This seems to be a problem in all weather conditions, mainly over 32 degrees F. A loud fan will come on and stay on as soon as the temp needle goes a tick over 80 on the engine temp gauge. It is really annoying! It slows down the car a lot and is very noisy. It does it with the A/C on or off. The coolant was flushed last year at 36k miles. Truck now has 48k and it has been doing this noise since about 28-30k miles. Any ideas what it is? It sounds like a school bus. btw, the temp gauge never goes above like 80-85.

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2003 C320 4matic (C2, C3, C4)
1999 ML320 (ML55 headlights, MY00 door seal update, ML500 grille, chrome bull bar)

I'd get it hooked up to an MB shop computer. They will be able to look at actual values and find out what is making the fan come on. If there is "no" reason, then I'd have to guess it's being caused by a faulty AAM (All Activity Control Module, known on other types of vehicles as the body control module (BCM).
This all assumes that the loud fan is the auxillary cooling fa in front of the radiator. Pop the hood, start it up, and see if the noise is the fan in front of the radiator.
Because the fan does switch on and off, just not at the appropriate times, I wouldn't think it would be the fan relay itself though.

Gilly

Thanks for the help gilly, I have brought it into the shop before for the same problem and they said nothing is wrong. Now the truck is out of warranty and I don't want to pay big bucks for a defective AAM. It is the front fan that spins off the drivetrain that is making the noise. It does not come on until it reaches around 80, till that temp the car actually has pep. So I assume the fan should not be on all the time? The gauge basically stays "pegged" at a hair over 80

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2003 C320 4matic (C2, C3, C4)
1999 ML320 (ML55 headlights, MY00 door seal update, ML500 grille, chrome bull bar)

The fan on the front of the motor turns with the fan pulley, so whenever the engine is running, it should turn.
It has a fan clutch, which will make the fan harder to turn (more force applied by the fan pulley to the fan itself) as the engine warms up. It's not electronically contolled, so forget the whole AAM thing.
Possibly the fan clutch is bad, it's one of those things I'd almost need to look at it to tell.
Is there another dealer who could examine it to give a second opinion?
The temperature the truck is running seems appropriate, 80 to 90 c is good.

Gilly

The fans in front of the radiator have been replaced twice, still makes the noise. I am pretty sure it is the large cooling fan that is connected to the fan clutch.

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2003 C320 4matic (C2, C3, C4)
1999 ML320 (ML55 headlights, MY00 door seal update, ML500 grille, chrome bull bar)

In your first reply to my post, I was led to believe you were talking about the engine fan:
Now it appears you are talking about what I first assumed, the aux cooling fans:
I would try to demonstrate to the dealer that the fans are running when the engine is cold, and shut off when the engine warms up, which seems to be opposite of what would be desired.

Gilly

Ya I will try to do so, I still think it is the first one u are talking about (fan spinning off the pulley). The electric cooling fans were changed twice due to poor A/C performance i believe. They replaced it once with a reman. unit, then it failed, then they put a new unit in. Thanks again for all your help Gilly! I'll let you know what they say.

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2003 C320 4matic (C2, C3, C4)
1999 ML320 (ML55 headlights, MY00 door seal update, ML500 grille, chrome bull bar)

I have a 202 ML 320
the electric fan is staying on after the car is turned off. We are now unplugging the fan connector when in the garage. and plugging it back in when driving ( and staying on when Driving) any thoughts.