W-140--Any way to test the coolant temp sensor for the a/c system - PeachParts Mercedes-Benz Forum

oldsouth · #1 12-23-2002, 12:07 AM

Is there any way to test the coolant sensor that controls the fans and does it have something to do with the a/c - heat system. All of a sudden my electric fans are running all the time and occasionally the little motor that runs the coolant for the residual heat function runs without pushing the residual heat button when leaving the car. Strange electrical gremlens.

mbdoc · #2 12-23-2002, 08:06 AM

The aux water pump is designed to run after the car is off & the engine temp is HIGH to allow the engine to cool down properly.
The BLUE 2 pin temp sensor at the front of the engine sends info to the A/C controller & the controller engages the aux fans.
The sensor isn't an on/off sensor but is a variable resistor that calls for different things at different temps.

oldsouth · #3 12-23-2002, 11:53 AM

Does the same sensor control both the fans and the aux pump. When the fans run all the time when the engine is on and when cut off the aux pump starts running when you shut the engine off AND the engine is not hot, what is the problem. It will do this in the morning at first start when I know the engine is cold.

Arthur Dalton · #4 12-23-2002, 01:16 PM

If the sensor is open [ check w/ohm meter] , the fans will run all the time..regardless of temp..
Same is true if it is unplugged.
When either of these condition arise, the computer defaults to ON fan mode....
I have a sensor ohm/temp reference chart, if needed...
The resistance goes down as the temp goes up... this is called NTC thermistor [ Neg temp] , so an ohms reading at gauge temp
camparison to chart will tell if the sensor is OK and in calibration...

oldsouth · #5 12-23-2002, 01:37 PM

Ok, I went out and got into the test mode. The number 6 which is the coolant temperature sensor reading is all over the place. It continually changes in wide swings, say 20-30 degrees. Not constant. Fans run all of the time, or cut on and cut off to immediately cut back on. When I unplug the connector to the blue coolant temperature sensor, the fans cut OFF and work properly. They only cut on when the refrigerant pressure reaches a little over 16 bar. I have no way to check if they come on at high engine coolant because it is cold here and would have to do a high speed run, if even that would get the coolant temp up high enough. But I guess there is no backup sensor for the fans and coolant temp.

Arthur, If you would send me the chart you are talking about. I am pretty sure I need a new sensor. I will go check the ohms now and post back. oldsouth@direcpc.com

oldsouth · #6 12-23-2002, 01:42 PM

No ohms reading at all. Open circuit.

I also could not get into reading the DTCs as K6JRF detailed. Would only go into the actual sensor values mode.

Arthur Dalton · #7 12-23-2002, 01:48 PM

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That is the problem..
That sensor feeds the control unit, so all funtions [including pump], will be effected by the sensor.[ see MB Doc post]
I will hold up on the charts as you have found a bad sensor.

oldsouth · #8 12-25-2002, 12:56 AM

Yep, new sensor fixed the problem. All is well with the 350 now. Merry Christmas all.

h_shek · #9 12-26-2002, 06:30 AM

hi there..

how did you check the ohm rating from the sensor? Did you unplug the connection and test the 2 pins with an ohm meter? Any direction you can give as to testing the sensor?

Thanks

oldsouth · #10 12-26-2002, 12:09 PM

Yes, that is how I tested it. The problem I was having is that the fans were running all the time and the REST water pump was also running. You can go into the diagostic mode on the ac and number 6 tells you what the sensor is reading. My reading was all over the place.