Control won't move from front blower to defrost

[KC Strip]

I replaced the a/c & heater control module on Saturday (in my 1999 Suburban) thinking that would solve my problem, but it did not. So I'm looking for some help.

My problem? I can't get the knob to properly work ... to move the blower from the front vents to the defroster. It's October and soon I'll need that defroster. One thought I had was that it could be the blend door actuator (or temp valve actuator or whatever term is used). Any help is appreciated!

 

[Chris142]

Is it vacuum operated?

 

[KC Strip]

That's a great question. How can I check to let you know (because I don't know off the top of my head)?

 

[emory]

On my old vacuum systems you could hear the air hissing as you changed from defrost to floor. Not sure if that's a reliable means to check.

 

[KC Strip]

In the recent past, it would make kind of a thunking sound when it switched between the front vents and the defroster. Now it only blows through the front vents.

 

[MrCritical]

Sounds like the blend door is stuck.

 

[KC Strip]

Let's assume you're right, MrCritical. What is my next step?

 

[dwendt44]

My beater car is stuck on mid-level, won't move, no vacuum leaks.
I just close the A/C vents in the winter and the fan blows air/heat on the windshield and heater outlet.

 

[KC Strip]

dwendt44 -

That might be a possibility if I were going to drive the Suburban this winter. However, my wife will be driving it no later than the end of the year. She won't want to deal with little tricks to do this and that. That's why I need to get this resolved ASAP! I'm sure everyone can appreciate my situation - right?!

 

[Merkava_4]

Quote:

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Let's assume you're right, MrCritical. What is my next step?

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That's where a factory service manual comes in handy for instructions on removing the dashboard. I don't have an FSM for my car, but I get by with using AllData online for most of the mechanical repairs I make.

I draw the line of my weekend mechanic endeavours at the dashboard because of the complexity involved. I wouldn't even turn one screw for removing the dash without an FSM; it's just too complicated and risky.

After saying all that, it's the vacuum operated blend doors that are the easiest to trouble shoot. These vacuum operated diaphragm actuators open and close the blend doors. All you have to do is check to make sure the diaphragms are receiving vacuum and holding vacuum; they're actually quite neat to watch in operation.

I've never worked on the electric solenoid operated blend doors; I imagine they would work similar to an electrically operated EGR valve. I would assume that would entail checking for power received by the solenoid actuators at the time when they're supposed to receive it.

KC Strip, when you replaced the A/C Control Module, was there just a wire harness connector or were there vacuum lines going to it also?

 

[KC Strip]

There were only three wiring harness connectors.

 

[GMGuy]

I can't speak for the suburban layout, but remove any panels over the passengers feet. Above that is the plenum and usually, all of the vacuum controled doors. Each (usually) is controlled by vacuum. Startup the vehicle, and turn the knobs to see which control moves which vacuum control. (in some cases, multiple vacuum motors will be activated for partial flap movement). When you narrow down what you think is the vaccuum motor for defrost/vent remove the vaccum line and put your thumb over it and feel for vaccuum as you move the control. If you feel vaccum, good. Your control head is doing something. Next, remove the vaccum motor from the plenum. This will let you do two things, 1)see if the vaccuum motor works (plug it back in!) if the arm moves, its good. 2)manually move the rod in the plenum and see if its movement is free, and feel if its stuck, or if it is broken free of its door. (Somebody may have dropped something down the vent and jammed the door). If its broken free, you'll just manage to move the rod without resistance. If the door is attached you'll hear that familiar thunk as you move it from one extreme to the next and gravity takes over to slap it shut.
Proceed appropriately for whatever you find amiss.

Alex.

 

[Triple_Se7en]

I had a similar problem with my GM S-10. Suddenly it started working again when my transfer case got replaced. There was a small hose that either cracked or punctured. If your Suburban is 4-wheel drive.... check that black vacuum hose underneath your vehicle.

 

[KC Strip]

It is 4-wheel drive, so I'll check it. Not sure where the vacuum hose is you are describing Triple_Se7en.

 

[Merkava_4]

Service and Owner Information
> Chevrolet & Geo > 1999 > Suburban > Shop/Service Information

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1999 Tahoe-Suburban/Yukon/Denali/Escalade Service Manual (Old Style)
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(English, Paper, GMT99CK)
Click here for detail


Helm Incorporated

 

[KC Strip]

Thanks for pointing me toward the service manual!

 

[Triple_Se7en]

Quote:

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It is 4-wheel drive, so I'll check it. Not sure where the vacuum hose is you are describing Triple_Se7en.

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I didn't do the major work on mine. All I know is that it's near the transfer case - or attached to it.

 

[KC Strip]

Thanks for the insight.

 

[KC Strip]

Here's the latest ... I tried to fix this myself, but to no avail. Mostly frustration (and thankfulness that my kids weren't around when I was trying). I took it to the dealer and found out that a wire harness had fallen down and was binding the HVAC linkage. So, problem now resolved.

Thanks for all the great help on this everyone!