I'm getting a yellow coolant error light (see picture below) pop up whenever I go on the highway or above around 70km/h or so. If I just hit the close button it will go away and not come back until I the next time I turn on the car. I don't have any overheating issues, and my coolant temp stays pretty constant (around 90-95C). It's very annoying though, and I've been trying to get rid of it for a long time.

When reading the DTCs, I saw P2B837A - The rpm of coolant pump 1 of the charge air cooler is too high. Leakage or a defective seal was detected. (stored code)

At first I thought it was from air in the coolant line, so I drained the coolant, pulled a vacuum and then refilled the coolant. Still got the error.

I thought maybe it was because I didn't do it right, so I took it to a mechanic who used to work at MB and has equipment specialized for MB cars. He hooked it up to his system, drained the coolant, held a vacuum, and then filled it with new coolant. Still got the error. I went back to him and he ran some diagnostics and saw that the RPM of the aux coolant pump was at the border of the range that it should be at, so he hypothesized that it was on its way out and would go out of the proper RPM range at higher speeds and trigger the code/warning light. So I ordered a new aux pump and put it in myself, and then drained/refilled the coolant under vacuum. Still got the error.

I then took it to another mechanic who hooked it up to his system, pulled a vacuum for what he estimates was about 15 minutes, and then let the vacuum suck the coolant back in (and added some extra). I saw that he overfilled it, so I took some out. I can't say for sure if it needed much coolant, but when I took it to him it was slightly below what I had put in a couple months before. Not enough to make me think there was a leak in the coolant lines though, as I would expect it to go down much more with all the pressure that builds up while driving on the highway in warmer weather.
I'm at a loss as to what the cause of the issue could be. The two mechanics I took it to didn't report any leaks in the coolant lines, and there should be no air in the system after what each of them did.

The only other thing I can think of is that it's caused by a used radiator that I put in a while back. I bought the car with it needing a new main radiator and aux radiator due to an accident, so I got a new main radiator, and a used aux radiator (from a GLA, but with the same part number). Could a used aux radiator cause this headache that I find myself with? I don't want to replace it for no reason and I'm tired of throwing money at this issue after the hundreds and hundreds I've already spent.

 

If it were me I'd check to see if it's overheating, just to be safe. Especially because the used radiator. If you get a cheap $10 OBD dongle on Amazon and a $5 app for your phone like Torque Pro you should be able to see the water temp and intake temp. Good to have that tool anyway. If those check out then I'd assume some bs sensor is the problem.
What engine is it?

 

I bet you went to these mechanics and said, "I get this yellow coolant light on my dash. Please check my system for leaks." Or something like that. People who do their own work often make that mistake. If you finally go to a mechanic, it is because your DIY efforts have failed. Just say, "I have this light. I have this code. Please tell me what is causing it and how much to fix." Then you can make the decision what to do.

I agree with Chevota that it is probably some bs sensor.

You do understand that the charge air cooler is your water-to-air intercooler and not the main engine cooling circuit. Our cars with turbos have separate cooling circuits -- one for the engine and one for the intercooler. The intake air temp sensor may be malfunctioning and telling the charge air cooler pump to work harder.

 

I read/posted using my phone, and I guess I didn't read the teeny text all that well. So here's a better reply:
It's still confusing about the charge temp code but yet freeway speeds trip it. In my car the "charge temp" only gets hot when stopped or going very slow in the summer. My record temp was In blistering noon summer sun in a new blacktop parking lot you could cook on while waiting for a spot, and it got up to 150F. It never alarmed, but the point is 150 was not easy to hit. On the freeway I'm usually around ambient to maybe 10 deg above it. Your eng is different but I can't imagine that temp would be much different.

