‘92 245; AC works and gets really cold, but the vents only blow in the correct position they’re set to for like 10 minutes. Then they’re uncontrollable, and it’s a guessing game as to which vents are blowing air.

I checked the black and white check valve off the intake manifold, and the vacuum connectors at either end of it were dry rotted and cracked. I blew into it and it only blows one way. Replaced the bad looking connectors, and reinstalled it - no change.

Where do I start from here? What are the more common issues on 240s with the HVAC vacuum system? I haven’t put my vacuum tester to the check valve on the manifold yet, I guess there is a chance it is not holding vacuum long enough; it does look original, or if not, very old...

Hi,

Where to look is fairly straight forward.
Take the right passenger side panel off the center console to locate a white vacuum bottle.
The hose from the engine compartment black and white valve goes there next.
The check valve holds the vacuum on that bottle.
From there it feeds the “control buttons” that select the vacuum motors that operate the vents for defrost, floor air and the recirculating flap located on the rear of the console up against the firewall.

It’s Really hard to replace but the good news, if you are really cold, it must not be bad, as I think they fail open and let outside air come in. It should be a safety thing to fail that way.
As far as the time delay happening could depend on engine loads as that changes the amount of available vacuum from time to time. It’s another reason for the vacuum “reserve” bottle.

If you have a sufficient enough vacuum leaks in the cabin the engine idle will be affected eventually.
It will be more noticeable on an initial start up until the engine can get the vacuum bottle evacuation completed.
Have you noticed any unstable idling at different times too?

A vacuum motors diaphragm can vary its capacity to leak because it folds over any cracks in different positions.
A hand operated vacuum can be used to check all this stuff out by locating different hoses to each.

Hope this will shed some light on the issue but a flashlight will give you lots of help too! (:-)

Phil

Hey Phil,

So after reading this I did a little more testing and dove into the center console stack probably as far as I will go. I was able to verify the check valve is holding vacuum with my mighty-vac; it didn't lose the vacuum at any time, and I cranked it up pretty tight a few times to be sure. The line going from the firewall into the center console vacuum bottle looks new, honestly, and I don't hear any hissing inside the car while operating any of this stuff. I did notice that the elbow connector off the vacuum bottle behind the switches looks aftermarket, like this has probably been chased/fixed in the past. Either way, I pulled the elbow out, inspected everything, and plugged it back in tightly to the rubber adapter off the bottle. I hooked the mighty-vac up to the firewall vacuum line, intake manifold side, and pumped it up and it seemed to build pressure. The gauge on my tester wasnt working correctly, and broke while doing this, so I couldnt get an actual reading. I was able to pump it to create the vacuum, and then operate the switches and hear the flappers move.
Lastly, I inspected the drivers side of the vacuum bottle, and the small metal u-shaped line that comes out of it. All seemed okay, and i plugged it back in tightly.

After doing all of this, it has been working fine, and I haven't lost air pressure out of the front vents on my short (15-20 min) ride to work since. Also, no issues idling or anything like that.

Moving forward, my plan is to keep an eye on it, as I'm not convinced I've done anything to really fix the issue, unless it's a possibility that the flap doors were stuck or sticking and applying a heavy amount of vacuum to them with the mighty-vac sprung them free? No sure. However, I have a spare vacuum pump for the cruise control off a Volvo 940, and my plan, if this wasn't an easy fix, was to hook this vacuum pump up to the line coming out of the firewall, and cap the nipple on the intake manifold. I would then wire the pump to ignition power so it runs with the car on, and see if that fixes my issues.

Hi there Sunapeelaker,

I was curious to what that meant and found plenty to read about that area in the northeast.

