87' 300TD NO A/C

Hello Everyone,
I just bought a 87' 300TD. The a/c is not working. I'm a ASE certified master tech but I have no experience with Mercedes. The climate control seems to be working OK. THe fan speed changes with the temp settings, the air blows from the correct ducts when swithched which usually indicates no vacuum leaks or sevo problems. But the A/C compressor clutch does not engage. I checked the high and low pressure switches on the high side a/c line near the sight glass and they are not getting any power to them so jumping them will not help. I've read that the a/c control cutout relay could be bad, but I don't like throwing new parts at a problem without verifying with a test 1st.
So can you tell me how to properly diagnos this ??
Thanks in advance for any help.
Art D
A+ Machine Inc