AC / vent vacuum distribution problem on 1992 740

[isaac]

Hey guys, thanks in advance for any advice. So I have a 92 740 blowing weakly from the front vents with this system:






It only has 2 vacuum diaphragms actuating the blend doors on the left, and I can’t find a video where there aren’t 3, so no help. Anybody know what those 2 do?

So while set to front vents only, I’ve tested the white hose in the plug full of hoses in the back of the selector dials: it pulls lots of vacuum, but no idea what "enough" is.

Reconnecting the plug to the back of the AC control dials makes the front vents blow a little harder but not hard enough. Meanwhile there’s a little air coming out of the foot and defrost vents at the same time, but not nearly as much as the front.

All of this is done at idle. The front vent airflow definitely gets worse when driving, but not by a huge amount.

So it's definitely sending blower motor air pressure where it shouldn't go, but no idea how much.

UPDATE 12:19 p.m. :

If I switch to defrost it switches a lot of the blower power up to there, but not all of it. It doesn't change as much airflow to the feet when I turn the dial to the feet.

Oh, and I found this hose disconnected, actively pulling vacuum:


It sends vacuum back to the engine bay, inexplicably, and as you can see goes down through the firewall in the same place as the main vacuum line that goes to the back of the AC controls. No idea what this hose does. Plugging it or connecting it to the intake manifold seem to do nothing.

Ideas? Does the valve mechanism inside the rotary dial actually go bad or is it always just the vacuum diaphragms on the side of the box? Thanks

 

[Pietercc]

Other 3 are drivers side

 

[Pietercc]

 

[isaac]

Other 3 are drivers side

But this is the driver's side. Believe me if the whole left/right thing was a problem for me I wouldn't be under the dashboard right now.

It's interesting that graphic doesn't show what any of the actuators on the left side actually do. Which of the three does which diverter door?

 

[isaac]

Well holy shit. I just looked at my own f'ing picture and found a third vacuum actuator buried up there.

That doesn't improve the problem whatsoever and gets me no closer to diagnosing this but it's there, above the other two. Red hose coming out of it.

 

[Pietercc]

Make sure all hoses are sealing well, listen around for hissing while you have the car running.

 

[isaac]

That's wassup

 

[isaac]

I hear no hissing in the cabin at all, even with the blower at the slowest position.

I did find a hose (pictured above) in the engine bay that's sending vacuum back into the engine bay that I can't explain, and also doesn't seem to do anything at idle when I plug it.

 

[NLRegina]

You need a vacuum check valve in the engine bay going to the cabin vacuum actuator supply. Yours is either missing or disconnected it seems. The part is still available and cheap. It's a small black and white piece. I can share more info later if needed.

edit: I'm blind

 

[Pietercc]

I hear no hissing in the cabin at all, even with the blower at the slowest position.

I did find a hose (pictured above) in the engine bay that's sending vacuum back into the engine bay that I can't explain, and also doesn't seem to do anything at idle when I plug it.

That looks to be for the heater valve

You need a vacuum check valve in the engine bay going to the cabin vacuum actuator supply. Yours is either missing or disconnected it seems. The part is still available and cheap. It's a small black and white piece. I can share more info later if needed.

You can see that pictured above

 

[isaac]

That looks to be for the heater valve

You can see that pictured above

So does the fact that the vacuum hose for the heater control valve was actively pulling vacuum mean it was trying to turn the heater on or off? The previous owner of this car removed a lot from the engine bay including the heater control valve so I can't check function.

 

[Pietercc]

Off, and it is loosing vacuum by being open. Plug it, see what happens

Also, AC will not work as well with no heats valve, just because there "should" be no air going through the heater core, doesn't mean it doesn't effect the air. My AC was kinda crap all summer, as I have a valve with no flap in it, and once I pinched the line it got as cold as it should.

 

[isaac]

Off, and it is loosing vacuum by being open. Plug it, see what happens

Also, AC will not work as well with no heats valve, just because there "should" be no air going through the heater core, doesn't mean it doesn't effect the air. My AC was kinda crap all summer, as I have a valve with no flap in it, and once I pinched the line it got as cold as it should.

Interesting. I appreciate it. I'll try it out and see what happens.

 

[isaac]

Well I went for a drive and the vacuum leak fix on the heater valve hose may have done a little, but not enough. I'm noticing that as soon as I'm on the throttle out there it's still losing significant blower force through the front vents so something's leaking somewhere.

 

[NLRegina]

I rescind my previous statement that your check valve is missing or disconnected, but are you sure that the check valve is functioning? Does it allow you to suck air in one way but not blow it back? Those valves can go bad eventually.

 

[isaac]

I rescind my previous statement that your check valve is missing or disconnected, but are you sure that the check valve is functioning? Does it allow you to suck air in one way but not blow it back? Those valves can go bad eventually.

Yeah I'll have to...check

 

[Pietercc]

I rescind my previous statement that your check valve is missing or disconnected, but are you sure that the check valve is functioning? Does it allow you to suck air in one way but not blow it back? Those valves can go bad eventually.

Agreed, thought it can be seen, doesn't seem to be working

 

[Pietercc]

The check valve makes it so that when the throttle is open, it keeps vacuum in the climate control system. There is also a vacuum reservoir, capable of keeping the system pointing to the right place for at least a min. Also, you said it doesn't blow strong to defrost? Defrost is open with no vacuum, so if there is a vacuum issue it should blow strongly to defrost.

 

[isaac]

The check valve makes it so that when the throttle is open, it keeps vacuum in the climate control system. There is also a vacuum reservoir, capable of keeping the system pointing to the right place for at least a min. Also, you said it doesn't blow strong to defrost? Defrost is open with no vacuum, so if there is a vacuum issue it should blow strongly to defrost.

When I actually select defrost it blows enough to be effective. Strangely it seems to blow on defrost even better after I did what I read in another thread and disconnected and plugged the yellow hose from the double-acting vacuum diaphragm on the left side of the box. The one with the blue hose that's also connected to it.

The front vents are my main concern, because they still don't seem strong enough. The front vents really have their job cut out for them here in Florida. It's the third day of fall today and the heat index hit 100º at 11:30 a.m.

 

[isaac]

Ok I checked the check valve and when I pulled the hose off it released the vacuum that it had inhaled 2 hours earlier when I was dropping my son off at school. So that's not it. Two days ago I replaced the tiny hose that connects the check valve to the hard line that goes through the firewall, and it was pretty worn out, so I know I restored some vacuum, and probably severely reduced vacuum loss, with that.

But the front vents still blow as little air on the fan's max "5" setting as I'd expect them to blow on position 2 or so. At this airflow level I am actively sweating in the car, even with the vent temps in the 50s.

It makes me wonder if maybe the evaporator has a ton of debris piled up on it (this car has 361,000 miles) and it's severely restricting flow. Is it possible to open that thing up without pulling the whole box?