Hello all,

I am having a hard time determining which coolant temperature sensor on my 87 300d sends the signal to the temperature gauge on my instrument cluster....is it the three-pin sensor on the top of the cylinder head, or the single-pin on on the side of the water pump?

Dash gauge temp sensor (one wire) - Sensor is located on the side of the head, below the injectors and intake manifold, behind the injection pump. Not easy to access. Hope this help.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

That helped immensely...babymog suggested that before I replace my suspect fan clutch I should check the coolant sensor. After reading your post I took a reading with an infrared thermometer right next to the sensor and the reading was pretty spot on to my gauge. Looks like she needs a new fan clutch

I believe your car uses the 3 pin sensor for the temperature gauge & ac cutout, and the 1 pin for the engine computer. I'd take a peek at the wiring diagrams and compare wire colors to confirm.

My 91 603 uses a 2 pin sensor on the head, near the glow plugs for the glow plug system and dashboard gauge. There's another 2 pin sensor for the climate control / radiator fans, and the 1 pin sensor goes to the engine computer.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Hmmm...I unplugged the two-pin section of the three pronged sensor and nothing changed...didn't pull the single pin on it though...going to look at the fsm...I did before, but it was not straight forward for my problem

Just checked the wiring diagram and the three pronged sensor goes to the aux fan and the compressor, not to the temp gauge. While I was there I tried reading resistance from the two pins of the sensor and got nothing...does that mean my sensor is shot? This makes sense bc when my aux fan is engaged, it is only ever engaged on high, never on low...

The 2 pins is not a sensor. It is a switch. It is open circuit until it reaches temperature. The temp rating depends on the color of the sensor. It could be rated 100, 105 or 110C. When the switch is made it will turn on the AUX fan at high speed. It is the design. The slow speed is controlled by the switch at the receiver/driver ( A/C circuit ).

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Whoops! Thanks ah-Kay! Got that mixed up. I ordered the red switch, as opposed to my gray one. I think the car could benefit from having the aux fan come on sooner