Air vent control not working

[mlipmd]

I got my AC working, blowing cold air now that I managed to recharge the system, but my problem still remains that my control buttons on the panel won't change the airflow out of the various vents. The fan works well, can change the speed, but it sounds like it's blowing but only a little air comes out of the vents on the dash or down below. This is probably due to a vacuum leak rather than a problem with the electrically operated manifold tucked up underneath the driver's dash, so I want to investigate the vacuum leak first. Can anyone tell me where should be the starting place to look for this leak? All the lines I see look nice and shiny with no holes or cracks in them but obviously I can't see them all. The vacuum pump on the passenger side below the headlight goes on for a few seconds if I turn the key on, then shuts off and does not keep recylcing. At one time it seemed to keep running on and off, sounding like a failing headlight motor but it doesn't do that anymore. I haven't checked the cruise control to see if that works, if not, that could indicate a vacuum leak too.
Thanks.

[QB93Z]

The Vacuum Pump by the right headlight does not any connection to any other systems except the fuel injection port throttle plates.

The vacuum source for the HVAC Dampers comes from the vacuum line connection on the passenger side of the Plenum. The vacuum line runs into a wire bundle and through the firewall on the passenger side. If you remove the passenger side knee panel and search around, you can find the tubing. It runs behind the radio enclosure and then to the HVAC vacuum programmer above the throttle pedal.

I found the tubing connections at the HVAC Vacuum Programmer to be very tight and was afraid to try to disconnect them.

My suggestion is to trouble shoot is to find the vacuum line on the passenger side. then cut it and connect a vacuum gauge to the engine side of the source, start the engine and verify you have good vacuum there. Then connect a vacuum source to to the HVAC side of the cut tubing and try to operate the dampers one at a time with the Key On and HVAC in Auto and test each button on the control head.

DON'T Forget to use a vacuum line connector to repair the vacuum line you cut.

I found the problem I had on my 1994 ZR-1 was a pinched vacuum line under the PS knee panel.

Jim

[mlipmd]

Sounds good, I'll have to go out and get a small hand operated vacuum pump and gauge set and try that, to establish first that there actually is vacuum. And then test the programmer.
At least that's easier than going straight to that manifold vacuum programmer on the driver's side and trying to get that out. And if there's a vacuum leak, that's a big waste of time and energy.
Is that vacuum line under the passenger side black like all the others?

[QB93Z]

Yes it is black and looks just like an 18 gauge wire. Be careful what you cut!!!

Sitting here in Avon, CO, I can't remember all of the damper/positions. Start with the defrost damper which is easy to see through the windshield. The other dampers may require removal of some duct work to actually see then move.

Jim

[mlipmd]

Wouldn't you know they are moving correctly if the air blows out the appropriate vent when you press the buttons on the control panel? No need to dismantle anything if it works.
18 gauge wire is 1 mm. in diameter, so even with insulation on it it is maybe 2-3mm. in diameter. That is pretty small. That vacuum line has to be bigger than that. But I'll watch for it.

[mlipmd]

Getting back to this thread while I await the arrival of my vacuum pump - QB93Z said "The vacuum source for the HVAC Dampers comes from the vacuum line connection on the passenger side of the Plenum. The vacuum line runs into a wire bundle and through the firewall on the passenger side. If you remove the passenger side knee panel and search around, you can find the tubing. It runs behind the radio enclosure and then to the HVAC vacuum programmer above the throttle pedal."
Why can't I just disconnect the vacuum line from the PS side of the plenum and check the vacuum in the line from that spot? If it's OK there without any leaks then there's no need to remove the PS knee bolster etc. to try and search for a vacuum line under there. If there is a leak then I have to travel along the line and all the connectors looking for it. And if I apply vacuum to that line and try and activate the HVAC buttons and nothing happens with the airflow out of the vents, then I know I may have to dig out the programmer under the driver's dash. I am aware of the problem with some of the later C4s having those rubber nipples that the vacuum lines attach to collapse and restrict air flow, but I didn't think the '90 cars had that problem because that connector was made out of a different material.
As I inspect the car, there are many vac lines running from the canisters on the driver's side below the headlight which go towards the cruise control. Aren't those the lines that also branch off to the HVAC programmer? The FSM is of little help here but I'll have to go back to search it again.

As an aside my car runs fine with no evidence of any vacuum leak but I have to try the cruise control. If that works, there really can't be any leak leading to the HVAC controller as the two are connected to the same lines.