300SD fan question

My 83 300 SD seems to be running a bit hot and I suspect the electric fan is not functioning.
I assume that the electric fan is controlled by a thermostat, where is the
thermostat located?
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I'm not familiar with that particular model, but can give you some info of how it works on my 1980 300SD, which may be helpful.
First, the aux fan only comes on rarely, during extreme conditions. An example would be after hard driving in 85F ambiant temps, and then suddenly coming to a stop and idling. Or being stuck in bumper to bumper traffic in hot weather. So, if yoiu have an overheating during more normal driving, it's not the aux fan that's the primary problem.
On mine, the fan is controlled by a relay. The relay in turn is driven by either the coolant temp goiing above a certain point or by the AC refrigerant temp going above a certain point. When I've had fan problems, it was either a bad connection where the fan motor connects to the harness or the fan motor was shot. The connection is after about 2 ft of wire from the motor, which puts it in a bad location to get road salt, etc.
Here's a simple test which works on my model and probably on yours too. I never needed to locate the coolant temp turn-on for mine, so I don't know where it is. However the AC overtemp thermo switch is located on the AC receiver/dryer by the radiator. There are 2 sensors on it. One is to detect low AC pressure, indicating loss of refrigerant and to prevent the compressor from coming on. The other is the high temp turn on for the fan. Assuming the AC refrigerant system is working OK, you can temporarily put jumpers across both sensors. Then, with the AC on and the car running, the aux fan should be running. If it's not, then you know you at least have other problems, eg connection, relay, etc. besides the engine coolant turn on sensor. There is also a fuse involved, so I would make sure to check them as well. Mine is located in the main fuse box under the hood.
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The auxilary fan is what you are talking about... in front of the radiator. First, make sure that fan works by providing power directly the the unit. The easy way to do this is by the AC dryer, there is a red or green switch attached to the dryer, touch those two wires together and the aux fan should come on... if not, remove fuse and check condition.
Secondly, that circuit is complex and is not just a thermoswitch that triggers it. If you turn on your AC, it should come on immediatly. If not, it is most likely you do not have enough refrigerant to trigger it. Too little refrigerant will not trigger the aux fan.
The last thermoswitch is on the engine... should be two prongs blue color... by pulling this out, fan should kick on high speed or simply on. This depends on what car... some has two speed and earlier one like yours are simply one speed.
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