Electrical Question - PeachParts Mercedes-Benz Forum

bio350 · #1 12-25-2008, 08:25 PM

When I did my engine swap (a 3.0 for a 3.5) I neglected to change out the Temperature sensor on the 3.0 for the one that was in the 3.5. The problem is that the temperature sensor on the 3.0 is a single pin, where as the 3.5 had a double pin. So, the receiver wire is a double pin receiver that goes to the actual temperature gauge. I can't get to the sensor without either removing injector lines off of the pump or the intake manifold.
So my question is this: Can I wire the single pin sensor to the double pin receiver?
I have done this much: I connected a wire from the sensor pin to one of the receiver slots and the gauge stays at the bottom. If I then add another wire to the available receiver slot and ground it, it pegs the temperature gauge all the way up.
Any way I can "jerry" rig this to avoid dismantling one side of the engine due to my lack of preplanning?
I thank you all for helping my solve my transmission problem after the engine swap. It was an adjustment to the Bowden Cable, not "reintroducing" the new engine to the old transmission as my Mercedes mechanic had suggested.

sixto · #2 12-25-2008, 09:03 PM

From what little I remember of my 140 and using my non-technical vocabulary, the reason the single pin sensor doesn't work with the 140 gauge is MB changed to a temp gauge that uses a reference voltage other than 12V. The temp gauge is calibrated to display resistance to a reference of 5V or something like that. The sensor has two pins because it doesn't use chassis ground. If your experimentation hasn't burned out the temp gauge, see if you can determine the reference by checking the voltage at each connector pin. Someone who knows electronics might be able to adapt the gauge to a 12V or 13.4V reference given the gauge's reference. Could be as simple as adding a resistor in series with the sensor.

If that's unappealing, here's Rube Goldberg's advise -

Do you have the temp sensor and fitting from the old head? If not, the part numbers are:

008 542 56 17 = temp sensor
601 997 01 72 = temp sensor fitting
N 007603 022100 = crush washer between fitting and head
N 007603 014100 = crush washer between sensor and fitting

I think I have a fitting to spare. I don't have a temp sensor.

Drain coolant through the block drain, remove the cruise actuator aft of the IP, remove the plug under the #4 intake runner or whichever plus is most accessible (plug takes a 10mm hex bit), attach the sensor and fitting where the plug was, extend the gauge wiring to the new sensor location.

IIRC the same temp sensor is used by the afterglow system. This is not a tragedy. Worst case is your glow plugs burn for the full cold cycle rather than cutting early because the engine is warm.

Sixto
87 300D

Oracle12345 · #3 12-25-2008, 09:11 PM

correct refernce voltage is always 5 volts on any computer controlled system of todays cars. The thermistor sends the iinformation and by the time it reaches the instrument cluster which has voltage regulator which is a transistor on the board. Its a ground sided switch.

bio350 · #4 12-25-2008, 10:19 PM

I have the old sensor. Thanks for the suggestions. I'll see what I can rig up.

bio350 · #5 12-27-2008, 09:01 AM

An update.
I disconnected a couple of the injector lines from the IP and was able to access the temperature sensor enough to swap them out. I have thrown nothing away so I even had the original lead. All works good now. Thanks again!!!!