can you identify this part?
 

I am trying to figure out what this part is - the pale orange part in this picture with wires attached at each end. It attaches in engine compartment right behind the driver side headlight in my 1987 300E with a small metal bracket (bracket not shown). It is made of some kind of ceramic or something and mine is badly cracked and corroded at one of the connections. Some kind of resistor? I'm thinking this may have something to do with the a/c electrics?

http://images.cyclones-world.net/mercpart.JPG

Aux fan resistor!

Thanks. Next question, what is its function exactly? It resists current on the aux. (for a/c use?) fan, why exactly?

It allows current to turn on the aux fan once the engine temp reaches a certian temp. Not sure what temp, but that is the basic function.

It is used to create the low speed fan function. The wiring goes around the resistore when it wants high fan speed.

The aux fan has two circuits
One is high fan and it is triggered by the engine coolant temp sensor.
The other is low fan and it is triggered by the a/c high side pressure switch. The low speed is accomplished by having this droppiong resistor in that circuit.

Thanks for all the info. The connection by the broken off part in the picture is so corroded that the fan stays on all the time. I ordered a replacement from Fastlane, around $20. But the screws appear to be corroded onto the switch, probably going to have to drill them out.

The resistor needs replacing, but it is not the problem with the fan staying on. It is not a switch, it is just a resistor .
It drops the voltage to the fan for low speed .
If the fan is always on , I would check the switch/sensors in the fan circuit.
Does the fan run on high speed or low speed ?
That will tell you which circuit your problem is in.

I see what you're saying. I was thinking there may be something that can tell there's no current flowing on that circuit (because its is open due to the corrosion) and defaults to turning on the fan. I will have to check but the fan seems to be running pretty fast.

No.. that is just the load/feed side of the fan circuit..if the resistor opens , the fan can not run [ low fan] as it can not get any power feed..so an open resistor results in NO fan..
The default circuit would be when a sensor opens on the high fan circuit.
The controler [ N22] then goes into default so there is no chance of engine overheat due to bad sensor .
The sensor for this would be the 2 prong blue one on the engine ... this is high fan .. check it with an ohm meter and see what you have .. the default will also appear if the connector to this thermistor/sensor is loose/poor connection..
A stuck relay could also cause this, but I doubt it cuz you said you can hear the relay contacts open/close..

We actually unplug the blue sensor as a test to see if the aux fan circuits relay and wiring/fuses are good. By putting the controller into default by removing this connector, the high fan should come on....same as if the sensor was bad or not connected ...

Arthur, I think you were on the right track about why the fan is always running. Following a tip I found on the MBCA website, a few weeks ago I put a resistor inline with one of the wires on that blue engine temp sensor to fix a rough idle problem. I am thinking now that may be the cause of the fan staying on all the time (just so we're clear, I'm talking about the fan right in front of the engine, not the one in front of the a/c condenser). I am going to take the resistor out and see if that's what it is. Thanks

<<just so we're clear, I'm talking about the fan right in front of the engine, not the one in front of the a/c condenser).>>

Are you saying you have an electric clutch fan behind the radiator..??

I'm not sure if it is electric clutch or how it works. There is one fan right behind the grill in front of the ac condenser (which is in front of the radiator). That one is not running all the time. The fan that is running all the time is behind the radiator, right in front of the engine. I am virtually certain that I have the factory 1987 300E setup as there is little chance the previous owner changed the fans around.

That should just be a standard Viscous fan .. they do run all the time ..
The fans I assumed you were talking about was the Aux. Fan in front of the a/c condensor.. that is the one that is controlled by the sensors.

Huh, I never noticed it running all the time before. I thought I would remember something like that, but then again I've rarely had the hood open with the car running. The confusion is my fault because I'm did not know which was the "aux" and which was the other fan.