2020 C300. Diagnostic from MB repair shop claimed AC compressor shot following loss of engine coolant. Replaced compressor. Held high vacuum for over 24 hours. When attempting to charge with the engine running and compressor clutch spinning, it appears to pressurize on the high and low sides to 70psi (can pressure). It appears not to be pumping, clearly.

Is there some magic Mercedes trick to getting this to work?

Thanks.

 

the modern compressors use a proportioning valve to set the piston plate. Thing its all new... can you review your AC Module data... any DTC's ??
It's not pumping down low side into high side pressure regardless the fact its spinning.

Double-check that replacement is really compatible, suspect new proportioning valve of new compressor is the wrong type (diode vs. no-diode)

 

No DTC. I very specifically was careful to ensure the replacement was specifically for the Mercedes part number that came out of it. It's possible the compressor or valve wasn't the issue to begin with.

 

Great: 99% it's the correct matching new compressor.

It is spinning but not building any head pressure.

Look at the governor valve duty cycle
Look at the clutch activity !
look at the condenser pressure sensor

Resist the urge to overfill. 75Psi should allow pumping.

 

I can't access that information, and I'm not going to waste any more money on this car by buying a scanner. I took it to an AC shop. When it's finished I'm going to sell it.
I don't see how I can overfill it when the compressor isn't pulling down the low side? It's reaching can pressure and that's it. I have no way of pushing more refrigerant in it.
Thanks for the info though!
I'll post what the ultimate issue was.

 

FYI: the sealed refrigerant system is strictly filled by weight using a precise scale... not measured by container and not by pressure.

With both Hi/Lo equal pressures your compressor is not pumping.
The A/C shop will get this sorted in no time.
Let's see....

 

The issue was a physical broken wire between the ECU and the compressor solenoid valve. The very expensive local Mercedes shop failed to hook the diagnostic computer directly to the AC compressor to test it, and so misdiagnosed it as needing a compressor. Or, they're scammers. But since the sale and installation of a Mercedes wiring harness is far more expensive than installing an AC compressor, I'd say they were just sloppy and incompetent. Gotta love a shop charging hundreds of dollars an hour and failing to perform basic troubleshooting properly.

 

Thank you for letting us know the final say.

There is something else very easy you can do in that department : waterproof the connections to your A/C compressor.
You can see compressor from the wheel well where it gets sprayed with road elements - The two connectors are not waterproof - Apply engine RTV where wire harness enter the back of each plastic connector.
Enjoy.

 

Tell me what's easy about applying sloppy RTV to all the 100 engine electrical connections on a 45,000 car? And how does this prevent a physical broken wire in the harness? Do you just supply random thoughts?

 

Revisit this thread next time your compressor harness quits again


Wheel well view of exposed A/C connections

This electrical failure is not as random as my thoughts.

 

Yeah I'd be hesitant to pt RTV on anything electrical related. I'd use a rubber boot or protect it in another manner. But RTV... not so much.

 

Do you have any harness run in your front or rear bumpers covers (radar sensors)?
These use connectors directly exposed to road spray.

Each individual wires act as a guide to channel water inside the connector onto the tinny oxidized pins. This guarantees poor connections.

I seal the external junction of wire to connector.
Whatever seals the water out...