I have recently aquired a 1981 SD. My first MB. My ac isn't working and I have ben trying to get to the bottom of this mystery. I'm looking for a wiring diagram and a componant layout for an 81SD. There are no markings for the relays in the fuse panel. The compressor clutch won't pick up on it's own, the pressure switch is good. There is a temperature regulator under the right side of the dash near the blower motor. there is another module close by. so where is the compressor relay? and where is the cutoff switch? if there is one on these. I picked up the compressor clutch with a jumper to the battery. the temp got to about 53 on the left vent. I can hear damper doors move when i vary the temperature on the temp wheel. I plan on following Mr. Patels troubleshooting step next week when i have a day off.

One of the things i have found is that the pin connectors tarnish a bit with age, and need to be polished with a scotch-brite pad. Found this to be the problem with the tach amp, and the cruise control amp. Also the pins on the ccu and temp regulator were tarnished, causing high resistance.

Also Has anyone tried alldata? Is there information indepth and accurate?
Thanks
Scott