3 pin coolant temperature sensor questions

[John5788]

I was looking through an online retailer parts listings for a coolant temperature sensor and I found this 3 pin sensor I've never seen (was looking at a 1986 300E):

Part number: 006 545 42 24


I tried looking in the manuals, but I can't find information on this sensor. Does this one sensor serve the purpose of the blue (aux fan switch) AND the single pin (dash gauge) sensors?

Part number: 008 542 45 17


Part number: 005 542 10 17




I'm trying to find a way to get an additional device reading the engine temperatures without throwing off the readings on the existing thermistors. If I can combine the blue 2 pin and black 1 pin sensors into a single sensor, this frees up a port at my water pump housing that I can use to install another sensor for a completely separate device to read from.

[Mxfrank]

You may be confusing thermoswitches and sensors. The three pin device is a thermoswitch, the two paths are either open or closed, depending on temperature. The black one pin device is a sensor, it contains a thermistor which has variable resistance based on temperature. I may be wrong, but I believe the two pin device in your photos is also a thermistor, which supplies a temperature reading to the climate control unit, which in turn controls the fan.

On those cars where the red switch is used, it serves the purpose of completing two circuits. The first circuit will close when engine temperature reaches about 100C. It engages the electric fan at high speed, bypassing the resistor. The second circuit engages at around 105C, and turns off the A/C compressor. There are a couple of versions of this switch, which engage at different ranges.

If you're looking to add a device, there are a series of 22mm bungs on the left side of the head. A couple of them are probably in use, either for sensors or for heater return water. Remove one of the unused plugs, and you can install a 22mm fan switch from a VW, Jaguar, or Porsche. Mercedes uses 22mm to 16mm adapters to install sensors in these bungs, that's another option.

[optimusprime]

John you look across the top of the block you should see an extra threaded brass plug you can remove to fit the new sensor in to. I use it to bleed the coolant system after a change And the the first sensor in pic is the electric fan clutch
Second is a coolant sensor ACC
Third is just a coolant sensor .

[John5788]

Originally Posted by Mxfrank

You may be confusing thermoswitches and sensors. The three pin device is a thermoswitch, the two paths are either open or closed, depending on temperature. The black one pin device is a sensor, it contains a thermistor which has variable resistance based on temperature. I may be wrong, but I believe the two pin device in your photos is also a thermistor, which supplies a temperature reading to the climate control unit, which in turn controls the fan.

On those cars where the red switch is used, it serves the purpose of completing two circuits. The first circuit will close when engine temperature reaches about 100C. It engages the electric fan at high speed, bypassing the resistor. The second circuit engages at around 105C, and turns off the A/C compressor. There are a couple of versions of this switch, which engage at different ranges.

I understand that the blue 2 pin sensor I showed is a thermoswitch. It turns the aux fans at 130C. So you are saying that the red sensor is a thermoswitch for 2 separate circuits then. This doesn't fit the criteria for what I'm trying to do then.

Originally Posted by optimusprime

John you look across the top of the block you should see an extra threaded brass plug you can remove to fit the new sensor in to. I use it to bleed the coolant system after a change And the the first sensor in pic is the electric fan clutch
Second is a coolant sensor ACC
Third is just a coolant sensor .

The engine I'm working with on this is the CIS M104. It doesn't share the same luxury as the M103 with spare plugs to remove.



I was hoping to consolidate the blue 2 pin and black single pin sensors into 1 hole with the red sensor, leaving me with a spare hole to add an additional sensor for something else.

[John5788]

I searched on this forum for "3 pin coolant temperature sensor" and found this post:1992 190E 2.3 Temp/Fan Switch/Sensor Questions

Originally Posted by BenzEnthusiast

I know that the black top 3 pin switch controls the clutch fan with 2 of the pins on it and the 3rd pin is for the temperature gauge. I know that it's for the temp. gauge because when I unplugged it the needle on the gauge drops to the bottom.

But then this poster says something else: 603 coolant temperature sensor location

Originally Posted by kelley312c

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...

Another confused poster: Runs on 3 cylinders(w124).

Mbdoc: 1992 Temp Gauge - 190E

Originally Posted by mbdoc

The 1991-93 2.3 uses what looks like an aux fan switch on earlier cars.

It is a 3 pole black sensor in the head,

the single pole is for the temp gauge

Maybe I'll just buy the sensor and try it out, it's not that expensive. I can't seem to find testing procedures for a 3 pin sensor in the manuals, only 2 pin and 4 pin sensors.

[John5788]

OK guys, got what I was looking for. Both halves of the story are right, the 3 pin sensor is a dual thermoswitch AND it is also a thermoswitch/thermistor, depending on which part number you buy:

006 545 91 24


This part is exactly what I'm looking for. It acts as a thermoswitch and thermistor. I can collapse 2 sensors into 1 body and free up a port on the water pump housing.

006 545 42 24 and 006 545 45 24


These two, red and green 3 pin, are both controlling 2 circuits and is a dual thermoswitch.

Same goes for the gray switch

006 545 61 24


This explains much of the confusion I was seeing across this forum. The crummy part is, we are accustomed to the different insulator colors being different temperature values, when in this particular case, the black insulator body sensor is a completely different functional unit.