A/C comes on in Vent mode

[austinado16]

This is a new one. The a/c has started to turn on (a/c clutch engages) when the dash controls are in "vent" position.

Do I have a failed control assembly? I've had the dash apart quite a few times (instrument bulbs, replacing the flap for defrost/vents control, etc. but I've yet to do anything to the controls.

I've also got a bad diaphram somewhere, and running down the road in max a/c mode, the flap over in the passenger footwell opens and closes depending whether I'm climbing a hill(manifold vacuum low) or just cruising straight and level(manifold vacuum high).

 

[89Suburban]

Damn this is a hot topic lately.

These threads have a lot of pics of those system components:

http://www.gmsquarebody.com/forum/showthread.php?t=1559&highlight=heater+duct

http://www.gmsquarebody.com/forum/showthread.php?p=57102&highlight=heater+duct#post57102

The controls run off a single vacuum feed line that runs off the rear of the intake manifold and enters the mid-right side of the firewall and into the controls. The heat/cold control is cable actuated. Then there are several other vacuum lines running back out to the appropriate locations. The door on the right foot well is the recirc/fresh air door. Then behind the ashtray/glove box area is the blend door for the vents, and on the trans hump where the heat blows out there is a diaphragm for the floor/defrost.

 

[austinado16]

Saw those threads and looked at the photos. I know what/where those components are, as I've been that deep into all of it.

Question is about the a/c coming on in Vent mode, and my guess is that the control lever's contacts are out of wack, damaged, clocked wrong, etc. Was hoping someone else had this issue and would have a few hints.

I probably have a torn diaphram in either the vac unit by the gas pedal, or the one on the back of the HVAC box. Haven't dug in yet, other than to check for leaks in the vac line starting at the intake and work my way into the cab and behind the box, isolating vac lines and testing for vac holding, using my Mityvac.

 

[Jims86]

Over time, a crapload of shavings clog up the electrical switch....same thing as in some ignition switches i have seen.

 

[89Suburban]

Oh I misunderstood, I thought you meant blowing the air.