mx3500

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.

kevinkpk

Have you looked at the belt?

oilcanhenry

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?

mx3500

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.