Mechanics

HOW DO I FIX A/C THAT WILL ONLY BLOW THROUGH DEFROSTER VENTS AND FLOOR VENTS

2003 Chevrolet Avalanche • 140,000 miles

My battery died, so I replaced it. Once I did this, now when the a/c comes on, it blows normally for 1-3 seconds then will only blow the cool air through the defroster vents and floor vents. We've tried several ways to re-set it - turning off a/c, disconnecting the battery, re-starting the car and letting run for 5 minutes before turning on a/c again. Tried this at least 5 times. Took it to a local repair shop and they tried to re-set the computer, but it would not work. They've said we have to go to the dealership for them to hook it to a machine to re-set. Is there no other option?
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Rrrsearson
July 2, 2012.




GM has designed in a lot of "got'chas" to cost you money after the sale, and this may be one of them. Volkswagen, BMW, and GM have the most tricks built in that cause these problems after simply disconnecting the battery. Newer vehicles are often worse.

Expect to make a trip to the dealer, but first check the owner's manual, or the service manual if you have one, to see if there's a procedure to reprogram the computer. The only one I'm familiar with was on the '97 and '98 Caravans and getting half of it to pass could be extremely frustrating. The procedure was initiated by pressing certain pairs of buttons on the control panel.

Caradiodoc
Jul 2, 2012.
GM, like almost ALL MANUFACTURERS of late model cars, including Chrysler, uses an electric actuator to operate the mode doors and a central module to operate them and either can be bad or one of the air doors can be stuck. It will take someone very familiar with the system and some expensive diagnostic equipment to diagnose it properly.

Wrenchtech
Jul 2, 2012.
In my experience with ese problems a small vacuum leak somewhere will cause the AC system to default to the defroster.
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Georgeduffy
Jul 4, 2012.
All good answers but the assumption is this is related to disconnecting the battery. Plus, it works normally for one to three seconds. That should rule out any mechanical problems.

Caradiodoc
Jul 4, 2012.
If this happened after disconnecting the battery, this is what happened. Every time the power is cut off the system loses memory and when the power is reconnected the system goes through a calibration process that moves all the doors to their extreme ends for calibration and many of them will get stuck when that happens. It very common on the temp doors. It may unstick itself and you may be able to help it along by pushing on the linkage under the driver's side dash.
There are no vacuum controls on this system.

Wrenchtech
Jul 4, 2012.
Thanks Wrenchtech. How long does that calibration process take and how do you it's done and / or passed? If an actuator sticks, then unsticks from bumps in the roads, will it self-calibrate then or do you have to do something to initiate it again?

Caradiodoc
Jul 4, 2012.
Sometimes they break free and sometimes they don't. It is initiated whenever the power is interrupted so a battery saver is a good idea if disconnecting the battery for any reason. It happened a couple times to the blend door in my own Blazer. One time it freed up on it's own and another time I had to resort to drilling holes in the housing and popping the door free with a long thin screwdriver but with the mode doors, the gears and detents are on the outside of the case so you can just grab them.

Wrenchtech
Jul 4, 2012.

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