HVAC SYSTEM 'MODE LOCK-UP' NEW MODE ACTUATOR RELEASED

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

MODELS: 1988-89 C/K 1, 2, 3 TRUCKS

THIS BULLETIN CANCELS AND REPLACES GMC TRUCK SERVICE BULLETIN 89-T-18, DATED JANUARY, 1989 TO UPDATE THE SERVICE PROCEDURE AND SERVICE PART NUMBERS. ALL COPIES OF 89-T-18 SHOULD BE DISCARDED.

Some 1988 and 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".

Previously, a jumper harness was released to correct this condition. An improved mode door actuator has been released which no longer requires the use of the jumper harness and the need to cycle the system into defrost. This "Mode Lockup" is caused by corrosion on the actuator contacts and the new actuator will eliminate this corrosion. The new actuator will also be about two times faster, as well as quieter.

SERVICE PROCEDURES, MODE ACTUATOR REPLACEMENT (Figure 1)

o Remove

1. Battery ground cable. 2 Instrument panel compartment for access. 3. Electrical connection to the actuator. 4. Retainer and linkage. 5. Mode actuator.

o Adjust

Adjust the Mode Actuator:

1. Install battery ground cable and electrical connection to the actuator. 2. Turn the ignition key to the run position and let the actuator position itself. 3. Turn off the ignition key. 4. Disconnect the electrical connection to the actuator and the battery ground cable.

o Install

1. Mode actuator. 2. Linkage and retainer. 3. Electrical connection to the actuator. 4. Instrument panel compartment. 5. Battery ground cable.

SERVICE PARTS INFORMATION

Number Description Quantity -------- ------------- ---------- 16141802 Mode Actuator 1

This new mode actuator has a blue background pressed on label with the part number on it. 1988-89 mode actuators will have either a white or yellow background part number label.

Parts are currently available from GMSPO.

WARRANTY INFORMATION

For vehicles repaired under warranty use: Labor Operation: D1807 Labor Time: 0.5 hr.

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.