HVAC SYSTEM LOCKS IN EITHER A/C OR HEAT MODE

SUBJECT: HVAC SYSTEM LOCKS IN EITHER A/C OR HEAT MODE

VEHICLES AFFECTED: 1988-1989 C/K 1, 2, 3 TRUCKS

Some 1988 and early production 1989 C/K trucks may experience a condition where the HVAC system will not come out of either the Heater or the A/C mode when another mode is requested. This condition is known as "Mode Lockup". It occurs due to corrosion within the mode door actuator.

In order to repair this condition, a jumper harness (P/N 12085272) that includes a resistor must be installed. The addition of the jumper harness will enable the customer to unlock the system by pushing the Defrost button. From there it will be possible to cycle the system into the desired mode.

This cycling procedure will wipe the corrosion from the contacts, and the HVAC system will then operate normally until such time as the corrosion reoccurs. If the corrosion reforms, it will be necessary for the customer to once again first cycle the system to the defrost mode, prior to going to either Heat, A/C, or Vent.

Perform the following operations to install the jumper harness and verify system operation:

1. Remove lower steering column access panel.

2. Remove three screws in radio receiver mounting bracket and swing aside in order to gain access to actuator harness.

3. Disconnect the vehicle harness from the mode door actuator by depressing the rear portion of the tab lock.

4. Connect the vehicle harness to the female end of the jumper harness (P/N 12085272) and then connect the male end of the jumper harness to the mode door actuator.

5. Using a tie band, route and secure jumper harness to actuator stand off mount and trim excess from tie band. (See Figure 1)

6. Cycle the system from A/C to Defrost, then back to A/C, ten times. This will help to remove the internal corrosion and allow the system to operate normally.

7. Ensure system performance by moving from the A/C mode directly to the heat mode and back again.

8. Reinstall radio receiver mounting bracket.

9. Reinstall lower steering column access panel.

IMPORTANT: The internal actuator corrosion that causes "Mode Lockup" is likely to occur after the customer has left the HVAC system in the same mode for an extended period of time. Customers who have this repair procedure performed should be advised of this "cycle through Defrost procedure" in the event their vehicle experiences "Mode Lockup' in the future.

Beginning with the following VIN'S, the resistor was added internally to the actuator and the actuator label color was changed from white to yellow.

Fort Wayne GMC 1GTDC14K4KZ510473 Chevrolet 1GCDC14K0KZ135780

Oshawa GMC 2GTEK14K3Kl5ll758 Chevrolet 2GCHK39NlK1125271

Pontiac East GMC 1GDGC24K7KE509954 Chevrolet 1GCFK24KXKE135231

PARTS REQUIRED DESCRIPTION QUANTITY -------------- ----------- -------- 12085272 Jumper Wire 1

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.