LOSS OF HEATING OR COOLING CONTROL (SERVICE PROCEDURE)

VEHICLES AFFECTED: 1990 Y-CAR WITH ELECTRONIC A/C (C68) OPTION

This bulletin has been revised to change the time in step 2 of the correction to 1 minute instead of 30 seconds.

CONDITION: The temperature of air coming from the HVAC system cannot be changed. A code 01 or 02 may also be set in the HVAC control head.

A secondary condition of false trouble codes being set in the ECM or CCM may also be present.

CAUSE: The air mix door or the Heater and A/C system will not move out of the full hot or full cold position causing loss of heating or cooling control. If the courtesy or radio fuse, which supplies power to the A/C control system, is installed with the ignition key in the ON position, the automatic A/C control system may lose its calibration.

Removal or installation of any fuse should always be done with the ignition key in the OFF position!

CORRECTION: Use the following procedure to re-calibrate the system:

1) Make sure the courtesy and radio fuses are installed in the fuse block.

2) Disconnect the negative battery cable for 1 minute and then reconnect it. The A/C system will go through a 2 minute calibration sequence, which keeps the blower fan off, when the battery power is restored.

(If a DELCO-LOC II security code has been programmed originally into the radio by the customer, the radio may display "LOC" when the battery cable is disconnected. If this condition occurs, the customer will have to re-enter enter his/her security code to enable the radio. Refer to Section 9A in the service manual for further information on this procedure.)

3) Verify the proper operation of the HVAC system.

Other causes such as wiring disconnects, air mix door motor condition, Freon loss, etc. may create these same conditions. If the above procedure does not correct the vehicle, refer to the 1990 Corvette Service Manual for additional diagnostics.

NOTICE: WHILE WORKING ON THE HVAC SYSTEM, DO NOT RUN THE AIR MIX DOOR MOTOR WHEN IT IS REMOVED FROM THE HEATER CASE. THIS WILL DAMAGE THE MOTOR.

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.