Hello ,
Where I can find the resistance values of the temperature sensor mounted on the water pump ( for additional instruments - temp gauge ) ? I retrofited a temp and petrol gauge on my 88 RS , the petrol gauge works perfect but the temp gauge jumps on red as soon as I switch the key in ignition lock. I checked the wiring , all seems to be ok ... The gauge I think is ok and the sensor faulty but I don't know how to confirm that . The parts were taken from a dismantled K and the seller states that the gauge worked without problems.
Thanks

For the sensor to make the needle to go straight to red that's the same as having a short circuit.

Check for any resistance in the sensor at air temp then drop it into water and you should see a reduction in resistance.

Sorry I can't give figures but that would mean taking mine out again!

If you have an M10 bolt about you can remove the sensor without loosing barely any coolant just keep your thumb over the hole when as soon as the sensor is out.

I recall someone here, it may have been Charlie99, saying that the VDO sensors are compatible with the motometer gauges so you may be able to get info on them or use one as a replacement.

ok, thank you , I will check the resistance of the wiring and sensor after removing the sensor from the bike. Regarding VDO sensors , no luck yet finding something equivalent .

See if the gauge stays at Cold if you turn on the ignition with the wire disconnected from the sensor.
If it still pegs, you may have a shorted place in the wire between the dash gauge and the sensor.

With (only ) the wire from the sensor disconected, the temp gauge stays on blue/cold. If I connect directly a wire fom the sensor to the gauge , the gauge jumps directly to red. Same when I connect the temperature pin on the gauge directly to the frame. My guess is the temp sensor is shorted....

You are correct. You will need to remove the sensor for tests and/or replacement. Above, someone mentioned the correct size bolt you can quickly put in the hole to keep the coolant in so you'll only loose a few drops while the sensor is out.
Once out, the sensor may actually have some (tiny) markings stamped into the metal that could give you a clue as to the resistance range it is supposed to function in, I'm not sure.
At any rate using an ohm meter if you have one, the resistance from the wire contact on the gauge to the threaded area should be high when its cold and get lower as it heats up.....you could attach a meter and watch it get higher in value as you warm it with a hair dryer for a general test.
We are quite sure however from your preliminaries that it's no good. I'm sorry I don't have the information to tell you exactly what the range is. So I'm not really helping answer your original question.
Here is a general spec I found for a VDO gauge:
Temperature Sender: Cold - 700 ohms
Hot (250 degrees) - 22 ohms
If you were really "crafty" you might find a little radio potentiometer connect it in series between the wire & ground. At the same time connect the ohm meter across the same two legs of the potentiometer........turn the pot. all the way one way then all the way the other way & watch the gauge. When the gauge reads "just" full cold, record the reading. then turn the pot' till the gauge just reaches the top. Record the reading again. Those 2 readings should give you the range you are looking for??????
Then with that info....you might go shopping for a proper sensor.
If we know the "brand" on the gauge, we might be able to narrow it down on the internet as well.

Thank you all for taking time answering to my questions. I will replace the sensor for further testing and most likely replacement. I am not sure if I will use a non OEM new sensor , I had a bad experience with a VALEO equivalent ( instead of BOSCH ) water temp sensor ( the one that feeds the temp signal to the motronic ). The only problem is that I have to wait 2 weeks for any BMW dealer order to be delivered since there are no parts for our old bikes at my dealer and all those parts are delivered directly from Germany. The temp gauge is OEM BMW ( Motometer ) . I will take measurements at different temperatures of the new sensor after I have it, maybe this will be of use for someone else here some day .

ibjman wrote:...the sensor may actually have some (tiny) markings stamped into the metal...

hi folks, got a replacement Bosch temp sensor but its got a different number stamped on it...the original one has 951 on it & replacement one is 742...will this have completely different resistance  or is it a K100 one?
thnx
Mark