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Call in pro and hope for quick A/C fix

| Friday, Sept. 13, 2013, 9:07 p.m.

Question: I've been a longtime reader of your column and finally have a question to ask. My wife has a 2002 GMC Envoy XL. How she manages to drive it around with the vents shut off is a mystery I won't ask you to explain.

When I ride in the vehicle, I have her turn on the air conditioning. Usually it is set to “auto.” The vents on the passenger's side work fine, but those on the driver's side blow hot air. We've been getting by with closing off the vents. This works well as long as I am not driving, but I do want to figure out what could be the problem and fix it.

— Rajeev Jog

Answer: Rajeev, your situation sounds similar to that of the Chevrolet Sonic we recently looked at, but differs quite a bit in how it needs to be fixed. Your Envoy's dual control automatic A/C system has two temperature control doors within the air distribution module rather than just one. Instead of cables operating the temperature mixing doors, remote control electric servos are used. These are similar, but larger than the kind used in radio-controlled model airplanes. Each servo contains a reversible electric motor and a position sensor. Five connecting wires — two for the motor, three for the sensor — allow the control panel to deliver position commands and verify the objective was met.

It sounds like either the left side air temperature door is stuck in the hot position, is being commanded there due to a system fault, or the A/C system is low on refrigerant.

A professional grade scan tool is needed, with significant understanding of its capabilities and system operation, to check for body control module diagnostic trouble codes, or DTCs; door position; and door response to commands. A nonmoving temperature door could be the result of door binding, a faulty actuator (servo), actuator linkage, wiring fault or failed control panel. In the event of a system fault, the control panel will command full hot, presumably to ensure safe functioning of the defroster, which is more important than operator comfort. Service techs report that your symptom often, and oddly enough, can be fixed by restoring the A/C system refrigerant level to specification.

The bottom line is this job requires the services of a savvy and well-equipped technician. I'd start with a scan tool check for vehicle DTCs, followed by a check of A/C performance and charge level. If needed, a closer look at scan tool parameter IDs, or PIDs, will indicate servo position, temperature sensor readings and other diagnostic info. An actuator recalibration procedure may also be worth a try.

Now the ooky part. Should the air temperature actuator or door require close-up physical testing or replacement, considerable disassembly (and reassembly) of the instrument panel is necessary, to the tune of five or more hours, plus the cost of needed parts. Let's hope the low-on-charge A/C theory pans out and a leak diagnosis, repair and recharge gets things right again.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net.

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