the AC/vent air through or heater coil air?

Ok, Im belly up on a vac pump. ANy way to use a shop vac or hosed vacuum cleaner to create that? I would think those have tooo much suction.

iceman2555 on Fri October 01, 2010 12:27 PM User is offlineView users profile

From your post, it seems that there may be several 'repairable' issues with this vehicle. Why not simply purchase a manual for your vehicle. This will offer wiring and vac diagrams along with other repair information. These are available from various sources.
A hand held vac pump is great for tested for vac leaks and component testing. These too are available from various sources and the cost will not break a budget.
This is available from Mity-Vac for less than 40 bucks.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

NickD on Fri October 01, 2010 4:51 PM User is offline

Manifold intake vacuum goes through a check valve to a reservoir that may be on either side of the firewall that powers the vacuum system. I ended up replacing all of my vacuum lines, was something like a dime a foot, your car may have a vacuum connector. Only takes me seconds to learn what is going on under the hood, guessing here.

You may find a cable operated valve in a hose feeding the heater core, tells you right away that is your temperature control, newer vehicles keep the heater core always hot and use a blend door for that same purpose. When I first got into my motorhome, forgot how long it takes those valves to respond so tend to over control, instantaneous with a blend door.

Yes, the climate control provides 12V all the time when in AC mode, but that goes through a cycling switch and maybe even a relay to activate the clutch circuit, here a circuit diagram is worth a million bucks.

These cars are pretty simple compared to todays stuff, entire ignition system is in the HEI distributor, four barrel carb with a heated air cleaner, got a name of that but forgot it, may be an exhaust heat damper to direct hot exhaust gases through the intake manifold. Have a buddy with a Corvette, he removed that as it burns the new paint on his intake manifold, but has to idle the engine for a couple of minutes before taking off. Fuel pump should be on the side of the engine, can change that in about 15 minutes with a ten minute break in between. So much easier back then.

Pitfall are those unplated brass Packard Electric spade connectors that turn green, use to have a bag of 5,000 of those, but used up the last of them on my motorhome.

Dougflas on Fri October 01, 2010 11:32 PM User is offline

I have both a wiring diagram and vacuum diagram for the 1979 Vette. Do you have a fax #?

Dave5701 on Mon October 04, 2010 9:21 AM User is offlineView users profile

WOOOOHOOOOOO!!!! You might have saved my life!! I can afford a manual though but I like this price better! Yes, 806 924 5217. Previous owner or one previous to that evidentally had issues they could not fix. I removed the console and dash panel and there are cut and spliced wires. Yikes!! I traced out someof the vacuum lines and it appears there is more going on the driver side of the firewall. No resivour that I can find on the engine side. AND thanks on the info on the vac pump. Looks like I could use one of those.

Dougflas on Mon October 04, 2010 4:09 PM User is offline

It's on the way. If you can't read it, I Can blow it up and resend it on Tuesday eve

Dave5701 on Mon October 04, 2010 5:00 PM User is offlineView users profile

WONDERFUL!! I just got it. I can read it 99%. THANKS SO MUCH!! I was looking at the vacuum line part, it shows dash panel and the harness assembly looks like it is mounted to that. Do you think the dash panel is the firewall?

Dougflas on Mon October 04, 2010 9:05 PM User is offline

That is what it appears to be. Good luck

NickD on Tue October 05, 2010 5:25 AM User is offline

Did you by any chance buy your Corvette from the same guy I purchased my motorhome from? Maybe the same guy that CyFi purchased his Supra from as well. Such a sweet old adorable couple I purchased that motorhome from, extremely low mileage, never had any problems with it, man was that thing a mess, even surprised I was able to drive it home.

When I dug into it, what a mess it was, was ready to take it back and park it up their butts, but paid such a low price for it and did call him, nicely to learn they spend a small fortune having some mechanic work on it. Probably why they sold it so cheap.

