JNT,
Thanks for the info.

As I said earlier in this thread, I read some threads referencing the coolant temp sensor and the EHA. I adjusted my EHA 1/4 turn CW and that made it idle worse, I then adjusted the EHA 1/4 turn CCW and it is idling fine again. However, it is still running rich since my plugs are fouling, I have black smoke when I gun the engine and I'm leaving black soot on the driveway. I've spent a great amount of time reading the MB shop manual CD today, section 07.3-0121 (testing electrical components of KE injection system). Page 59 references the 4 pin connector that contains the temp sensor input. Page 60 references the volt and resistance values for a properly working temp sensor and page 64 references test setup of 4 pin sensor that seems to contradict information on page 59 since there is a vacant pin referenced on page 59 which would make a measurment setup like that on page 64 worthless. So I'm back to the original question.

Does anyone know definitively how to test the coolant temp sensor?

Thanks,

Bob

The 4 pin B11/2 sensor is actually 2 sensors in one case .. They should both read the same ..they are internally wired diagonally opposed , so any diag set of pins gets you one resistor measure. That would be pins 1/3 or 2/4. You are looking for 2.5K ohms at room temp [ 20C] or 325 ohms at coolant running temp of 80C.
This is a thermistor , so that is why there is a temp/ohms variable ..decreasing resistance as temp increases [ Neg Coefficent Thermistor,,, NCT].
That is if you have the 4 pin...these two sensors are electically isolated from each other, so you can not get a double resistance value from this 4 pin sensor ,,

If you have the two pin , [ which you do, if I am reading your post correctly]then each resistor is internally wired from one pin to ground, so each will be the same if you test across each pin to ground individually. If you just go across the two pins , you will get double the resistance b/c each resistor shares the same ground, so you would then be actually reading two resistors in series across the two pins. [ R1 + R2 ..ie....double the resistance of each individual resistor.]
Your chassi can have either sensor , depending on chassis #..
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU2241BH3G9&year=1987&make=MB&model=300-E-002&category=C&part=Water+Temp.+Sensor&appEngines=_any

Arthur,

Thanks so much. You read correctly, I have the 2 post sensor. Are there 2 circuits using this sensor both reading a resistance to ground? Or is there only one circuit reading the resistance across the 2 posts?

Thanks,

Bob

Two different circuits..it is actually 2 seperate sensors , but they share the same physical case ....just picture/draw a 'Y" with the bottom leg being grounded and each of the upper legs being a resistor. That is what the internals look like on a wire schematic of the sensor. So, you can see that from the tip of one leg to ground you have one resistor, but if you go across the tips of the upper legs, you are measuring both resistors in series
[ resulting in R1 + R2 for value.]
But b/c each has it's own circuit, that 2R value is never used by any inputs....
Get it ??????
I can tell from your reply that you do, so.....just FYI , et al...........
The ONLY difference between the 4 pin and 2 pin sensors for this chassis is the 2 pin uses the sensors case for ground of the two resistors to complete each circuit to ground.
Whereas , the 4 pin does not use a ground, but rather has each resistor individually wired across 2 pins, making each resistor completely seperate from one another electrically..they still share the same phyical casing, but that's all. The circuits on 4 pin are completed by power going in one pin, through the resistor , and back out the other pin to complete the circuit.
You can not get a R1+R2 value reading on this sensor b/c they are not joined electrically. This chage was made b/c there was an electrical design change made in the chassis that required isolation of the sensors ..... both styles still have the same values and they both have 2 sensors in the single case.....it is just how they are wired back to the the controls that is different. The 4 pin has IN/OUT connections/pins, the 2 pin has IN only, getting the completion of the circuit to ground through the case [ thus needing no OUT terminal]
In other words , if you unscrewed the 2 pin from the engine , eliminating the ground, their would be an OPEN sensor circuit...but if you did the same on the 4 pin, you would still have a completed circuit...

Ok,I don't want you to get mad,but do you know that when you changed the EHA setting you have to change the mixture too?You said it ran rougher so you put it back as it was! When you lean the EHA you need to reset the mix control, by going just a small turn of the three m/m Allen to blend the mixture. So when it got rougher running you turn the mix (allen 3m/m) c/w,let it set to level off.JNT