No AC Pressure after compressor replacement
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
No AC Pressure after compressor replacement
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.
Is there some magic Mercedes trick to getting this to work?
Thanks.
Out Of Control!!
Joined: Apr 2019
Posts: 12,058
Likes: 6,857
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
PWM PROP VALVE
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.
Is there some magic Mercedes trick to getting this to work?
Thanks.
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)
Last edited by CaliBenzDriver; Feb 23, 2026 at 03:25 AM.
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
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.
Out Of Control!!
Joined: Apr 2019
Posts: 12,058
Likes: 6,857
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
clutch and valve
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.
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
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.
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.
Out Of Control!!
Joined: Apr 2019
Posts: 12,058
Likes: 6,857
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
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.
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.
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....
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
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.
Trending Topics
Out Of Control!!
Joined: Apr 2019
Posts: 12,058
Likes: 6,857
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
No repeat... fix!
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.
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.
Last edited by CaliBenzDriver; Feb 27, 2026 at 03:55 PM.
MB World Stories
The Best of Mercedes & AMG
New Electric Mercedes-AMG GT 4-Door Coupe Unveiled: 10 Things You Need to Know
Verdad Gallardo
6 Mercedes Models That Did NOT Age Well (But Are Somehow Still Cool)
Verdad Gallardo
Manual Mercedes? 6 Times Sindelfingen Let Drivers Have All The Fun
Verdad Gallardo
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Verdad Gallardo
8 Classic Boxy Mercedes Designs That Have Aged Like Fine Wine
Verdad Gallardo
Flawlessly Restored Mercedes 190E Evo II Heads to Auction
Verdad Gallardo
Electric Mercedes C-Class Unveiled: 11 Things You Need to Know
Verdad Gallardo
Mercedes EQS Gets A Major Update: Everything You Need to Know
Verdad Gallardo
5 Underrated Mercedes-Benz Models That Don't Get the Love They Deserve
Verdad Gallardo
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
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.
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.
Out Of Control!!
Joined: Apr 2019
Posts: 12,058
Likes: 6,857
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
IGNORANCE'S A BLISS
Wheel well view of exposed A/C connections
This electrical failure is not as random as my thoughts.
Last edited by CaliBenzDriver; Feb 27, 2026 at 08:24 PM.
Out Of Control!!
Joined: Apr 2019
Posts: 12,058
Likes: 6,857
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
Nearly Waterproof Connectors
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...
Last edited by CaliBenzDriver; Feb 28, 2026 at 03:24 PM.
