I'm hoping someone can give me some tips. I have a 94 and when the AC is on, it only comes out through the floor and windshield. I cannot get it to come through the vents. Is there a vaccum leak somewhere? A friend told me that was the default location it would go to if there was an issue. Where do I start? Any help would be appreciated.

 

my 1990 is doing the exact thing; I havent found any answers yet and will be watching this thread

 

Look at the F shaped vacuum fitting under the passenger side fuel rail cover. Bet it is cracked and leaking.

 

Locate the vac line to the firewall in the engine bay , behind the dist. The line originates at the rear of the plenum, runs around to a check valve, splits and enters the cabin. Part is hard plastic, probably broken. Patch with same size tubing. Don;t break it off so close to the firewall there is nothing to patch...

If its Opt-68 a/c there is a programmer that directs all the vents to operate. That has vac as well. There are also a couple vac lines in the dash near the floor that run with thr harness wires.

 

aminnich diagnosed my same problem last year, very easy fix for 3-4 dollars.....

 

Thanks for the advice guys. That gives me a couple places to start. Will check it out this weekend.

 

I found the following posted on Corvette Action Center. Also, Digital Disaster and BVSmith on this forum have some excellent posts as well. I completed the repair on my '96GS last week. My problem was in the A/C Programmer vacuum manifold. Hope the following helps:
Originally Posted by '6 Shooter' at www.corvetteactioncenter.com

I will go through the steps I went through, although I just knew it was a vacuum problem from the very start. I didn't technically know how the whole system works so I had to do trial end error with a lot of help from people on here and other forums.

Vacuum essentially controls which doors up inside the vent system open or blend to shoot air out which vents in your system. With Electronic Climate control, the Control Head which sits in the dash center console with the control buttons, is simply a little computer that senses outside temp, inside temp, and combines that info with the button you push lets say the auto button, and the air temp you set, to determine which doors to open or close to deliver air to the appropriate vents. It has no vacuum lines attached only electric and data connections.

Over the data lines the control head sends info to the Vacuum Programmer, the one above the gas petal that says open or close the appropriate doors for proper venting. The actual movements of the doors come from vacuum applied through solenoids inside the vacuum programmer as a result of the data sent from the control head.

Vacuum comes from the engine. On the passenger side of the engine near the fuel rail and under the plastic injector cover (it just pops off) is a black valve connected to the engine and containing 2 vacuum lines one of which goes directly to the vacuum programmer.

There are other components but in this case where the airflow is simply stuck blowing in form the heat and defrost vents these are the most important ones. I found out that the default position is heat and defrost when something like this is wrong.

First I would check the trouble codes. To do so you press the fan up and fan down arrow buttons simultaneously and hold till the outside air temp read out LCD, says 00. If no codes pop up then press the fan button, the one between the fan up and down buttons. If still no numbered codes, like 09 or 15 whatever comes up then the on board computer has found no faults in either the Control Head or the Vacuum Programmer. This is most likely good but under some circumstances a code will not show up. If you have a numbered code pop up then you will have to go on line or somewhere and match the code with the symptom and proceed from there. But lets assume you had no trouble codes, just as I did not.

Pop the plastic cover off the top of the engine on the passenger side and you will see that valve. I think there are 2 valves under there but this one has the 2 small vacuum lines going back toward the firewall. I don’t know exactly what that valve does, but it is a valve and serves some function other than splitting the vacuum into 2 other lines. You should have engine vacuum when you pull the lines off. I went ahead and replaced it as it is only about $5.00 and people say they go bad all the time, so get a new one and be ahead of the game when it decides to go out.

So now we have no trouble codes and you are sure the proper vacuum is leaving that valve on the engine and going down that vacuum tube to the controller. Turn over upside down, first grab a flashlight or drop light to take with you. Put your head on the floor up close to the gas pedal. Among all the wiring harnesses, relays and other stuff under there you will see a black box probably about 6 or so inches wide and 1 ½ inch deep. You will also see up in one corner of the box a connector with several colored tubes attached to that box. That connector connects the black vacuum input hose from the engine through the box and out the colored hoses to control the doors. You will quickly see there is almost no slack in those vacuum lines so be patient, as you have to work on the connection up under the dash.

I did not do this initially and I would highly recommend that you do it. You really need a partner but can do it alone. With the engine running, put your fingers on the green vacuum line connector where it connects to this box. Apply pressure to that connector; squeeze it really tight to the box. The objective is to simulate a tighter normal connection between the lines and the box. If the climate control is on auto, you have the temp selected to 60 and it is hot outside the air should shortly (maybe 10 seconds) shift from blowing in your face on the floor to the dash A/C vents. While holding that connection really tight have your partner push some other buttons on the control head and see if the air flow shifts to other vents as indicated on the buttons. If this happens you have a bad or loose vacuum line connection just as I did. IF this procedure does not make the air shift to other vents then you may have another problem.

Lets assume this procedure does simulate fixing the problem. (Even if you get a momentary flash of air from the A/C vents this indicates a connection problem). Now you have to get the box off and out of the car. Ultimately you do not have to take the box out to get a better connection but I found the main black vacuum line to be partially blocked with gunk so it gave me an pportunity to clean it out. Also with the box out, you can take a visual inspection inside to make sure nothing on the circuit boards is burned up.

Where the vacuum hoses connect to the box you will see a silver screw sticking up through the connector. It has one of those flimsy push on retainers I have already talked about supposedly holding the connection tight. (Someone told me there should be a nut on there but now 3 people I worked with have had the push on retainer). If it is a retainer instead of a nut, you are going to have to dig around with a small screwdriver to get under that retainer and bend it out, and then you can grab it with long nose pliers. However, you need to pop that retainer off. When you do the vacuum lines will fall right off the box. Then you will have slack to disconnect the electrical lines.

Get the box all the way out and pull the pack off, it is cardboard, yes cardboard. You can release the vacuum line board inside by removing two screws. Then you have remove the other board with two other screws. Make a visual inspection to insure nothing on the board is cracked, broken or looks burned.

Next focus on the vacuum connector with the male parts. Remove the screw that holds the assembly to the box and ultimately will hold the connection tight. Replace the screw with a longer one but make sure the screw head is exactly the same size as it fits into molded plastic to keep it from turning as you tighten it up from the outside. Make sure you have a real nut that fits the screw threads. Put your new screw in, tighten the nut holding the vacuum piece to the box, and tape your cardboard back on. I cut a spacer to slip over the screw, which would press tightly on the vacuum connector at the connection and allow me to get the nut on the screw at the tip of the screw. (it makes it easier). Plug the box back into the wire harness then get back on your back, crawl back under the dash. Put the box back almost in it’s installed position and fit the connection to the box. Slip the spacer over the screw tight with the connector base then put the nut on the screw head and tighten with a socket. Doing this upside down is easier said then done but be patient. Tighten the nut pretty sung, start the car and check it out.

NOTE - The system will go through a series of re programming itself before it does anything. Wait a minute or so and if still nothing, turn it off, then back to auto, then press the fan buttons and make sure the temp is set to 60. If all this works you should soon have nice cool air blowing out the ac vents in your face. If it does not try to tighten the nut on the connector a little more, remember you have to have a really tight connection there.

If this still does not work, take the top off and drive it one last time as a convertible before you drive it into the lake.

Seriously, if all this does not work you probably have a faulty control head or vacuum programmer box and the system just didn’t pick up any trouble codes. From here you’re on your own to figure which box to replace first.

 

If the fitting isn't broken, then it could be the AC/heater programmer. I replaced the programmer and all was well.