1998 Buick Park Ave
3800 VIN K

First problem is the cruise control. If I turn the cruise control to on and let OFF the gas and hit set the cruise light will illuminate for a split second and then drop out. If I hit set while keeping my foot on the accelerator, it will not illuminate at all. I have tried all different speeds and the cruise control fails to lock on and stay on at all speeds. A month ago it would actually work periodically right but other times wouldn't work at all, So it in essence slowly died.... I have checked and replaced all normal failing parts for the cruise control. The only difference I can see is that on cold mornings before the engine gets up to normal temp, the cruise "may" stay on a short time longer but will always drop itself out.

I have replaced the following with NO change:
Brake Switch
TCC Switch
Throttle position Switch
Vehicle Speed Sensor (on transmission)
Multifunction switch stalk
Cruise control module (and throttle cable)
All brake lights work properly and bulbs are good.
Have checked every fuse

I have also checked all 10 prongs on the cruise control module plug and they all show 12v (or ground) when the corresponding button or switch is pressed. The only ones I haven't seen voltage on is the enable and inhibit circuits that go back to the ECM. I did however unplug the ECM and did a continuity test between the enable and inhibit to their corresponding pins ( 55 and 72 ) and neither were shorted or grounded out. From what I can gather is the relay to enable the servo cruise control motor is actually in the ECM, and it'* almost like the relay isn't holding, but I could be wrong. But like I said, I have checked and rechecked and replaced and rereplaced all the obvious problems and answers that I have found. I am not the only one who has posted something to this effect, but every post I read either doesn't have an answer, or is something easy, that I have already tried. The only thing I haven't tried is that I thought I read a post on this site that somebody said they replaced the ERG valve and then their cruise control started working again. Not sure how the 2 could possibly be connected but maybe?

2nd problem is my A/C compressor and fans. I know that whenever the A/C compressor runs the fans are also told to run, I got that. My issue is that I start my car with the HVAC controls OFF and fans do not run and compressor does not run. BUT within seconds of turning the HVAC controller ON ( doesn't matter if its AUTO, HEAT, AC, FLOOR, DEFROST, etc,etc) as long as its ON within seconds the AC compressor clutch engages and the fans start. So if you can imagine it is trying to heat and cool at the same time and it pumps the freon. I have not yet tried a different or new control head unit (that'* my first move) but I do understand that the head unit sends the signal to the ECM and then the ECM send the ground signal to the AC clutch relay which in turn engages the clutch. So based on all this info, am I getting the to point where it may be that my ECM is taking a crap? Or can anyone at all shed some light on these issues, or give ideas (no matter how obsurd) that I can try to fix these issues. I think I have included all points but please ask questions if I have not. I am very mechanically inclined (just no expert) and if I don't have the answer, I will get it for you. To try to rule out the ECM as a possible culprit, I will try to replace the HVAC head unit this week.

the hvac control unit is kind of expensive just to replace, the outputs of the hvac and cruise can be seen with a scanner but has to be a higher end scanner like a tech 2 or hpt so if you can find someone willing to check it out you may be able to narrow it down or confirm the pcm as the issue.

From the sounds of it, he has the electronic HVAC controller. If you can, unplug it while the engine is running. If the A/C compressor turns off, then you have an HVAC controller issue. In the meantime, you can disconnect the electrical connector to the compressor.

I do believe your cruise is controlled via the BCM then to the PCM.

Have you scanned the car for any codes lately? Even though you do not have a CEL, you could still have one that doesn't illuminate the light.