My 1998 E320 auxillary cooling fan runs all of the time - even at start up. I believe that it is supposed to kick on after a certain temperature is reached
(100 degrees). Can I just replace the thermostat or could it be a sensor that needs replacing and how would I go about checking? New thermostats are relatively cheap - $20 or so and I would imagine relatively easy to install. Thanks to anyone that can help.

it will be somewhat a challege to diagnose. I say start as follows:
1) Check Radiator and Antifreez level, Radiator could be goign bad, or antifreez could be low. See if not fixed then,
2) Check engine oil level if low or not.
3) Replace thermostat. If not fixed then:
4) Possible senstors. If not
5) Take it to the shop

Thanks E320Star,

My oil and coolant levels are fine, the engine runs great and does not get hot at all. I'm quite sure that the problem is either the thermostat or a sensor telling the fan to kick on even though it is not supposed to yet.

Im assuming that you dont have the air conditioning on right. Because I know they automatically run full time when the ac is on. My mechanic had a hell of a time putting my thermostat in. Took about 45 min. Good luck

No A/C is off, fans still run non-stop.

what is the mielage on your car? If high and Thermostat was never replaced, it is time sounds like.

Car has high mileage - 130,000.

when mechincal repairs been you done on the car the past 60k miles like:
1- Have u ever replaced thermostat?
2- Flushed radiator?
3- Replaced Water pump?

....etc jst to get a history of the car?


Thanks

Bought the car with 110,00 miles. I've flushed and drained radiator, changed oil and other normal maintenance. Does anyone know if changing the thermostat is an easy job? Thanks.

Unplug the Cooling Fan sending unit aka 212 switch @the Engine and see if the fans stop..if so the switch is defective....seen several do this

Thanks Mark Cummins. Is that the part right in front of the radiator with the electrical connections? I thought that was the thermostat or are they both together? Thanks.

I am not sure on the benz, but it may have a relay in the fuse panel for the aux fan. Sounds like a relay stuck. I think the fan on the w210 is triggered by the same temp sensor that gives the computer information. I would think you would see other problems if that sensor was bad.

Wow. Sensors, switches, relays. Where to start - I might just leave this up to my mechanic to figure out.