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Been having problems starting and idling till engine is warmed up. went thru plugs wires vacuum lines, etc. and have read thru the forum and have come to the engine temp sensor as the next thing to try changing, but the one I find online to order is a 3 pin and mine is a 4 pin. The car is a 1990 300e 2.6. any suggestions on this?
 

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About a dozen 1988, 1989, 1990, and 1991 sedans, wagons, 4Matics and 1 coupe
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I assume that you mean engine COOLANT temp sensor. Where are you looking online? Doing a google search for the part? Go straight to one of the online suppliers that specialize in MBZ parts. I use Rusty.
Went there 30 seconds ago and found the 4-pin sensor that you may need for about $30. Your choice of two brands even. You do need to know your engine number which almost certainly has a 12 prefix indicating that it was mated to an automatic transmission.
It does sound like the ECT is bad on your car but there are other possibilities. The fuel pump relay controls cold start injector operation and there is the cold start injector itself. The ECT is easily tested with an ohm meter. Measure resistance between terminals 2&4, then between 1&3. At any given engine temp the resistance values measured between those two pairs of terminals should be the same or close to it. That's a first step.
 
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