If your pump is not working, or not flowing, it can raise the temp so maybe that's what it's detecting? Not really hot just hotter and it thinks the pump died or something. My example temps above are with my pump Off, and On is much cooler when stopped/going slow, but on the freeway I can't really tell if the pump is on or not.
I would remove the cap on the water tank and look for flow. On mine it's obvious when the pump is on, the water is swirling about the tank. My pump is On/Off only, but apparently some, likely newer models, have pulse width modulation. If it's On/Off you can find its relay and force it On to check like I did. Mine basically refused to turn On at idle, and basically only came on under boost. Yours I don't know, just fyi.
And I would not worry about vacuuming the system, but I'd still wonder about the used radiator. So if no flow in the tank then I'd feel the pump to see if it's running. I assume you know where it is. Maybe the accident broke a wire or something, or they forgot to connect the ground or who knows what. Or maybe it's just a fuse, if the wire got pinched in said crash. If the pump spins then I'd check for no kinked/pinched hoses to block flow. Than I'd be looking hard at that used rad rad. You can simply pump water into it to check. You can do that with the entire system btw. Just pop a hose somewhere and pump water into it. Obviously not direct 100psi from the garden hose, but one way I've done it is use my hand as a pressure relief. Car hose and garden hose an inch apart with my hand wrapped around them to make the connection, if that makes sense.
Another thought is I read if you mix the wrong MB coolants together it can gel and clog the rad. I can't imagine, but that's what I read. If so, the previous rad owner may have? Just a thought...


That simple OBD tool I mentioned: The app reads your data as you drive and saves it, so you can review what does what and when. Very helpful for a variety of things.

 

Yeah that's what I said the first time, which is when he did the coolant flush. Then the second time I said the code was there and asked him to diagnose it, which is when he pointed to the aux coolant pump needing to be changed.
This last time I told the other guy that the code was saying that there might be leaks in the system, and he just did the vacuum/refill thing and said I'm good to go. I thought he might do some more diagnosis, but I guess he just wanted me in/out quicker.

Anyway, the radiator I put in was the aux one that cools the turbo/transmission, and the pump I replaced was for that (not the main coolant pump). I'll look into the sensor. It's just odd that it only gives that error above 70km/h.

 

I don't think the coolant system temperature is the issue. That temperature is displayed on the dash, and it's pretty locked to 90 degrees as I mentioned before, and only ever goes a few degrees above that before coming back to 90. I also tried what you mentioned earlier and used my Topdon to display the speed of the "charge air cooler circulation pump" in real time as I was driving. It tended to hover around 20% as I was driving, and when accelerating it sped up (but never over 70%). I went on the highway a couple times to trigger that code, and both times I looked at the graph and it didn't look too high. The first time the warning light came on the pump speed % was in the 50s, and the second time in the 40s. So it looks like it is spinning and circulating coolant, as also evidenced by the coolant temp indicator sticking firmly in the middle at 90.

I think your theory of a sensor or wire might be something to look into. The pump is under/behind the radiator (I changed it myself), and there's one electrical connector that goes into it. I didn't see any sensors there though, or visible damage to the electrical connector. If there was damage, then wouldn't the coolant temperature or speed of the pump not get reported correctly?

Oh and in answer to your other question, the engine is 260.920

 

I just FINALLY fixed this issue. The problem was with a second coolant pump that is located close to the one I changed. I didn't know my car had another one. I thought it was just the one in the thermostat, and the auxiliary one. I initially changed the one with part number 000-500-36-00-80, but the problem was with the one with part number 000-508-38-00.

It makes sense that the RPM of the good pump was too high, because it had to compensate for the other one that didn't work.

In any case, I just wanted to put the solution here in case anyone else has the same problem.

 

Thank you for sharing the solution :
new coolant pump!

Can you confirm: engine type/year ?

 

Yeah. It's weird that the dead pump wasn't throwing a code itself, and I only got info about the other pump running too fast.
The engine is 260.920 I believe. The car is a 2020 A35 AMG.

 

The coolant pumps are also known for leaking hot coolant into the wiring harness - Inspect the connector for signs of salty crusties. There may be pigtail protections available...

The basic coolant pumps use two wires only.
More fancy pumps have multiple wires to control Rpm. Presumably they die by over rev'ing when temps get too high or by flooding their motor.

This pump is going to be a very a popular failure item.


> ROOT CAUSE TIP...
-- Does your engine feature an oil pump solenoid to limit cooling under 3000Rpm?


oil pump control harness

-- Your premature cooling pump failure may be caused by dry pistons storing extreme heat ??