Steve, thanks for your insightful technical explanations. So my 94 C220 have a fixed, cycling compressor. Back to my original question, the compressor seems never visually disengaged once AC is on but the PBU #7 reading does change back and forth from 10.x to 16.x in engine idle condition. Besides, the lowest evaperator temp is only 39. It's not as cold as you said 32. Does it mean I have to flush and recharge the system then the compressor may work the way it should be.
Thanks a lot