Son and I purchased a 19' 86 Bayliner from another sweet old couple with only 370 hours on it last spring, but both of us spent seven hours looking over everything. Things we wanted to do, but no surprises after when we purchased it. They did have a second radio installed in it, but was just mounted loosely on a shevle, no holes were even drilled to mess up the dash. I knew I had to go over the entire electrical system because it was 24 years old, but mainly all clean up work. He had a friend rewire his trailer, that was a mess, ripped everything out and put in all new stuff, but knew about it that reflected my offer.

With my 92 DeVille, was in desperate shape at the time and the general manager lived a block from me, let me have it for three days, only thing I found wrong with it was the AC relay was replaced, but not clipped in correctly, plus he let me have it for 1,500 bucks lower than the trade in price. Just wish recently I unsnapped all those plastic clips for the brake an AT fluid lines and put some paint in there, had to replace all those line due to rust. Also got my 88 Supra below trade in price back in 1990, saw were someone used a hydraulic under the fuel tank to lift it, dealer put a new Toyota tank in the hatch, said I would replace it. Really don't like nasty surprises. Really have to look before buying, especially from a sweet old couple. Son purchased his Bravada from an extremely attractive woman, he was taken for a ride by her, more than had our share of nasty surprises.

Dave5701 on Tue October 05, 2010 9:47 AM User is offlineView users profile

That diagram is worth a million, (not that I can pay you that). I immediately found a problem, still did not make it work though. I am going to invest ina MityVac and see if I can get things going. What do you know about the heater valve??? I always thought the "hot water" always circulated through the heater coil. Apparently not from this diagram. Is there a reason for this? THanks again so much for this, it REALLY helped a lot.

Dave5701 on Tue October 05, 2010 9:53 AM User is offlineView users profile

Mr NickD, you are so right on surprises. I got the Vette for 3K under book, so I have some room to work. And allwinter to get the AC going. Still though, it is sad when people cant just be honest. Right now, i wish they would just talk to me and tell me what they tried, changed or punted on so at least I know they looked at that and what they did. But, alas, I ask them about things and they worked fine when they sold it to me?? How strange they dont work for me now. I am so glad you guys are out here on the forum. I can do things, but sometimes, like this time, need a little help in understanding and heading in the right direction.

NickD on Tue October 05, 2010 11:40 AM User is offline

Originally posted by: Dave5701
That diagram is worth a million, (not that I can pay you that). I immediately found a problem, still did not make it work though. I am going to invest ina MityVac and see if I can get things going. What do you know about the heater valve??? I always thought the "hot water" always circulated through the heater coil. Apparently not from this diagram. Is there a reason for this? THanks again so much for this, it REALLY helped a lot.

History time again, when did they change from heater water valves to always hot heater cores with blend doors? Actually the blend door dates way back into the 60's with automatic climate control, but they also used a vacuum operated heater valve in addition to the blend door that would only open if the system was calling for heat. These were either fully opened or closed, but always a delay as when that heater core was ice cold or red hot, would take time for it to heat up or cool down since dealing with coolant rather than air.

The manual valve types have a lever on the valve that opens or closes with the temperature control, that cable has to be properly adjusted so that lever follows the temperature control. Easiest way I know of is to open the hood and look.

Looks like yours should have a manual blend door with a vacuum operated heater control valve just looking up this valve on the web.

Here I am guessing when you put your climate control in the Heat position, vacuum should be applied to that nipple on this valve to open it.

See you can also buy a separate blower motor switch, normally you have to buy a new instrument panel just to get this switch.

Edited: Tue October 05, 2010 at 11:41 AM by NickD

Dougflas on Tue October 05, 2010 8:22 PM User is offline


check your fax sent a better copy

NickD on Tue October 05, 2010 10:28 PM User is offline

Nothing beats a scan and an e-mail. Gosh, my fax is twenty years old already, but I think I finally got it paid off. Last time I used it was to a government office, not that they don't have the best computers in the world, they just don't know how to use them.

