i have the electronic controls for the heat, a/c. etc. i have been having annoying issues with it for a while. sometimes when i start the car, the a/c will come on by itself. other times the heat will come on and in the summer that is annoying. most of the time if i shut the car off and restart it it will be ok for a awhile.

i have tried different head units in the dash but it still acts up every now and then. the other day was really annoying as it was cold and i couldn't get the heat on. my question is, is it possible to switch the system to the "manual" system.at least this way, i would have the control of the system.

are you sure there isn't a short some where. or a bad ground at the heater contol box. i would think you would have to change over everything to make it manual. 

Chances are the real problem is in the CCM(central control module), otherwise also known as the BCM(body control module).
Swapping to a manual system would be a MAJOR hassle, since you would have to change the CCM, wiring, head unit, and who knows what else.
Have you tried to pull any codes from the system? I have heard many tales of HVAC system ghosts in those cars...can't remember off-hand what the #1 problem usually is, tho....I'll see about pulling some TSBs in the morning.

Several years ago I was having similar problems with a twist.... the display for the fan speed and temperature would dance and flash all kinds of crazy values. I took a shot in the dark and it somehow did the trick. Pulled the unit and set it on the bench face up and hosed it down with CRC electical component cleaner, followed by squirting canned air every angle I could get on all of the buttons. Re-installed it and it's been fine ever since (knock on wood)

Number:	90-101-1A
Section:	1A

Date:

December 1989

Corporate Bulletin No.: 931107

Subject:

LOSS OF HEATING OR COOLING CONTROL

Model and Year:

1990 Y CAR WITH ELECTRONIC AIR CONDITIONING (C68) OPTION

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 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 his/her security code to turn on 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.

Labor Operation Number:	T1456
Labor Time:	0.2 Hours

This is prolly not your problem, but it appears to be the only TSB that even comes close...and it's info for others, just in case someone IS experiencing these symptoms. I'll keep lookin...


|UPDATED|12/17/2012 10:51:18 AM (AZT)|/UPDATED|

i was looking at some tsb's about this . there is an out side temperature sensor for the ac . it shorts out and can cause your problem. mater of fact there are several tsb's for your problem. unfortunately my dealer would not let me run copies or bring them home. i did see the one that Adam posted . i hope you can figure it out. however changing it over to manual would be very expensive. and a major job.

The C68 electronic controls are not integrated into the CCM other than the back light displays can set CCM  lighting codes. So that's not a concern. Finding the parts to do it is.

Installing manual control HVAC in a 90, it was available in 90-91, before C68 became standard in 92. A quick survey of the Black Book shows 1,149 90's, and 1,136 91's for a total of 2,285 manual control HVAC systems in 90-96 Corvettes.

Good luck finding one.