Sometimes I have cold A/C, Sometimes not

This was a 2000 Nissan Frontier. The complaint was that sometimes the a/c seemed cold, sometimes not. The air felt cold enough when I drove the truck into the shop. A quick test to find max cool temperature is to turn the temp control to coldest, fan to low speed, max a/c or recirc setting and put a thermometer in one of the center vents. Generally speaking, a good system should output 50F or colder when driving at road speeds and set as written above. Automatic temperature systems will change output temps once the cabin reaches the setting you have in place. Until then it will output the coldest air.
The vent air on this truck was moving between just below 50F to just above, as the compressor clutch would cycle. I wasn't driving at road speed. I was just in the shop holding rpms at about 1800. The clutch was cycling a little more often than I would have expected and the output temp was just at acceptable range so I decided the refrigerant charge, while it could be slightly low, wasn't the cause of the complaint.

I let the truck continue to run with a/c on for several minutes before the problem appeared. The air had stopped cooling and the compressor clutch had disengaged. 

The system had sufficient pressure according to the gauges. 

Testing at the compressor clutch relay showed no command from computer to close the relay. 

I needed to see if the computer was aware there was no command to the relay so took a look at scan data. The computer was seeing a/c request but was keeping the relay off. Verified by scan data.

Since the computer was intentionally turning the compressor clutch off I would need to look at other data items for something that could be causing the problem. I checked stored codes and the check engine light was on from the beginning. None of the codes seemed related to the a/c problem. Going back to data items though I saw the engine coolant temperature was above normal. 

The computer was turning the compressor clutch off to prevent engine overheating. I added a half gallon of coolant to the radiator and the same amount to the recovery tank. I found no leak even with a pressure test but will be keeping an eye on this one. The actual refrigerant charge was 3oz less than a full charge so did make a difference in the a/c temp as well. Here are the final pics after the work. 

Thanks for reading!!

Kenneth Hayes

G&G Auto Repair

kenny@ggauto.repair