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Problem; air blows only through defrost vents; HELP

techno13_24 on Thu May 06, 2010 7:10 PM User is offline

Year: 1987
Make: Chevy
Model: G30
Engine Size: 350
Refrigerant Type: r-12
Ambient Temp: n/a
Pressure Low: n/a
Pressure High: n/a
Country of Origin: United States

My ac doesnt blow cold. But thats the least of my worries. My priority is to get the air flowing through all vents!

No matter what the control switch is set to, it only blows through the defrost vents up on the dash. I have replaced the control switch with several good quality others. So it cant be that. I had a diagnostic on the vaccume lines from a shop and they are okay. So it cant be that. And I still have the old R-12 running in my system (though its not blowing cold right now) however I do plan on switching that out later with a retro kit.


mk378 on Thu May 06, 2010 8:01 PM User is offline

On a vacuum operated system, this is usually because the main line under the hood is disconnected, check valve blocked or installed backward, etc. Do you have vacuum at the control switch with the engine running? If you do, it could be the line to the defrost door actuator or the actuator itself (blown diaphragm).

Also don't use the retro kit in a box, it would be much better to stay with R-12 if possible. A proper retrofit takes more work than just the "death kit". If you even look at a R4 compressor the wrong way it is prone to seize on you, they don't take any abuse.

Edited: Thu May 06, 2010 at 8:03 PM by mk378

techno13_24 on Thu May 06, 2010 8:06 PM User is offline

How do I tell if I have vacuum at the control switch?

mk378 on Thu May 06, 2010 8:10 PM User is offline

Disconnect the switch, start engine, see if one of the lines is sucking air.

techno13_24 on Thu May 06, 2010 8:34 PM User is offline

what is the check valve? and from what components does the main line run from and to?

Oh, and I just checked, I dont have vacuum where the switch is.

Edited: Thu May 06, 2010 at 8:38 PM by techno13_24

mk378 on Thu May 06, 2010 9:05 PM User is offline

I'm not sure exactly how that truck is made, but generally it starts at the intake manifold, passes through a check valve (which lets air flow TOWARD the engine). Then there is usually a reservoir-- sort of a plastic tank. The check valve and reservoir are necessary because running at wide open throttle, manifold vacuum drops severely. In those conditions the check valve closes and the vacuum in the reservoir keeps pulling on the control system. The line from the reservoir goes through the firewall to the control switch.

techno13_24 on Thu May 06, 2010 9:33 PM User is offline

Ok I'll tear it open and check for a disconnection and go from there. Hopefully I can get some more information, and hopefully by tomorrow I can get it posted up on this forum. Any assistance further from here is greatly appreciated! Thanks.

Dougflas on Fri May 07, 2010 5:34 AM User is offline

I have an OEM shop manual for the 88 model. Should be the same. if you need help, I can look things up for you.

NickD on Fri May 07, 2010 12:08 PM User is offline

Sometimes a guy is lucky, find cracks in the end of the hose into the intake manifold, cut off about a half an inch and then you are back in business again.

Other times not so lucky as the leak is under the dash, I use a hand vacuum pump with the climate control turned to off to check for leak down. At times, by disassembling the climate control vacuum switch, find hard grease, soak the rubber in denatured alcohol and put on fresh silicone grease, or plastic hoses with cracks, seldom ever found a leaking actuator.

The vacuum on these systems is extremely weak, nothing compared to the vacuum on your power brake booster, the tiniest pin hole will give you grief, so the leak down test with a hand vacuum pump is the way to go. And you won't hear a rush of vacuum either especially with the engine running. A leaking check valve would let your system work normally, until you hit the gas, then it switches back to the default mode, which is the defrost, by the way. That tin can of a reservoir can also be the problem, again a leak down test by using a thumb on the outlets pressed hard.

techno13_24 on Fri May 07, 2010 5:05 PM User is offline

Well I found the problem. It was a blocked check valve. The main line was disconnected under the hood. I wound up replacing the check valve (which was a two way, with one channel plugged off) and some hose.

NOW, the other problem I have is that most of the vacuum hoses were disconnected. I have studied the vacuum line diagrams, but I still have difficulties. For one, I can hardly tell which components which, and what valves are what under the hood. I am only able to identify the EGR, a few of the check valves, the carburator(obviously) and the canister. Oh an the distributor, which doesn't even have a line connected to it or even a nipple to connect to. However the diagram for my vehicle says that a line goes to the distributor. Whys that?

I just plugged what vacuum lines I thought went where, and now I have control with the vents again! Though this solves my vent problem, could this lead to a bad thing elsewhere? Like with the EGR or the carburator?

techno13_24 on Fri May 07, 2010 5:06 PM User is offline

One more question, does anybody know what the abbreviation "EFE" stands for in the vacuum diagrams? I checked the glossary to my book and no answer.

mk378 on Fri May 07, 2010 6:19 PM User is offline

Connecting the vacuum line for the controls isn't going to mess up anything related to the engine.

Probably you have a mismash of parts accumulated over the years, not all of them correct for your truck. Generally when it's a carbureted engine and it's running OK, leave it alone (take a look in awe at the nightmare there, close the hood and back away slowly). Dougflas did offer to help out with the manual though I think all of the 1988's were fuel injected.

Edited: Fri May 07, 2010 at 6:21 PM by mk378

Dougflas on Fri May 07, 2010 9:49 PM User is offline

what engine do you have? the 88 305 was a throttle body. Give me some more info and I try to have an answer for yu on Sunday as I have a bunch of work for saturday to do.

