19. TITLE - Loss of heating or AC control.

YEAR(S) - 1990-1991

PROBLEM - The following information comes from page 47 of the 9/10 issue of the Registry Newsletter from Gordon Killebrew:


First off, you may or may not notice your radio controls acting odd as well...

"...the cause was a so called "logic loss." The memory programmer in the AC programmer lost track of where it was. Just like a home computer when the power is removed from the system, then re-powered up, the computer did a re-learn and again performed as it should. A bulletin was released to all dealers telling them if this occurred to removed the radio and courtesy fuse for 15 sec. with the ignition off. This did a reset of the AC logic. A defective radio data signal could cause a AC logic loss, or vice versa.

CORRECTION - In 1991, the AC data lines were removed from pin J ALDL data line to the radio. The #1996 circuit then became number 461A and 461B going from the AC control head to the AC programmer. The pin J ALDL #1996 wire became the #720 circuit going directly to cavity C6 in the radio control head's white connector. This resolved the AC radio interface problem, the root cause of the logic loss that was coming from the new AC air mix motor. All 1990 Corvettes have a BULER flat pack air mix motor (part number 16145772). It was causing the logic loss in the AC system. The motor did not have built-in stops and if the tip would break or wear, the motor would go beyond its travel range and was extensively damaged; thus causing the AC system to flash lights and fail. The fiber tip, GM part # 52452753, did in fact crack and wear on many cars. As the cars get older this may happen more often.

In the mid-1991 GM released a new motor with stops, part # 16124952 made by Calasonic.

In 1990, bulletin 90-378-1A was released with the part number for L98 as well as ZR-1 programmers with capacitors to prevent logic loss. In 1991, bulletin 91-200-1A was released stating the corrected programmers for the early 1991 cars.


If you think you may have a logic loss on your AC system review this information and look for the presence of a Buler flat-pack motor worn tip or broken tip. If the indicator lights flash and you do not have the correct AC control, do the following test:


1. Turn the ignition key on, then turn on the AC on Auto.

2. Hold the up and down fan speeds buttons both in at the same time for about 5 seconds, then release.

3. In the digital dash, "-00" should appear . If a number appears, you have a system fault code. Press the FAN AUTO button to show any fault codes stored in the programmer. (Pg 1C1-2 of 1990 service manual.)


Code 01: Air mix door actuator circuit fault.
Code 02: air mix door actuator circuit fault.
Code 03: outside (ambient) air temp. sensor circuit open.
Code 04: outside (ambient) air temp. sensor circuit short.
Code 06: Inside car temp. sensor circuit shorted.
Code 07: Solar (sun load) sensor open.
Code 08: Solar (sun load) sensor shorted.
Code 09: low freon detected.
Code 10: UART or E&C data buss line failure.


Most commonly, a 01 or 02 code will be present. Sometimes there are no codes present."

SERVICE BULLETIN/RECALL - 90-378-1A and 91-200-1A