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Hi folks! I recently inherited a 1985 500SL. The cooling fan is not running, resulting in numerous problems, including the engine overheating, loss of power, and no air conditioning. I've tested the fan by connecting directly to the battery with jumper cables and the fan works just fine. This leads me to believe it is the relay. I found the location of the relays above the fuse panel, but I don't know how to determine which relay it is. Also, when I find it, what is the best method for testing continuity? I appreciate anyone's advice, and I hope I can be of some help to someone in the future on this board!
 

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1983 380 SL
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The relay is not the only thing that can cause the condition. There is a temperature sensor on or near the thermostat housing (at least it is on my 380) there is also a fuse you can check.

Good luck.
 

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1989 560SL
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Hi folks! I recently inherited a 1985 500SL. The cooling fan is not running, resulting in numerous problems, including the engine overheating, loss of power, and no air conditioning. I've tested the fan by connecting directly to the battery with jumper cables and the fan works just fine. This leads me to believe it is the relay. I found the location of the relays above the fuse panel, but I don't know how to determine which relay it is. Also, when I find it, what is the best method for testing continuity? I appreciate anyone's advice, and I hope I can be of some help to someone in the future on this board!

This photo may help some. I’m not certain what year it is from so consider that it may not be exactly right.
Usually, an A/C pressure switch will turn the fan on at a lower speed via a resistor block in the circuit and an engine temp switch (100°C) will turn the fan on at full speed.

https://www.benzworld.org/forums/attachment.php?attachmentid=311482&stc=1&d=1275507185



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1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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1989 560SL
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That picture is from an '86 560SL. Different.

Here is the info from an 85 380SL. That should be the same.
With relay #6 removed (lower left as you face the fuses and relays) you should not have ground on pin #5(terminal 85). If you do, then either the receiver drier switch or the 100°C switch is shorted and will need replacing. You could unplug each of them while the fan is running to see if one of them switches the fan off.





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Discussion Starter #7
To update, the cooling fan relay ended up being in the position of 3rd from the bottom on the right. I appreciate the diagram and picture though, mbsbms. I purchased a new cooling fan as the old was off balance. After replacing the relay, the fan is humming like a bird. Unfortunately, the car engine is still overheating. I realized there is no coolant in the reservoir, so I purchased genuine Mercedes Benz engine coolant (part number BQ 1 03 0004). Hopefully this is the right stuff. I will top it off, check the fuse as mentioned by Jyuma, and post with updates. Could you provide any more detail as far as the location of the fuse?
 

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I realize this is from an old post but I too am having issues with aux fan not kicking on—1981 380sl. I have changed out the aux fan, thermostat, water pump, aux relay D, and have a coolant sensor on order now (where thermostat housing is located). When the car is traveling it lowers its high temp which leads me to believe its a cooling issue in the front. Short of changing out the radiator I’m getting pretty stumped. Also, just for kicks when I was switching out the relay I noticed a fuse missing where the relays are located. I tested to see if it has current and it does, but not sure what fuse (if any, goes in that spot), see image attached circle with question mark above it. Any help is deeply appreciated.

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