Hi everyone, I'm new here. In October, I picked up a 1996 C4 Collector's Edition 6-speed. I've been going through the car over time fixing a variety of problems with the goal of having a semi-daily/weekend driver that I can also take to SCCA autocross and track days. I've got the car just about ready to give it a shakedown, but I'm a bit puzzled about a problem that would be nice to solve but isn't a big priority.

I just picked up the car from a shop where I had the weather-stripping replaced around the windshield and halo (after trying and failing miserably to do it myself) and mentioned that the HVAC on the car is exhibiting somewhat unusual behavior. When the car is idle, or at speed, or under gentle acceleration, the AC blows cold just fine. Under moderate or heavy acceleration, it will temporarily cut out and just blow warm air. My initial suspicion from past experience is that this is a low refrigerant problem, but when I mentioned this to the shophand, he suggested the problem was 'likely' the HVAC controller. I'm not inclined to fix the HVAC controller considering the work required to solve what I would describe as an intermittent problem since the only time the AC isn't blowing cold, is under acceleration. I also can't through my research (which included searching this forum) find anyone describing a similar issue. HVAC issues seem to be tied to 'it never works', or 'it only blows cold when idle but not when driving' or 'it stops blowing out of vents.' None of these describe the condition I'm experiencing.

I'm just wondering if anybody else here has specifically seen this kind of problem before and if so, if the source of the problem was not low R134a. I'm still going to check the refrigerant pressure when I get to trying to diagnose this problem, but it would be nice to know ahead of time if the pressure is fine, whether or not this is a solvable problem without inducing a hernia trying to remove the HVAC controller.

Car for reference.

 

Unsure about that year, but I was under the impression that in any vehicle the AC clutch turns off under heavy acceleration or WOT. Dont think it should instantly blow hot air, but it cycles on and off is normal.

 

Yeah I don't think this is that. If I baby it, it'll keep blowing cold, but even at 50% throttle it'll almost immediately stop blowing cold air for a couple of seconds then kick back in. I can't say I hear the compressor disengaging because it's an old noisy car but I can't really confirm it isn't either. Usually when a compressor kicks off it's more like 15, 20 seconds then back on. Not two or three.

 

I would look at your vacuum lines. If you have a leak especially at the vacuum reservoir-with throttle the vacuum drops and could be letting a damper door change.

 

either WOT or low vacuum could be the cause. I would start with a home made test wire to tap the AC compressor and run to a light/led by windshield or a vacuum gage run the same way. Then drive it, hit it and watch what drops/cuts off.

 

I had the same exact problem with another car and it turned out that it was just a little low on freon so I recharged it and it immediately fixed it.