Running Max A/C up long hills the air diverts to defroster and floor.

With engine running on Max A/C the vacuum motor on the blower motor housing retracts properly. Plastic white hose running from it into harness appears good visually. All the engine vacuum hoses have been replaced with rubber hose but not the A/C past the check valve/tee.

Getting out my MityVac the vacuum motor tests good. White hose at vacuum motor leaks somewhere. Reconnected it.

I disconnected the black hose coming out of harness at the the check valve/tee and tested it. The black hose leaks on Max A/C. However, testing it and moving the selector **** to anything other than Max A/C it holds vacuum fine. So the black to the selector **** has no leak. With engine on, the vacuum motor only retracts when using Max A/C. The leak must be somewhere between the selector **** and the vacuum motor.

Bottom line, the question, is what is the best way to replace the white plastic hose with rubber hose to the A/C selector ****? The white runs in the harness through the blower housing into the cab.

Since I figured out the vacuum motor is only used on Max A/C I plan a test drive with Normal A/C up a long hill to see if it diverts to defroster. However, I would like to have Max blow in my face in 100+ degree weather.

Thanks everyone.

 

Underhood, try unwrapping the harness tape a bit to follow the white plastic vac hardline into the harness. Mine had a deteriorated spot just a couple inches INSIDE the tape. I just spliced that section and all was good.

If your problem is indeed in the long run of the white line, you could abandon it, and run a new hardline in parallel with the routing of it, using short bits of rubber vac hose as connectors on the ends of it. You can get the same diameter hardline at O'Reilly Autoparts, it's out on the floor in the help! section. I think it is 36 or 48 inches long or something like that. Color is black. Bring in a bit of your old line to compare inner diameters, though I think it seemed obvious to me at the time. It's been a while...

Your idea of testing up a long hill with the selector in regular A/C, not using Max, is a good one. If it defaults to defroster then, then that shows that the white line isn't the real problem. Good thinking!
If it fails then, then that points to some small leak in the system that can be handled with enough engine vacuum, but when vac is low with throttle open, vac reserve is depleted. So a leak somewhere in the regular vac circuit, or the check valve doesn't seal tight, or leak in the vac reservoir or the line to it, etc.

Yeah, this time of year around here, we need ALL the A/C we can get!

 

Thanks. We are definitely on the same page.

I did a road test today but without the trailer I had to really get on it up a hill to duplicate the issue. I finally got Max air to default to defroster then made the same run again with normal A/C and it did not go to defroster. Also, the manual temp gauge dropped about five degrees without the vacuum leak.

Two hours later when all was cooler (a 102 day today) I decided to test the check valve. When I pulled off the black hose to the harness at the check valve I heard a PSSSS.s.sss..sss..sss...and put my finger on the end of the hose and felt the vacuum even though the engine had not run for two hours. No need to test. It held on Normal A/C vacuum.

Now to work my way down the harness looking for the white hose leak. If no luck thinking maybe propane or ether there and under the dash where I have not looked.

 

Today I double checked the vacuum motor and it holds vacuum forever.

Removal of the glove box makes it easier for old folks to see the black and the white hoses coming in under the heater box over to the right where there is a hose connector. Disconnecting it and drawing a vacuum I found a slow leak in the white one going under the hood and also the one running to the selector. Removing some of the harness tape under the hood did not reveal anything nor did the propane test.

To be continued after a vacation week...

 

I found one leak. Pin head sized hole in the white hose 1/4 inch from the connection on vacuum motor. I missed it before. Now the vacuum holds forever from the connector behind the glove box to and including the vacuum motor.

Next! Now the white leaks from the connector to the control. I tested it at every control **** position and it never held vacuum.

How do I remove the control **** assembly? The back side appears inaccessible.

 

If the vaccuum switch holds in AC but but not in Max AC.....you have a switch problem

 

I think that is OK, if I am understanding you correctly: At the inline connector on the white recirc line, with it disconnected, you are applying vacuum back towards the control head, and flipping the control through all positions.

