corellian vette

So tonight I decided to replace the heater valve because my A/C is weak and I can still feel heat when the A/C is on. Good thing, as the internal diaphragm was leaking even with vacuum applied.

OK so new heater valve installed, turn A/C on and the lever doesn't move.

I disconnect the line and hook a vacuum gauge directly to the white supply line. Turn the A/C on again and the line is drawing no vacuum. So there is the other problem.

I pull the cowl cover and see the line is going under the wiper motor and disappears. Looking up under the dash I don't see where it comes in or what it connects to.

Anyone know the location of this supply line and how it functions? I would love to bring my A/C back strong!

zekgb

All HVAC vacuum lines are fed from the solenoind bank behind the control unit in the center console. Dwayne's writeup covers it,

FredR

For starters it always helps if you mention what model year you have as there are differences even in the a/c system [not that it impacts your problem too much].

The a/c system is actuated by a number of solenoid valves located under the central dash panel that are normally very reliable. When vacuum is needed the solenoid is activated and then vacuum should be available to the actuator. if there is no vacuum available on the inlet side there will be non on the actuator side of the solenoid valve.

The vacuum is sourced from the vac reservoir located in the front driver side wing. You need to check the reservoir is holding vacuum and that the nylon tube taking vacuum to the solenoids is also holding vacuum. Once you know this to be good the problem will then drop out to whether there is a problem with the solenoid valve [possible but not likely] or an issue with one of the other actuators not holding vacuum. If one of the actuators has a split diaphragm it can bleed the system of vacuum if the leak is greater than the rate of replenishment.

This loss of vacuum can be a bigger problem on later models that also need the vac reservoir to actuate the flappy [resonance] valve.

Testing requires a Mityvac kit or equivalent and the first step is to isolate the reservoir from the a/c system by disconnecting the vac line at the multi way splitter located next to the brake servo. check that the little blue/black valve is connected and holding vacuum and that the reservoir is holding vacuum. If that part of the system is holding the problem is likely in the hvac manifold area under the central dash [could also be the flappy on later models].

Assuming the problem lies in the HVAC system actuation, you have to get at the vac manifold and systematically test each connection until you find the culprit- it is often a failed diaphragm but could also be a connection that has come loose. This is not difficult to do when you fathom it but rather fiddly with difficult access to the vacuum manifold and solenoids on a stock setup.

Rgds

Fred

corellian vette

OK I made an interesting discovery!

First I have an early '86 my apologies.

I did the first set of tests as described in the writeup. The black feed line coming off the brake booster for the HVAC holds vacuum! So looks like I have no leaks in the system. Yay!

Here's what's strange - I measured the vacuum output right at the booster valve and I was like 25+ HG at idle. Great. That small line then feeds a Black/Blue check valve which looks like it feeds a few different systems. When I measured the vacuum at the port that feeds the black line, there is like 3hg. Almost none. So at the brake booster port, great vacuum. On the other side, no vacuum.

Is that blue check valve supposed to be a vacuum limiter of some sort? What should the tested vacuum be on to the line that feeds the HVAC system. This is more than likely my problem - I have no vacuum at all to the HVAC system!

corellian vette

Figured a picture is worth a thousand words. Here's my setup with call-outs.

Would like to figure out where some of these capped lines go to.

The open line in the picture is what feeds the black hardline to the HVAC. Using my vacuum guage I'm only getting a few Hg out of the other side of the check valve, even though I'm getting 25+ at the booster.

zekgb

I believe it's just an ordinary one way check valve. Did you test vacuum coming off the check valve directly? If not there is a vacuum reservoir in the front fenderwell that gets fed from the same valve and leaks along that path are possible.

edited: just saw your post, that Y splitter is going to the vacuum reservoir and cruise control actuator.

corellian vette

I did not. Good to know. I supposed if there is a leak on the other side, that could be my problem. Will check on that tomorrow. Would be great to find out I simply have a bad vacuum reservoir or something. That might also explain no cruise control.

zekgb

You might have a vacuum problem but almost all of the original cruise control brains are dead/dying. Lots of us tried reflowing the solder joints on the circuit board with various levels of success (didn't work for me) and now Roger sells rebuilt units that seem to be bullet proof.

zekgb

BTW if your check valve is in fact restricted you can replace it with a generic FLAPS part.

GT6ixer

FWIW here is what my '84 connector looks like.






The Y-lines go to this pass through plug in the left inner fender.

FredR

Sounds as though the check valve may be blocked/non functional.

You should be able to blow through it one way and not the other.

corellian vette

Thanks for the pictures Nate!

Can someone do me a favor and tell me how much vacuum you are getting at the HVAC port of the rubber X after the check valve?

Here's my problem. Check valve is working properly and I'm getting 20 Hg on both sides.

At the ports off the rubber X however, I only get 4-5 inches. Its like there is some leak or restriction in that X. If I connect the HVAC line directly to the back of the check valve the system works great.

So I'm perplexed. If the system doesn't use full engine vacuum I don't want to run it that way. But the restriction in the rubber X is not providing enough vaccum for the system to function.

Pretty sure my Vacuum Reservoir is working however since I tested that and it holds a vaccum. So the problem is in that rubber X block.

Thanks!

James Bailey

You can temporarily wire the heater valve closed for a few months until you are ready to attack the pods under the dash...

Bill Ball

Are the "capped off?" lines really capped off?

WallyP

Four-way:
1) In from black/blue check valve
2) Line thru firewall to HVAC
3) Line thru left front fender to vacuum reservoir and cruise control
4) Blocked off

Make sure that the blocked-off port is actually blocked.
Make sure that the line into the fender holds vacuum.

There is no restriction in or at the four-way connector. You should have the same vacuum before and after the four-way. If you don't want the cruise control, you can bypass the four-way, but you will lose the reservoir - not a major problem.

Sometimes the vacuum reservoir fails...