NickD on Sat May 08, 2010 5:18 AM User is offline

Did you just buy this truck? When I purchased that 82 Chevy P-30 motorhome chassis, it was a mess and is covered by the Chevrolet Light truck manual. Had to purchase an 80, 81, 82, and 83 manual to have a complete set, more of a unit repair manual with many different trucks included, including the G-30. And they didn't say what was used in what, had to go out to the motorhome and see what I had for components, then try to find that particular component in one of the four manuals. These were the GM doesn't know what in the hell they were doing years. My emission diagram, fortunately on the air cleaner was also incorrect.

My thing has an EFE (Early Fuel Evaporation) Valve on the air cleaner and controlled by a TVS thermal vacuum switch. When the engine is cold, a vent draws air from a tube connected to shroud on the exhaust manifold to vaporize the fuel quicker, then it maintains a near constant 160*F inlet temperature to the carburetor. These never did work worth a crap, have to idle the engine for a couple of minutes before I can take off or it will die.

My distributor is the older type HEI with self contained centrifugal and vacuum advance, comprehensible, but not in the 82 manual, found the specs in the 80 manual with the spark advance curves I needed for a sharp tune up. Somewhere along the line, they switched to an electronic spark control eliminating both the centrifugal and vacuum advance and adding a MAP sensor and kind of a computer, but the manuals did not keep up with these changes. GM had that dingbat CEO back then, don't recall his name, that just wanted to make the stockholders happy overlooking details like accurate manuals.

Also had an electronic tach on my engine the was suppose to energize an electrical vacuum switch to an actuator for a throttle pull off, that was suppose to gun the throttle when decelerating from 2,000 to 1900 rpm. Only the tach was there, could tell by the still painted holes, the rest of it was never installed. I am familiar with this from flying, called clearing the engine to prevent carburetor icing when coming in for a landing, just rev it about every 20 seconds. But GM didn't need this especially in a heavy vehicle while trying to brake and with the EFE, so took it out. But why did they leave in that tach and why didn't they correct the emissions label?

Just saying you have your work cut out for you.

Have no idea where Chevy came up with that climate control, mine did not have a vehicle connector, but like 19 spade lugs, all unidentified and really messed up. Had to take is apart and draw my own schematic making sure all the inputs and outputs were going to the right place. Found dried up grease in the vacuum switch, the idiot that owned this thing, tried to fix it by switching wires. Have to admit to not really looking it over, but did get it practically for nothing.

techno13_24 on Sat May 08, 2010 6:31 PM User is offline

Well after working on it for a few more hours I found that this vehicle has an electric distributor (meaning no vacuum lines connect to it) I replaced it once, but one was installed before I got the vehicle. Ive got it all running nice now. The air flows everywhere with control. Just need to make it cold now -_- lol.

For the longest time I thought I had a 350 v8 5.7L engine in there. Turned out I read some specifications wrong and I recently realized I have a 305 v8 5.0L engine. Couldn't ever figure out why that extra .7L wouldn't fit. Hahaha! And this was to my discovery after buying and installing a new water pump, power steering pump, alternator, starter, etc. for it. But for those two different engines, most the parts are the same anyway.

Well I have had this van for a year and a half now. My father gave it to me. It's the only vehicle I've ever had. A big project as well. He gave it to me half of what it is now. I have already installed captain bucket seats, tons of interior trim (sun shades, dash board, door handles etc.) , half the engine belt components, all sorts of wiring and switches (like the neutral switch, and the turn signal switch) locks, and even things like an exterior tire mount and a latter rack. Ya see, he use to use it as a work van and I use it as a camp van.

My future plans for this thing is to convert it into a 4x4, slap a 350 vortex in there, get a paint job, lift it 6", and finish up the interior.

NickD on Sun May 09, 2010 6:04 PM User is offline

I have a Chevy 5.0L in my Volvo Penta sterndrive, Well Volvo won't admit its a 305 CID Chevy, just say it was made especially for them in the USA. EPA was quite different for boating requirements compare to land vehicles, not a single vacuum line on this thing anywhere. Volvo went to Prestolite for the starter, alternator, and distributor. Distributor only has centrifugal advance 8* BTDC at idle and 30* at 3,000 rpm, and uses ignition points! Really hate to upgrade to solid state, as getting a solid fat blue spark at a half an inch. You won't see that with any solid state ignitions system, plus its fun having points again. But yet they recommend the spark plug gap at 0.028" where the HEI is dying at 0.062"

Alternator is of the self starting type with only one wire going to the battery, none of that ignition startup stuff and idiot lights running more wires to the IP. But the IP was a real voltmeter, so you know whether its working or not. I love this kind of simplicity. Volvo uses a special cam ground that closes the exhaust valves early leaving in some exhaust gas in the cylinder for the next cycle, meaning no EGR valve to go bad or full with. Still need that to burn unleaded gas.

Volvo had Chevy custom drilled the bell housing, so the starter does not require shims to fool with. There are only seven wires between the engine and the instrument panel with an ignition switch, fuel, temperature, oil pressure, and the volt gauges. I can deal with seven wires.

Volvo added a seawater pump that directly feeds the engine water pump and the water cooled Volvo made exhaust manifolds. You can lean on those without getting burnt, but don't recommend you try that. Uses a four barrel Rochester carburetor with real idle adjustment mixture screws, none of that plugged up stuff. Damper pulley can handle an additional AC compressor but can jump off into Lake Michigan in 55*F water to cool off, so really don't need that.

Just pure simplicity, like that and it runs very well.

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