Diagnosing HVAC control vacuum leak, glovebox removal tip

[Wade Nelson]

I've had a vacuum leak SOMEWHERE under the dash that was causing problems with the HVAC controls.

Specifically, when the engine was under load, vacuum would disappear, and the AC would blow out the wrong vents. Other HVAC settings didn't seem to work quite right, AC only worked on the coldest, recycle setting, not on the FRESH setting, and VENT didn't seem to work at all.

(This was after my earlier fiasco with the rotary valve in the HVAC control head being mis-aligned, detailed in another thread)

Another commenter here suggested checking the ends of the vacuum lines where they attach to the vacuum storage cannister. (Photo 1) They were OK.

To CONFIRM the problem was in the dash, and not with the cannister or its integral check valve, I put an additional check valve in-line just before where the vacuum line goes through the firewall. (Photo 2)

Since all the vacuum lines were attached, correctly, and the OTHER vacuum motors seemed to be working properly, I suspected the problem was with the vent motor above the main blower motor. (Photo 3)

Getting to it required removing the dash lower (kick) panel, and the glove box. After removing 9 screws around its perimeter, and the screw supporting the lamp, the Damn box didn't want to come out, the mount for the glovebox light was interfering.

Eventually I figured out the glove box is actually 2 pieces, hinged, with pins to hold it in the deployed position, and the top folds down for easier removal. Seeing details in black plastic is hard with 55 year old eyes. (Photo 4)

Aha! There it was. The vent door vacuum motor (Photo 5) Sucking on the 2 lines, one operated the motor, the other just bled air.

These particular "vacuum motor" lines had splices in them that allowed me to attach a longer hose and suck on them to confirm operation without first having to pull down the actuator.

Voila, I've found where the problem is. A vacuum leak AND explains why vent operation didn't work properly.

The vacuum motors on this car all seem to have TWO vacuum connections, one which gives partial "throw" and a second which completes it, so you get three positions for any particular door, open, shut, and half-way.

The other problem is the main vacuum source line is just falling apart, I found it split on both sides of the 1-way valve I'd installed. Time to replace. 1/8"???

In what can ONLY be described as a miracle, Autozone actually STOCKS 7/64" (2.88) mm vacuum/windshield washer line, P/N 50209 "Armor Mark" with an OD small enough to actually fit on the barbs on the vacuum reservoir and elsewhere.

The vacuum motor, secured to the air box by two phillips screws, is secured to the vent lever by a jesus clip, as in, JESUS CHRIST, where did that fall to after I picked it off. Mine will never be found, hopefully I can re-use one from the junkyard motor I'm going to go get tomorrow.

Beneath the Jesus clip are some nylon washers, careful or you'll lose those too!

(I attempted to hold a magnetic retrieval tool next to the clip while I removed it but didn't succeed).

I'd asked to borrow a smoke machine from a shop in town, which would pressurize the HVAC vacuum lines to 1-2 psi with smoke. Look for the leak. But I found the leak before the machine arrived.

Hopefully all this helps someone else fighting HVAC misbehavior.

Finally -- the big picture --- having the Z-car's dash all torn apart....AGAIN!

There's one extra picture there, looking back TOWARDS the steering wheel but up behind the dash, I took it just to see what was up there and where the AC vent line (the thing that actually carries cool air) plugged it.

Edited July 22, 2012 by Wade Nelson

[Wade Nelson]

Here's my earlier thread about the HVAC control head unit being messed up --- and simple to repair!

http://www.classiczcars.com/forums/thread45800.html

BTW, the vacuum lines to the fresh/recirc vent door "motor" are black with one, and two green stripes, respectively.

Here's a diagram from the FSM...

Edited July 22, 2012 by Wade Nelson