What is this sensor?
 

http://mediaservice.photoisland.com/...4106457740.jpg
On my '86 300E it's between #4 & #6 cylinders.
http://mediaservice.photoisland.com/...8066805181.jpg
On a warmed up engine there is infinite resistance between the terminals and between ground. I was testing the coolant sensor to diagnose a high idle speed and thought I would check this too. What is it for?

aux fan / temp gauge
 

looks to me like the sensor for the aux fan (at high temp, normally 110 deg C as stamped on the copper part of casing), switches a relay that lets the aux fan kicks in + the temp gauge's output (single pin on the device).

I'm basing this on what I have on my W124 200E 4 102 engine ...

The 2 pins side by side are to turn on the electric aux fan, the single pin is an overheating signal to the "klima" relay.

The "klima" relay that MBDOC alluded to is the AC cutoff switch to prevent overheating (I believe).

So this sensor has dual protection -- kicks in high speed aux fans and cuts out the AC compressor to prevent engine overheating.

~Paul

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'91 300E, 208k miles