That’s the coolant reservoir for the water cooler on turbo. I believe it takes the same coolant as the engine. Maybe a more experienced owner can confirm this but all I’ve seen do.

 

Yes it does. I had to figure it out on my M276 (2016 GLE400). It for the turbo system. I think there on a separate system and not on the same system as the main Engine cooling or it could be the circuit.

The turbo coolant reservoir, I found mine was low but never triggered the low coolant warning message but the main engine coolant was full.

My coolant is pink color on my GLE. My old 2004 CLK500 was the blue coolant. Don't mix up the color of the coolant...

 

My M278 has the spot where the small reservoir goes but doesn’t have it. I wonder why some do and some don’t.

 

Did a bit more research, it started at 2014 for M278 and M157 engine. It's for intercooler.
What I don't understand is that coolant is yellow, not the blue coolant. Wonder if I can mix it if need to refill, or have to drain and fill with blue.

 

I wouldn’t mix it just in case. There might be a certain coolant for it. I’ll check what colour the coolant in our C43 is it has the same reservoir as yours.

 

My 2018 GLE550 with the M278 does not have that small reservoir. My 2014 E550 with the same engine did.

 

I did a radiator hose change yesterday and before putting the blue coolant back in my radiator, I noticed this smaller reservoir was low. However It was not low before the hose change so they must be connected somehow. Mine is a 2013 S550.

 

The question is, Where can I find the correct Pink Color coolant?. MY 2014 ML550 has pink coolant. and Blue coolant for radiator.

 

Q 1 03 0004 blue (G48)
Q 1 03 0005 red (G40)

 

BQ-1-03-0005 Mercenes-Benz OEM Anticorrosopn/Antifreeze (Pink Color, G40)

 

I Order mine from Amazon or ECSTuning.

 

It appears that earlier M278 didn't have separate water cooled circuit for the intercoolers; Then they added water circuit for them a year later. On my 2013 it's all one circuit with only one reservoir, and uses blue coolant like Zerex G-48.

 

I have two reservoirs on my 2013 S550, one on passenger side for topping off the radiator and this same smaller one atop the engine and they are connected as when I drained the radiator, the reservoir atop the engine also drained. I have blue coolant in mine. As I refilled the radiator reservoir, it also refilled the smaller reservoir atop the engine. It must have been due to some tweaks during the 2013 production year on the S550 which would explain why there are two different lower radiator hoses that come up in a part search for 2013. The dealer ordered my lower radiator hose and when they delivered it to me, it was different, so I took mine to them and they then saw there were two different ones made for 2013. The one for my car has a Y that spits off with two smaller hoses, one that goes to to a metal pipe that disappears into my wheel well and the other goes into the front fender area behind headlight somewhere. The one they first ordered did not have the Y or the heat insulation wrapped around it. My guess is one of the small hoses coming off the Y must go to the reservoir atop the engine somehow. Its a mystery but one thing is certain, after 9 years many of the plastic connections are rotting and falling apart when I touch them.

 

I came across this video on YouTube on flushing coolant out of intercooler.

 

This kid is a real COMEDIAN! very good video very detailed going to do my gl450 soon. Why was your engine so noisey? Loudest mercedes i ever heard! GREAT VIDEO!

 

Interesting
My 2014 (late) ML500 has the intercooler reservoir.
It was low on fluid, and didn't change level when i flushed and refilled my radiator today ( new radiator )

I have topped up the level - bit of guess work as there is nothing i can find on a required level ??

Have started and run the cur up to temp.

Now for the crap part....
With my new scan tool, have found i have a P023A00
"Coolant pump of air charge cooler"
Not sure if its been something that has just occurred before as it doesn't throw a dash engine warning.

From what i can see, the coolant pump is situated in the drivers side down beside the radiator.
Its pipework runs back up to the reservoir and off to the intercooler BUT seems to T off with pipework that involves the main radiator lines.

In my efforts, i have damaged the Crankcase Breather Vent Line and pumped a bunch of coolant onto my garage floor !!! time for a Beer !!!!!!