A/C Vacuum leak help request

[jrhende6]

My car is a 88 GM US model with a automatic climate control system.

Bypassed the heater lockout switch and plugged those vacuum lines.
Disconnected and plugged up the vacuum lines for the cruise control.
Plugged the vacuum line for automatic park release.
Replaced the T check valve at the firewall and tested using a vacuum gauge.
Verified my vacuum reserve can can hold a vacuum for a good while (AZ cars have no rust).
The vacuum lines all seem fine in the engine compartment but hard to check all the lines in the dash and the EATC itself.


With A/C On the vents switch to defrost at 25MPH. When going down to 45MPH and not giving gas the A/C blows cold again through the vents.

Have scrubbed these forums pretty well. Any hints?

[kishy]

Welcome.
The most trouble-prone single line that I can think of, which would be a good place to start, is the one that runs from the check valve (mounted on firewall behind the thermal lockout switch you mentioned), over to the vacuum canister on the fender which you tested.

Disconnect that line at the check valve and try to test (assuming you have a hand pump for this) the vacuum reservoir through that line. If the reservoir held vacuum without the line, but won't hold it with that line, then the line has a leak. I found multiple breaks in that line on my 84, in spots where it was heated and rubbed on stuff.

If that line checks out my next go-to would be the check valve, but you did that, so...

If the reservoir, line, and check valve all come back OK, then you might need to chase it inside the car.

[gadget73]

Unplug the vac line into the dash from the check valve and see if you can pull vacuum on it. If that doesn't hold, the leak is somewhere in the car, or possibly to the line going to the thermal blower lockout. It splits off the main vac supply line in the dash. Not really any easy way to check that stuff short of pulling the controls out of the middle of the dash and pulling vacuum on the various air doors. COuld also be the control unit itself. I don't really know how to test that other than eliminating everything that can be tested. If the under hood stuff and the in-car stuff is good, and the lines are all good, basically the controls are all that remain.

something I should mention, the stuff in the dash does leak somewhat even when working correctly. If it won't hold vacuum at all, you've got a leak. If it slowly bleeds down, that would be normal operation.

[sly]

Make sure the all the lines to the actuators (vents/blend door) behind the ash tray and glove box are securely in place. Also, the check valve for the AC is just behind the engine on the center of the firewall and looks like an F. Hopefully that's the one you mean and you replaced it with the Motorcraft YG-337 part. Most other check valves don't seem to work in that location. If all of that is fine, there's probably a hole in a line or some fitting that's loose (like on the back of the vent controls where all the lines plug in).

[Toploader]

Sir, I feel your pain! I have been chasing a similar situation on my '86 Tudor for several years. I replaced the vac check valve and the line feeding it, then replaced the vac 'coffee" can and found it to have rusted out in a small place on the bottom.....according to the Ford emission manual, the vac can must be able to hold at least 15-20" of vac for one minute, if after reaching 20" watch the gauge, if any leaks cause the needle to drop in less than one minute..the "coffee can" is shot and has to be replaced. You have come to the correct place, many members have assisted me in my quest to keep my car on the road...,keep asking questions, it's better than being in college

My car is a 88 GM US model with a automatic climate control system.

Bypassed the heater lockout switch and plugged those vacuum lines.
Disconnected and plugged up the vacuum lines for the cruise control.
Plugged the vacuum line for automatic park release.
Replaced the T check valve at the firewall and tested using a vacuum gauge.
Verified my vacuum reserve can can hold a vacuum for a good while (AZ cars have no rust).
The vacuum lines all seem fine in the engine compartment but hard to check all the lines in the dash and the EATC itself.


With A/C On the vents switch to defrost at 25MPH. When going down to 45MPH and not giving gas the A/C blows cold again through the vents.

Have scrubbed these forums pretty well. Any hints?

[jaywish]

All of the above will find the normal issues. I went through all that and had Ivan get involved. In my case the hose between the check valve and vacuum can had collapsed. So it held vac fine in all the tests but it did not pass vacuum between the can and the car.

[jrhende6]

Attached are pics of investigation. Give me a moment to ask more questions.

