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Old 04-08-2002, 02:49 PM
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Resistance specs for Coolant Temp Sensor?

My 89 300E (127K) has recently lost its "choke" when cold. Starts right up and immediately the idle drops to 400 rpm +/- and is very rough. If I hold the accelerator to 1500 rpm for 30 seconds while it warms, it will idle and drive just fine.

I'd like to check my coolant temp sensor but cannot find the resistant specs in my UK version Haynes manual. Can someone please post the specs? I'm assuming this sensor is the culprit and I believe it's the green two lead sensor closest to the firewall on the cyl head. Pls correct me if I'm wrong. FYI....OVP, wires, plugs, dist, rotor, fuel filter, coil, etc., have all been replaced recently.

I also believe there's another sensor (ignition?) located nearby but I've heard that if it fails the car just simply won't run. If you believe that this one is worth testing, please post the specs also (if you have them) it the next two prong sensor moving toward the front of the engnie (there's a small one prong sensor in between them)?

Many thanks.
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Old 04-08-2002, 03:45 PM
Registered User
Join Date: Dec 2001
Location: Yorkshire England
Posts: 70
Hi Mike
I recently posted a thread about this.
I had similar symptoms and found a bad connection on the sensor.
If your 300e's sensor is the same as my E200, and I think it is,
then it should be 325 Ohms @ 80degrees C
1200 Ohms @ 40C, and
6k Ohms@0C.
Mine went totally open circuit, infinity Ohms, led the ECU to think the coolant water was minus God-knows-what, and went to full rich. As soon as the lambda probe caught up, its sorted things out.
So it started fine from cold, but hot starting was really difficult.

Hope this helps

Good luck
E200 1994 108k miles
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Old 04-08-2002, 05:44 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Also look for 3.0K at 15C. That would be a normal cool weather running value
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