NO AC following coolant flush and battery disconnect
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
NO AC following coolant flush and battery disconnect
So I performed a coolant flush, and also disconnected the battery to disconnect some harnesses to replace a coolant pipe. Following this, the AC will not blow cold. The clutch is engaging. I didn't touch the AC system. Any ideas?
Senior Member
Joined: Mar 2018
Posts: 323
Likes: 103
From: Central Florida
'15 C400 and '15 ML350
I had this happen on mine recently after washing down the motor (maybe not related and just coincidence). Ended up being the AC valve in the compressor. You will need to ID the right valve to match the one you have, drain the system, replace the $50 part and refill. Worst part is labor to R&R the R134a.
I got mine on amazon but this is the valve.
https://www.rkxtech.com/collections/air-conditioning
The compressor could also be toast, but if it happened suddenly and low/high side is the same, this seems to be the common part to fail.
I got mine on amazon but this is the valve.
https://www.rkxtech.com/collections/air-conditioning
The compressor could also be toast, but if it happened suddenly and low/high side is the same, this seems to be the common part to fail.
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
I had this happen on mine recently after washing down the motor (maybe not related and just coincidence). Ended up being the AC valve in the compressor. You will need to ID the right valve to match the one you have, drain the system, replace the $50 part and refill. Worst part is labor to R&R the R134a.
I got mine on amazon but this is the valve.
https://www.rkxtech.com/collections/air-conditioning
The compressor could also be toast, but if it happened suddenly and low/high side is the same, this seems to be the common part to fail.
I got mine on amazon but this is the valve.
https://www.rkxtech.com/collections/air-conditioning
The compressor could also be toast, but if it happened suddenly and low/high side is the same, this seems to be the common part to fail.
I'm guessing these AC units are all made by the same supplier.
I've replaced this valve before I'll likely do it again. Thanks for the info!
Going to German car repair guys tomorrow because my daughter is freaking out but I'm not paying them if they confirm it's a compressor.
Thread Starter
Junior Member
Joined: Sep 2025
Posts: 19
Likes: 2
So the German shop confirmed it's the compressor valve. What they didn't say is why it failed. On my VW, it was loss of radiator cooling and no high pressure shutoff circuit. In 2020 Mercedes has no way to monitor and prevent AC refrigerant overpressure? That's absurd. This car was low on coolant several times but the AC worked for a while.
Alternately, the car also did dump a ton of coolant on the AC compressor wiring plug....which is sealed. Seems weird that would cause the valve to go bad, but it did on yours.
Ordering a refrigerant capture machine and tackling this.
Thanks for the link to the parts!
How did you R&R the refrigerant? Did you find a shop willing to do it? I didn't even ask these guys, they wanted nearly $450 to change the engine oil.
Alternately, the car also did dump a ton of coolant on the AC compressor wiring plug....which is sealed. Seems weird that would cause the valve to go bad, but it did on yours.
Ordering a refrigerant capture machine and tackling this.
Thanks for the link to the parts!
How did you R&R the refrigerant? Did you find a shop willing to do it? I didn't even ask these guys, they wanted nearly $450 to change the engine oil.
Senior Member
Joined: Mar 2018
Posts: 323
Likes: 103
From: Central Florida
'15 C400 and '15 ML350
The valves seem to just get stuck open with age/time. Its not the pressure switch on the compressor, but the variable valve. Should have nothing to do with the cooling system, but if the coolant/water was getting into the plug then it could have caused the failure that way. I did wash my motor down a day or two before mine failed, but ive also done that every year for the past 10 so...
I had a shop R&R the R134a, but with a manifold gauge set and vac pump you could DIY it. just be sure to measure out the R134a by weight and not use pressure since with a variable valve you dont know the compressor demand % (or use Xentry to command it 100%)
I had a shop R&R the R134a, but with a manifold gauge set and vac pump you could DIY it. just be sure to measure out the R134a by weight and not use pressure since with a variable valve you dont know the compressor demand % (or use Xentry to command it 100%)
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