[jrhende6]

Vacuum line to can checked out.

There are 4 vacuum lines going thru firewall. 3 go thru at same place. 4th goes thru nearer to driver and is for the parking release. That is blocked off and not an issue. 2 of 3 lines are used for TBL and #3 must be the suppyfor HVAC. Using vacuum gauge #3 holds no vacuum. if #2 is blocked off I see vacuum held at #1. If block off #1 and test vacuum at #2 I see no vacuum. I assumed that #1 and #2 which is used for TBL are fine and left them blocked off. So I focused on #3. Don’t have a smoke machine so (risky) injected air to #3 at firewall. Hear air escape at temp sensor above glove compartment. Blocked the small black hose at temp sensor and continued to look for leak.

[jrhende6]

Can hear air escape at EATC. Disconnect EATC and block the supply there. Still cannot get a vacuum to hold at firewall. there are about two inline T’s in dash area from #3 supply from firewall. So I moved on and started testing actuators. there are 3 actuators under EATC, one of which has two vacuum lines in a push pull configuration. So there are 4 vacuum lines and I have tested 3 of them. The lowest actuator holds a little vacuum but leaks quickly. And this is as far as I have gotten. The vacuum lines all look pretty fresh. I think I will get a smoker to blow into my vacuum lines to look for leak. In the meantime does anyone have any observations?

[sly]

IIRC, one of the source line Ts goes back out to the thermal blower lockout. You said that was plugged, so there should be no leak there. The other T goes to the cabin air temp sensor just under the dashpad. The line for that one looks to be plugged too. If you plug both of those and also the point of source to the ATC controls, you should be able to pull a vacuum. If that's leaking out, find the leak and fix that and the system will probably work again.

[gadget73]

Dunno if this is of any help, but this is the vacuum routing for the climate controls in your car.

88 Grand Marq climate control.pdf

[jrhende6]

IIRC, one of the source line Ts goes back out to the thermal blower lockout. You said that was plugged, so there should be no leak there. The other T goes to the cabin air temp sensor just under the dashpad. The line for that one looks to be plugged too. If you plug both of those and also the point of source to the ATC controls, you should be able to pull a vacuum. If that's leaking out, find the leak and fix that and the system will probably work again.

Plugged as you suggested but could not get a vacuum. Maybe its been awhile.

[jrhende6]

Dunno if this is of any help, but this is the vacuum routing for the climate controls in your car.

[ATTACH]50261[/ATTACH]

Thank you. Have been looking for something like this.

Retested the 3 actuators under the EATC. The flaky actuator fixed itself. It is the partial panel actuator according to the diagram. The first time testing it it would leak real bad. Now it holds very well. Could just be test operator error.

According to the diagram, in every mode selected (off,vent,ac,hi-lo,floor,mix,def) there should be a vacuum at source port #4 of the 8 port vacuum distribution panel. However, only get a vacuum when lever selects defrost. It is a good vacuum (no leak) in defrost mode. Note that the electrical connectors are disconnected while I probed with vacuum gauge. Can anyone recall testing this device? Diagram says it is part "valve assy (atc) d9ah-190961". Is this my problem?

[gadget73]

port 4 is where the vacuum source connects from the firewall. To test that you'd have to unplug the connector, at which point the slider doesn't make any difference at all. Defrost requires no vacuum at all to work.

If you have proper vacuum from the firewall, but the system leaks in every position except defrost it sounds like the control valve is shot. I've never fooled with one, its possible you may be able to open and repair it but I honestly have no idea. I would expect to hear the valve hissing if its leaking bad enough to cause you problems though.

[jrhende6]

port 4 is where the vacuum source connects from the firewall. To test that you'd have to unplug the connector, at which point the slider doesn't make any difference at all. Defrost requires no vacuum at all to work.

Understand. Please see pictures of the device I am testing.The vacuum gauge is attached directly to the nipple of port #4 of the valve assembly. According to the diagram the device is working correctly. Thank you again for the diagram.


This is great I just need to test for vacuum at ports 2, 3, 7 and 8. Those have vacuum just fine. So the problem is somewhere between firewall and along one of the lines in the 2 inline T's.