Dave5701 on Wed October 06, 2010 9:39 AM User is offlineView users profile

Well, I used the diagram I received to dig around. Bought a vac pump and low and behold, found a problem. It is a good one. Previous "repair techs - owners?" monkeyed with the vacuum switch in the slide controller and did not put it back together with all theparts. It is loose and wont hold a vacuum. I took all the parts I could find in the switch and tried to get things to seal. It cant, there is a 1/16 gap in there. Do you have any suggestions on where I can find some parts for that? Does anyone make an after market vac switch that might work here? Im looking on the internet to find something. I fear that when you mention the word "corvette" the price is automatically 10 times higher. Surely there is an Impala or station wagon in that era that had the same type controller.

Dave5701 on Wed October 06, 2010 9:42 AM User is offlineView users profile

I sent this same info to NickD, maybe between both of you, I might get this going. Cutting to the chase, it looks to me the vacuum switch in the slide control was taken apart at one time and not properly put back together. It is loose, like a 1/16" gap in it. Wont hold vac. Know where I should go to look for a switch or part? I might consider an after market vac switch if something like that exists.

Dave5701 on Wed October 06, 2010 11:10 AM User is offlineView users profile

OK, found it, Ecklers online. $12. Thanks for the blown up diagram, I for sure can read that.

TRB on Wed October 06, 2010 2:05 PM User is offlineView users profile

How are you all emailing each other? I thought the PM system was turned off on this site.


When considering your next auto A/C purchase, please consider the site that supports you:

NickD on Wed October 06, 2010 2:08 PM User is offline

Post a photo of that, could be a rubber gasket is missing. I have repair broken gaskets with DAP silicone glue, can still cut it open, but seals tight. Also show a photo of your climate control panel, back then, GM was using more standard parts from vehicle to vehicle. Did find that blower switch from for under ten bucks, but they don't list the vacuum switch.

Can also do a google search on "1979 Corvette A/C Heater Control Panel" will find a brand new reproduction panel complete for 110 bucks.

Dougflas on Wed October 06, 2010 4:17 PM User is offline


I faxed the info to him; no emails

TRB on Wed October 06, 2010 4:23 PM User is offlineView users profile

I read that. But poster said he sent the same info to NickD. I did not think Nick provided an email or fax number. I've had complaints years ago about people getting PM'ed. So the PM system was turned off. Was just wondering if it was getting bypassed is all.


When considering your next auto A/C purchase, please consider the site that supports you:

NickD on Wed October 06, 2010 8:01 PM User is offline

I didn't receive any faxes nor e-mails, really don't need the diagrams, do not have 79 Corvette. But did peek at a climate panel on the internet function wise, almost identical to the one on my 82 P-30 motorhome.

Didn't post a link of it as our host made it clear not to post links to what may sound like an inlet of water. But if I were interested in it, would insist on photos of the top, bottom, sides and the rear to make sure it is identical to the OE unit. But knowing me, would just repair the OE unit like I did on mind, have a little machine shop to do things like that.

If the poster wants help with his, seeing is understanding, would have to post pictures of his.

Dave5701 on Thu October 07, 2010 9:20 AM User is offlineView users profile

Sorry if I did something not inline with protocol. Did not want to get you in hot water NickD. What I meant when I said I sent same info, was that I hit the reply button on the post. Not all forums work the same and I was thinking that a reply might go directly to him rather than just a post on the site. I wanted him to know I appreciate all he has done and had another question. I dont have anyone's email nor have I asked. What I have received has just been great help from some great folks. Back on the part, What was at issue on the missing parts was that behind the vac switch is a flat 6 fingered star-like spring. I can see that it applies pressure to the switch internals to keep them sealed and thus holds the vacuum in place. the spring was not bent/thick enough to apply pressue to hold the switch plates together. I found a picture of thepart and there was the spring with a button type piece in the center which made it thicker. That button piece is what I need. Since the switch is 31years old, I just decided to order the part and get the spring, button and a new vac switch with it. I sure hope that is the fix, but I am not expecting that is the end of it. In the words of Arnold, ILL BE BACK!! Im sure there are other issues.

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