I recently had wear metal in the system so I replaced the compressor and the usual things, flushed, evacuated and recharged. It worked great for about 5 miles and the evap temp started fluctuating wildly and then the compressor stopped. After much back tracking and checking temps/pressures, I can't find anything wrong with the compressor job. This morning I plugged the water to the heater core and that cured most of the problem - broken blend damper. Now, something is still shutting off the compressor and it requires a manual rest to get it going again. I can watch the suction pressure and temp see the compressor cycle at ~31 degrees suction temp - must be from a temp sensor on the evap. coil cuz there's no suction switches under the hood. There's a switch on the compressor discharge line and I think that switch is the problem, now. I ran a thermocouple to the inlet of the condenser coil so I can watch the temp going down the road to see if it's going crazy and it's staying very cool - 137 max as the compressor cycles on evap. temp.. I don't know if that switch senses pressure, temp or both. The switch has a 3-wire plug so I doubt if it's an ordinary binary switch - I'm thinking it's a pressure transmitter cuz it sure doesn't look like a temp sensor. Before I figured out the broken blend door, there was no ryhme or reason to the crazy temps but now it's very consistant. I can go ~ 10 miles now before the compressor shuts off and it comes back on after I cycle either the fan or compressor switch. A puter somewhere must lock out the compressor after it sees a failure. Sooo... I think I'm getting very close to the problem. Does anyone know exactly how that discharge switch functions and can a dealer or someone with a good scanner see what's going on? TIA! Craig

 

Wish I could offer suggestions, but I'm still working up the courage to replace my comp but keep seeing problems like this afterwards....

 

I've been doing some searching and it's called a trinary switch. It kills the comp on low pressure for loss of charge protection and on high pressure, in case a belt breaks, clogged condensor etc.. Some of them can also control fan operation based on hi side pressure. Now I just need to know how to troubleshoot it. I hate not knowing this super simple stuff! I can handle anything that happens to the 100T RTUs at work but my little truck AC is intimidating me! I was scared of the comp replacement/cleanup but it was cake. I don't have an electronic scale so I weight the charge in with a fish scale. I know the charge is on the low side (system calls for 30 oz.) but the superheat was at zero (filled evap) with only about 24 oz. in it so I stopped. I bet it's still low and the trinary is doing it's thing. That would explain why the comp quit at about 130 degrees disc. temp when I slowed down for a turn after it was doing fine while going down the road at about 135 degrees. Me getting schmarter! Craig