The electronic a/c system in my 90 vette has been operating with a mind of its own since I've owned it. The system for no apparent reason will turn on by itself, reset itself to 75 degrees, lock up (no control by the panel until restarting the car), The display panel will switch from outside air temp display to inside pre-set temp when coming to a stop or slowing down - not all the time - 50% of the time. The a/c programmer, control panel, ccm, have been changed. Nothing seems to change this problem. I have cleaned and checked all of the grounds for the dash and underhood. Anybody have any suggestions or similar problems?

 

For the C68 climate control, the best way to trouble-shoot this kind of problem is by using a high-quality (expensive: Vetronix Tech-1A or MasterTech) scan-tool with the appropriate software designed for the job.

Unfortunately, there is a setup procedure that has to be done too -- this procedure is in the shop manual.
Basically, you have to connect the serial-bus that the climate control "head" uses to communicate with the "Programmer" to the serial-bus for the radio. The reason for doing this is to allow you to use the Vetronix scan-tool to access the ALDL connector under the dash with the climate control.

Once this is done, you can use the scan-tool to simulate the control head and send commands over the serial-bus to the programmer -- by simulating the control head, if the problems go away, then the defective part is the control head -- if they don't go away, then the problem is the programmer.
Also, you can monitor the commands on the serial-bus (in English) to see what commands are sent from the control head -- this way, you can see if a command was actually sent to change things. If the command was never sent to the programmer but it changes anyway, then the Programmer is defective.

The difficult thing about all this is you need the expensive scan-tool and have to jumper wires to connect the two serial buses to diagnose the problem. And, the jumper has to be removed when done with it. I put a switch on my jumper so I can diagnose problems with the C68 climate control with a flip of a switch.

This really beats guessing.
As your post indicates, it can be more expensive and frustrating to swap parts with no results than to actually do it with the scan-tool.

Tom Piper

 

Tom's right - the easiest way to diagnose the '90 and up Electronic a/c is with a bi-directional scanner - however, you've replaced everything and it's still acting up. The only thing remaining is power and ground and I'm pretty sure one or the other on yours is intermittent. It's like hitting an on/off switch and it's not only turning it off (and then back on), it's erasing memory in the process - so when it comes back on, it's lost the prior setting and it has to go through the relearn procedure (anywhere from 30 seconds to a couple of minutes) before it works again. My bet would be on the ground (though if it's power, you only need to look at whatever is supplying Memory; ie, the circuit that's hot with the key off, and since that circuit is probably shared with a bunch of other stuff, anything else that isn't working will give you some clues as to where to look). Unfortunately, I don't have the '90 schematic, but once you know what the circuits are, re-examine the wiring. Ohm out the ground(s) from not only the Control Panel, but the Programmer too (and if they're spliced to any other controls that are working, you only need to look from the connector to the splice). Another test would be to have everything plugged in, the key on, and then wriggle the connectors, harness, etc to see what it does. The intermittent only needs to be a nano-second to cause the failure, so you might also try a short finder, though ripping out the dash and then having to unbundle harnesses is not fun work. So hopefully you'll find it at a connector - in fact if it were me, I'd probably spray them with some contact cleaner and hope that the simple stuff fixes it.

 

Did you ever find the problem? The memory on my 1991 A/C has gone out, also. The A/C resets to the default 75 degrees when I turn the car on (crank it) & off, but holds memory if I don't crank it (using acc only).