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?

I would remove the PB control unit & apply 12V directly to the wire that goes to the compressor.

That wire is at pin #25 of the right side connector & is blue w/ green stripe.

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- Checked resistance of compressor clutch voltage supply line - 0.5 ohms; line is intact.

- Applied 12.4v to clutch supply line... 12.4 volts present at the end of the line. Again, line is intact and there is no parasitic loss.

- Connected the voltage supply line to the compressor clutch and applied voltage - clutch is activated.

So, with the vehicle's original PBCU (210 830 22 85) installed, the voltage out from pin 25 is only 8 volts.

Is PBCU renewal warranted?

Installed a used PBCU from a 98 E 300D (210 830 32 85) that was functional when originally removed; clutch activation voltage remains at 8 volts.

reminds me of a grounding problem I once faced in a Mitsubishi Galant with touch screen A/C controls. The control units ground wire in the wiring plug was quite dirty - and cleaning it out solved it.

cant tell about your 210 though

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Since the A/C problem began immediately after the glow plugs were replaced I'd recommend looking over the work very carefully. If the intake manifold was removed, I'd suspect a mistake was made during re-assembly. Look for pinched, crushed, mis-routed wires.

It's ashame that the guy who took your money doesn't want to have anything to do with fixing a problem that he most likely created.

Located and corrected the following faults:

1. Insulation loss on smog sensor wiring harness, resulting in the 4 wires shorting against each other.

2. Low freon charge.

3. Grossly out of spec evaporator temperature sensor resistance.

4. Grossly out of spec heater core temperature sensor resistance.

5. Elevated resistance in W1 ground point.


Correction of above faults resulted in return of 12v in compressor clutch harness.