1997 E300D - Incorrect Voltage to A/C Compressor Clutch

[jgl1]

1997 E 300D (210.020 with OM 606.912), 150K miles.

Vehicle had functional A/C when taken to repair shop for renewal of glow plugs and relay. New plugs and relay # 210 540 03 72 were installed. After service, the A/C no longer worked. Returned the vehicle to the shop, requested A/C diagnostics and was given some mumbo jumbo and advised that the diagnostics would be very expensive.

Took the vehicle to another shop, who found that the front plate of the compressor clutch assembly and retention bolt were no longer present.


The entire clutch assembly has now been replaced but remains inoperative.

- Further inspection reveals the clutch line voltage is only 8 volts... regardless whether the pushbutton control unit (PBCU) is off or on.

- Defrost and economy modes both give 8 volts.

- Ignition on, engine off, PBCU off... still get 8 volts.

- M-B Diagnostic Manual states the voltage for the clutch should be 12-14v and that the clutch is not controlled by ground interruption.

- Installation of a PBCU of known quality failed to resolve the problem.

- With ignition on, PBCU off, removal of the left wiring harness from the PBCU did not effect the 8 volts present at the clutch power plug... but removal of the right wiring harness reduced the plug voltage to zero.


Any insight as to the cause of this fault?