1990 Corvette: Service Bulletin: Loss of Heating or Cooling Control

Subject: Loss of Heating or Cooling Control
Model and Year: 1990 Y Car with Electronic Air Conditioning (C68) Option
Source: Chevrolet Dealer Service Bulletin
Bulletin No: 90-101-1A (Corporate Bulletin No: 931107)
Section: 1A
Date: December 1989

TO: ALL CHEVROLET DEALERS

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 of 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 poser 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:

Make sure the courtesy and radio fuses are installed in the fuse block.
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 his/her security code to turn on the radio. Refer to Section 9A in the service manual for further information on this procedure.)
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.