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1999 C280, 2015 C300
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Discussion Starter · #1 ·
Last week I had to replace my radiator due to a leak. I took my C280 to my Indy and brought a gallon of OEM Mercedes coolant (BQ 1 03 0004) and distilled water. After the radiator was replaced and coolant added, the low coolant light kept on coming intermittently. Before I knew, the Indy went ahead and added some BMW OEM coolant. That did not fix the problem and the car was running hotter than usual. He said to keep driving and the coolant will mix and the light will be gone. After I came home, I noticed that the Indy had forgotten to connect the electric fan. I connected the fan and also squeezed the hoses to get the air bubbles out.

Now the car is running fine at normal temperature and the coolant light is not flashing any more.

I have two questions for this learned community.

1. Why was the coolant light coming and going when the coolant was almost full?

2. Will mixing BMW coolant with Mercedes coolant cause any problems? They both seem to have similar ingredients.

Thanks in advance.
 

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1995 C220
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First thing I would suggest is find a new indy mechanic. Anyone that will tell you that the light will go out once the coolant mixes doesnt know what the hell he is doing.

The light can only come on for two reasons that I know of. First one and most obvious, is that you are low on coolant. Second reason would be a faulty sensor in the coolant bottle or faulty wiring going to the sensor.

The coolant used can be a touchy subject around here, but the fail safe answer, is you cant go wrong if you have Mercedes coolant with distilled water. Its not the cheapest route, but if you are worried about it, this is the way to go.

Personally, I would not be bothered by having BMW coolant mixed with it, because it would really surprise me if either coolant was drastically different than the other. Unless it was the old school green antifreeze, our cars are old enough that any of the new anti-freezes will cover you. The issue isn't necessarily in the cooling/anti-freezing properties, the issue is whether the coolant is compatible with the type of metals and seals that are in our engines. It seems like there is always a new spec on the anti freezes from the Euro manufacturers, but I think all the new specs are compatible with the older engines, so you should be fine. If it bothers you though, find out who is making the antifreeze for Mercedes, and order some on Amazon or Rock auto, and swap it out. It will be half the price vs going through Mercedes. If I remember right, it was zerex G05. Get that and you should be good.
 

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1999 C280, 2015 C300
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Discussion Starter · #3 ·
The light can only come on for two reasons that I know of. First one and most obvious, is that you are low on coolant. Second reason would be a faulty sensor in the coolant bottle or faulty wiring going to the sensor.
The coolant was more than half full in the expansion tank and the light would come on and off. It would remain on for 1-2 minutes and then go off for 1-2 minutes. Could it be that the electric fan was not connected. After I connected the fan, the light has not come on.
 

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1996 C220 2007 ML320 CDI
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Coolant-wise Mercedes OEM or Zerex G05, and as others have said I do not think it would be an issue to mix the BMW coolant. Just stay away from the green stuff. What the coolant needs to have is adequate protection from galvanic corrosion between the iron engine block and the aluminium (yes that' how you actually write it) cylinder head. If BMW has iron blocks / aluminium cylinder heads in their engines then their coolant will have similar properties.

One other possible reason for the low coolant light to come on is a bad connector for the windshield fluid sensor (they share a single circuit). If you unplug the windshield fluid sensor, then you will get a low-coolant light (open circuit triggers the low-coolant light, short circuit triggers the low windshield fluid light).
 

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1999 C280, 2015 C300
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Discussion Starter · #6 ·
One other possible reason for the low coolant light to come on is a bad connector for the windshield fluid sensor (they share a single circuit). If you unplug the windshield fluid sensor, then you will get a low-coolant light (open circuit triggers the low-coolant light, short circuit triggers the low windshield fluid light).
I think in my car, the windshield fluid sensor is on a different circuit then the coolant level, because I have separate warning lights for the coolant level and windshield washer. Also based on your suggestion I unplugged the windshield washer plug, and started the car, no lights at all, not even windshield washer light.
 

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I think in my car, the windshield fluid sensor is on a different circuit then the coolant level, because I have separate warning lights for the coolant level and windshield washer. Also based on your suggestion I unplugged the windshield washer plug, and started the car, no lights at all, not even windshield washer light.
It's very possible that the circuit has changed for the facelifted models, but on the older ones, the two are connected. I also have separate lights for coolant level and washer fluid level. I should look this up in the Bentley manual.

At least that eliminates one potential cause for you. Might indeed have just been a lack of coolant. The proper coolant level is where the reservoir's black top meets the translucent bottom part, if you have it higher than that, when the coolant expands it will flow out of the overflow tube.
 

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