I've read very many of the threads in the AC Problems "Sticky" but haven't come close to finding a solution to an AC problem. Details follow:

Car is an 84 300SD (no Klima) and I the 3rd owner, finally getting around to making the AC work. Heat, fans, vacuum servos all work correctly but the compressor clutch will not pull in with CCU control.

1) I solvent-flushed and air-dried all lines, evap., condenser, exp.valve, etc. Replaced all o-rings with new, lubed w/refrigerant oil. New rcvr/dryer installed.
2) The CCU was replaced with a rebuilt unit from the Boca Raton FL company.
3) I took the car to a reputable MB indy to have the system evacuated and recharged.
4) Vacuum pull-down was good, running for an hour. Held 26" Hg with no loss. No obvious leaks.
5) With the CCU set to max cool, a new compressor clutch relay in position 6, the pressure switch on the receiver/dryer jumped, the clutch would not pull in while the engine was running. Time-delay was well beyond time with still no clutch engagement. Fuses 11 and 10 are good, contacts are clean.
7) Clutch coil resistance measured 3.8 ohms. No high-resistance short to ground in clutch coil windings.
8) The system was then charged with 2.3 pounds R134a.
9) The shop technician (not the owner) then jumped clutch relay socket contacts 80 to 87 to make the compressor clutch pull-in. The compressor clutch then pulled-in.
10) Now the interesting and puzzling problem: WITH CLUTCH RELAY PIN CONTACTS 80 AND 87 MADE COMMON TO FEED 12 VOLTS TO THE CLUTCH COIL, FUSE 11 BECAME VERY HOT, MELTING THE SOLDER FROM AN OLD-STYLE 25AMP BUSS GLASS BODY FUSE. At this point work stopped and the refrigerant was recaptured.

I do not have a wiring schematic for the AC system and am wondering what the contacts in the compressor clutch relay connect to. The fuse 11 overheating clearly indicates over-current draw somewhere, but where?

Any ideas or comments as to where the problem could be will be much appreciated. Thanks too for time spent trying to assist with ideas to work with.

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Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951