Glad to hear that you got to get around to that job that can suck if you have to get into some of those tight places.
So you cranked up the vacuum fairly high I gather because of the gauge not working. About 10 inches of vacuum is about all you need to create to have things work.
You should try to take it apart, as it is already broken.
They are a simply made device inside but at the same time rather fragile.
There is a flat Bourdon tube with a geared linkage lever attached.
It cannot take a jarring and In most cases that’s what harms them. Hey, it is a portable tool thrown into an open and closed drawer of a portable tool box.

When you pumped the system up that hard you probably did pull some stretch into those diaphragm materials and pulled the hoses on tighter. Metric versus inch again if they were too loose or spilt from being stretched.
You might try a bread tie or thin gauge wire to twist on those in question. Some people do use a dab of rubber cement or silicone too.
Yes, you will have to expect another occurrence in the future but no prediction can ever be made over aftermarket’s materials and age of original.

The console switch can leak some at times on certain function because of dust infiltration into the valve segments. It the only reason I have ever had one fail on me.
That is mainly because I tried to clean the shuttle mechanism and I think it made it worse. As I remember it’s a riveted or a heated plastic affair. The strongest thing that I can advise to use on one of those is denatured alcohol or water withe just a drop of Dawn dishwashing liquid seeped into it and sucked out.
Hopefully you will not have an issue with it. It’s in an aggravating place of the console.

The idea the cruise control vacuum pump connected to the reservoir sounds like a good “work around.”
Just like the electric windshield wipers over the vacuum operated ones.
We can observe over the decades what happens when more capital investment is made into our mass produced consumer products but then that is another double edged sword.

Anyway happy you posted back that you chased it down.

Phil

Well it happened again, so it’s not fixed. It seems to be an issue strictly relating to the passenger and driver side blend doors, doesn’t seem like it’s losing vacuum. When it happens, the drivers side loses air, but the passengers side still works fine; you can still select floor, vents, and defrost and they all work, just only out of the passenger side vents. So… the issue has to be with the drivers side blend door getting stuck or losing vacuum or something. I’m not sure how it works?

Hi,

To the best of my knowledge there are only the vacuum motors on each side of the vents system.

The two center vents get air all the time. It comes right of the top of the two turbine fans located on each side of the console. Both impellers are driven by the one centrally located motor.
All the air comes from the rear of the consoles opening governed by one flap that brings air from the outside unless in recirculating mode with the right side button, of the three, pushed in.
The fan housings have three outlets each.
One is directly to the top center vents with no controlling blend doors.
Another out the side but still upwards towards a main duct on each side. No control either.

On that UPPER side, on both sides of the car, have two vacuum motors.
One vacuum motor goes to defrost vents and another one over farther but down the duct goes to a floor vent right above your feet.
All air is coming from that angled side port off the fan housing.
If both turbines are turning air has to be going out

The third outlet is out the bottom of the fan housing. It’s sending air out lower ducted vents that go under the seats and empty in front of the rear seat floorboards. This duct also has a flap control motor. It doesn’t blend anything it’s open or shut depending on, I guess, the floor button.
I have never studied what it does, when?
One would think that if both were open, one to your feet up front and one to the rear passengers, less air would be available to the dash and the defrost vent.

That could be a scenario or less like the left fan impeller is not spinning but you are getting air only from the right impeller. That would dimmish your left output.
I have never read about that happening unless “a clip” that holds the impeller onto the left shaft came off.
I doubt though that it could fix itself? (:-)
There should be an odd noise, if this were possible, when it was loose and spinning on a shaft.
Has anyone ever worked on a motor replacement? Just asking.

I think you are probably looking at something is wrong with the three button controller, if it’s putting you into floor mode.
How it’s doing that only on one side is beyond me as much as an impeller spinning loose.

I admit you have a strange thing happening. Fixing itself or breaking itself is not the choices normally.
The only thing I can suggest is to feel around for fan air flow, while manipulating the buttons for all the outlets I discussed. Feel both sides for correct operation.
One should tell on the other.
You could try closing the vents at the upper registers to amp up the feel of forces below.

Keep us posted on your progression.

Phil