LOW CHARGE INDICATOR FLASHES DISCONNECT CHARGE DETECTION

Subject: AIR CONDITIONING LOW CHARGE INDICATOR FLASHES

Model and Year: 1991-92 C/K TRUCKS

Some owners of 1991-1992 C/K vehicles may comment that the A/C letters in the middle of the control head face are flashing. The HVAC control system used on 1991 and 1992 C/K vehicles incorporates a low A/C charge warning system. If a low charge is detected, the control head will not allow the compressor to engage and the A/C letters in the middle of the control head face that usually signity A/C operation will blink.

The HVAC control head determines a low charge by monitoring the circuit from the pressure cycling switch. If the cycling switch will not allow the compressor to run more than 1.5 seconds, ten times in a row, the control head assumes a low charge and disables the compressor until reset.

On some vehicles, it has been found that the wire from the pressure cycling switch to the control head is sensitive to EMI (electro magnetic interference) and gives false compressor cycling information to the HVAC control head which then disables the compressor.

If you encounter a vehicle with the control head A/C indicator flashing the low charge signal, but can find no reason for the flash using normal diagnostic procedures; including checking the wiring harness for chafing; remove the pressure cycling switch feedback wire from the control head connector using the service procedure outlined below. This process disables the A/C Low Charge detection feature, however, the customer will still be alerted to a low charge by excessive compressor cycling and poor A/C performance.

SERVICE PROCEDURE

1 . Remove the instrument panel trim bezel.

2. Remove the A/C control head.

3. Disconnect the control head electrical connector.

4. Remove the dark green circuit 59 wire located in harness connector socket number one. Do not cut the wire, remove the terminal (Figure 1).

5. Fold the wire back along the harness and tape it so that it cannot short when the control head is installed.

6. Install the control head.

7. Install the instrument panel bezel.

WARRANTY INFORMATION

For vehicles repaired under warranty use:

Labor Operation: D0362

Use applicable labor time guide for labor hours.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.