In normal operation, in MAX, vacuum is applied from intake manifold, through the control head, to the white recirc line and on to the Recirc vac motor. So inside the control head, there is a path from vac line coming in from the firewall, through the head, and on to the vac motor. So if you disconnect the white line at the connector, and with engine off try to apply a vacuum backwards towards the control head through that path, you will just be sucking air into the intake manifold from the atmosphere.

Next case, do the same with a vac source on the white line back towards the control head, but put the control in any position BUT MAX. Here again, I don't think you will be able to hold a vacuum there, as the white line to the Recirc motor is purged (connected to air) when in any position BUT MAX. Reason for this is to purge vacuum to allow the Recirc motor to de-energize when not selected. If this wasn't done, then vacuum could be trapped in that circuit after being deselected, and the Recirc vac motor would stay in the Recirc position until minor system leakage would finally release it someday.

I don't remember how far the connector is from the control head, you would want to rule out a leak between those two points.

I don't know how to pull out the control head, never needed to do it. If no one comes up with the method, I can look it up in the shop manual tomorrow.

 

Thanks and thanks again.

You understand perfectly. All that makes sense. I have not road tested since replacing the white from the vac motor into the harness. The white is still in the harness. I will need to be towing up a long hill, heavy throttle with Max Air set to duplicate the issue.

Since my last post I finally found it in Haynes. Remove trim that runs over control head and glove box to expose screws. Figure shows four vacuum hoses coming from the control to a four hose connector somewhere. The two hose (black and white) connector is behind the glove box to the right of the air box.

No need to look up removal. Unless it fails the road test I'm not pulling it out.

I greatly appreciate your help.

 

You're welcome!
Towing up a long hill with heavy throttle is a real acid-test for the vacuum system.

Our trusty Mopar minivan (replaced by the Escape) started to have the A/C air flow shift up to the defrost vents by the top of some of the hills on 45 down to Aggieland into a stiff wind. I wondered if old Bessie was getting low on performance due to high mileage. But I checked out the vac system anyway and all was OK. And on the next trip it did it again. I checked it out again, but this time the check valve didn't seem to seal closed once, then it was OK. Put it back in. Next trip, the same thing again!
I replaced the check valve, and after that it was fine. Seems it didn't always seal tightly closed, so the vacuum stored in the reservoir was bled back to the intake manifold when manifold vacuum was low for a prolonged time. It was super-simple to get to the check valve on that car.

 

I replaced a bad check valve 3-4 years ago. The white hose hole was only the size of a pin head. Now that it is replaced I guess I could test by setting on Max A/C, removing the hose from the check valve and putting my MityVac on the black hose going into the harness. It tested good on all the other settings.

 

Torky2 thanks for sharing your knowledge. I was trying to overthink this. All is well now.

In hindsight and for the benefit of those with a similar problem here is how I will test in the future:

Disconnect the black hose that comes out of the harness from the check valve. Hook up MityVac on the check valve and it should hold 18 inches of vacuum. If not, replace.

Hook MityVac to black hose and test at each of the control **** settings.
If none hold the black has a leak. If all hold 18 inches except the Max A/C the leak is either the white hose or the vacuum motor it goes to by the passenger side hood hinge. It takes several pump strokes to engage the various vacuum motors.

 

Sounds good!

I broke my gray plastic-handled Mity-Vac some years ago. The plastic snapped right through by the hinge. I really couldn't come up with a way to fix it since it was broken at a high stress point, so I upgraded to the metal Silverline Mity-Vac, which also has a vac/pressure gauge mounted right on it. Now having two sets of vacuum fittings/adapters/converters helps a lot.

Ya know, this vacuum motor stuff is going the way of the dinosaur. New cars are all electrical/electronic servo instead. Which take up less space, doesn't use engine vacuum, but are much more expensive to replace!

I hope that the blend door motor module on my pickup never goes bad!