Automobile heating/AC controls?

Paul Fredricks_3

We had a problem with the defroster on our 96 Suburban, only sort of in reverse. The floor vents worked, the dash vents would come on, but the defroster wouldn't. If we left it in the defrost position it would eventually switch over, though it might take an hour. We figured the flap was sticking and ignored it. Eventually it fixed itself.

Jeff_74

I may be off topic here, but...

Our family van's heating/AC system is stuck in the 'defrost mode.' In other words, regardless of where I set the control knob which directs the air to the desired floor vents, face vents, or defrost vents, etc, the air will only come out of the defrost. I know the electronic switch works because the AC condensor will come on when the control knob is
in the correct position. I"m sure the problem lies in the vacuum lines. I can take the main hose off, and draw air with my mouth and eventually the mechanical flaps will redirect the air to the face or floor vents. Any one have any experience with this as to how I can get enough vacuum to control the flaps? Is there a small motor or some other devise that controls/supplies
the necessary vacuum?

Sorry for the non hyrdronic/stream heat question but I'm stumped!

Thanks,

JC

Joe_75

Jeff,

I have used a huge amount of info from this site and since I am a mechanic I can now post some with a good amount of knowledge. First we need to know what the make/ model/ year is and then I could be more helpful. It sounds like you have a vacuum leak. If you have a mechanics stethoscope(take the metal end off)or a piece of tubing you will be able to hear the leak better, sometimes it is in the engine compartment. Trace all the vacuum lines and check for vacuum. If this does not work give me the make and model and I can help you more.

jeff_75

THanks Joe

Joe, I spent a couple of hours looking for a leak. I found nothing but I couldn't get access to all the vacuum tubes inside the dash. The car is a 2000 Chevy Astro w/AWD.

Thanks very much.

JC

Mark_46

Jeff

Make, model, model year and system specifics all play a big role in diagnosing. You really need a few things to begin digging into this problem. 1) a special tool to read DTCs (diagnosic trouble codes) 2) electrical and vacuum diagrams 3) vacuum gauge 4) hand vacuum pump. Without these anybody would be considered just hunting and pecking. DTCs especially. You need to know what the integrated system diagnosis is telling you. Keep in mind, this is probably a microprocessor controlled system. After code retrieval, there could be an already known solution from the manufacturer based on the symptoms, etc. Check with a local retailer.

With that said, the fact that the system is stuck in defrost mode is PROBABLY part of what is generally considered FMEM ((Failer mode effect management), this feature can vary in name by manufacturer. some may not even call it anything at all). But the design intent is to default to defrost mode for safety reasons when there is some sort of failure in the climate control system...helping to ensure a clear windsheild for safe driving. (This is achieved through spring tension in the actuator motors. Without vacuum applied, they have a naturally extended or retracted position.)

Phwew, OK. Now, to answer one of you questions, the system uses engine vacuum as a vacuum source. Unless its a diesel which would use a vacuum pump of some sort.

Yes, as you say, the problem could be in the vacuum lines. But also connectors, actuators, vacuum reservoir, an electronic sensor or the control itself. It is also possible that the problem is not near or part of the problematic system! It is also possible to find the obvious cause by visual inspection but more often than not it doesnt happen that way ). I would assume nothing and be careful not to 'throw' parts at the problem until its fixed.

As I said first off, you need to retrieve codes first to get an idea of what the system diagnostic capabilities are indicating, if anything. Then go from there.

ed wallace

auto a/c

I had the same problem with my 1999 astro 2 yrs ago my mecanic found the vacumm tube from control panel to the engine had fallen onto the engine block and melted if i remember correctly the line runs from control panel toright front of engine when looking at engine from the interior of the van back then the part cost was under $10.00

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Ericjeeper

Starting fluid

I might catch some slack for this.. But I have used a can of starting fluid.. Keep an extinguisher handy. Spray in short burst at all vacum fittings. and hoses.. when you hear the engine change speeds.. either rev or slow.. you have found the leak.
I am not sure about the astro vans but on a lot of the full sized vans.. attached to the hood is a round ball shaped vacum canister.. often this line gets pulled off. Good luck

Plumdog_2

are you insane?

I always use propane to look for leaks. It's much less flammable. (joke)