I'm baffled why the AC on my 2007 Chevy Express 3500 (4.8) cycles on/off but only on the highway. If I keep it below 55 mph it doesn't cycle on/off and blows cool air fine. The pressures at Idle were 250 high side & 50 low at 90 degrees outside. Problem started when I adjusted the temp dial back & forth, but might be a coincidence. Also the vehicle sat for a while before I acquired it.

 

Have you looked at the belt?

 

Did the coolant pressures change or the compressor cycle when you revved it up to the RPM's when it's at 60-65 MPH?

 

The belt is good

We checked pressures at Idle but didn't rev up RPM"s so that could be checked. I seen a pressure chart that showed 250/50 is normal at 90 degrees, but it didn't say at what RPM. Tested 3 times in the afternoons heat (105+ degrees) & it blew cold air on the highway without cycling, but this morning when it was 85 degrees it cycled on/off as soon as the speed got up to 60+. Are chevy's ac's controlled by outside temp? Some said they're supposed to cycle on/off to prevent freeze up, but others said no. Update: after a 4th test this afternoon temp is 107 degrees, ac did not cycle on/off. btw: The on/off cycling is subtle so if not holding the hand in front of vent its not that noticeable, but never seen a vehicle do that before.