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Old 04-05-2001, 11:04 PM
oldsouth's Avatar
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I have a 1995 S-350 diesel. The car has an 80 degree C. thermostat in it. From when the car was new until last summer the two electric fans in front of the radiator would come on only on high speed. The engine temperature would rarely climb over 100 C, even in the hot summer. (In the winter it runs just over 80 C) Last summer the dealer told me the car is supposed to have a resistor that would allow the fans to come on at a low speed first then as the temperature rises would go to high speed. He said my resistor was defective and replaced it. Well, after that the car runs much hotter in the summer. It runs about 100 C and has to get over that before they ever run on high speed. Is this normal and good for the engine to run at this high of a temperature before the fans go to high speed?

136,000 miles and climbing
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Old 04-05-2001, 11:13 PM
David C Klasse's Avatar
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DO a search on this. It is all normal!!
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Old 04-06-2001, 09:38 AM
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Though it may not be comforting to run the engine consistently where the coolant temps exceed 100 degrees C, it is not a problem for a properly tuned vehicle. The red zone is at 120C for a reason.

That said, given the history of head problems with the OM603.97 engine, I would be just as interested as you in keeping coolant temps under control. The fact that the problem worsened following a supposed fix by the dealer necessitates a return trip.

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Old 04-06-2001, 11:29 AM
oldsouth's Avatar
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Thanks for the imput. I did a search and found that this is normal. I am still considering taking out the resistor where the fans will come on high speed when they would normally turn on low speed. That will keep the temp down like it was before. Makes more noise but I barely can hear it inside the car anyway. I think the low speed is controlled by the ac head pressure AND engine temperature, but what temp I don't know. I never know when the fans turn on low. High speed comes on about 110C.
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Old 04-06-2001, 11:35 AM
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All our MBs have the same temperament.
... Kerry

126 tailed by a 203, 129 leading the pack.
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Old 04-06-2001, 12:34 PM
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Location: Florida / N.H.
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I believe you will find that the aux. fans have a dual funtion/speed that works like this:

Low speed-controlled by the high pressure the reciever/drier of a/c. The purpose is to aid the condensor at extreme high side readings. The resistor is how the low speed is attained.

High speed is controlled by the aux fan coolant temp sensor
and cuts in around 105C. This funtion is for engine temp
cooling assist and is independent of a/c system.
Different relays for each system.

May vary from model to model, but this is the basics.
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