I am working on a friends car. The ac does not engage. The system appears to have enough freon in the system with no evidence of leaks. I can power the ac clutch and compressor on at the ac clutch relay and have swapped with a known good relay. I see the relay is energized by the ecm, but I am looking for information on how to test the ecm's inputs. I know it has an evaporator switch and hi output switch. Are these common problems?