On my '86 300E it's between #4 & #6 cylinders.

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?

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.

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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