Have a problem with the temperature guage on 1990 K100 LT. When i turn the engine on cold (or any time) the needle jumps to the red and stays there. I've changed the temp sensor, relay, and the guage itself. It still does it. Any ideas? What am i missing?

i think the temp sensor is a double ended unit and goes to the efi computer and the temp sensing unit---thingy ..... i would expect the output from the temp sensor thingy drives the warning lamps and fan .

? does the temp meter sit on the incoming signal side ....or the out going side or the temp switch unit ?

would be good to find out huh ?

As you know from changing a few things it's a single wire from the oil/water pump area where the sensor lives and runs up through the timing cover and below the radiator and up the right hand side of the bike to the harness (it's not the same sensor as that which runs the fan, Charlie)...any chances the wire is pinched somewhere along its path and is 'earthed' or a connector is damaged?

I would say that looking for a chafed wire miking an earth or something similar is a good bet. The engine temp sensor for the ECU doesn't run that gauge till the K1100 came along. It could be the same on late 16v RS as well but I am not sure on that.

Or water in the rubber boot over the sensor?? An earth fault will give full scale. Start by measuring the connector under the tank of the sensor to earth.

read my mind rene ..........thanks twb .....im overworked and "loosing it" brother ......

hope you find a solution roamer ...these guys above are awesome

Good stuff. Looks like i'm going to have to tear down the faring and tank again to find the short. I was hoping for a quick fix so wouldn't have to do that. I have cancer (and getting older) and the drugs make me goofy so i have a hard time remembering how to put the hardware back! I could ignore it but things like that drive me nuts. I have a wiring diagram for the motorcycle but not to the fuel and temp guage. Isn't there a rheostat somewhere in line with the guage or is it in the guage itself? I though i saw somewhere on the internet, a wiring diagram for the cooling system and the guage. Thank you for your help. I agree that a short makes the most sense. Just wish I had a genie to show me right where it's at.

I'd be looking behind the 5-screw ignition cover for a pinch or chafe, then, finding it all good there, following that single wire behind the radiator up to its single connector buried on the right hand side upper frame rail. It's in parallel with the oil pressure sender's wire and perhaps follows the same path as the ignition wires (from the pick-ups) too. You should only have to tear off the right hand fairing panel and possibly lift the tank to access and observe the wire's path. Have a bright mini-torch (flashlight) in your teeth and some extra-magnified glasses on and unless it's buried further into the main harness bundle you'll be golden. Good luck with it.

First have a look that there's no water in the little rubber booth over the connection on the sensor.
There is no further resistance or reostat or similar in the system. Most gauges are set up to work with the sensor. There are normally two types of gauges 40-120 C or 50-150 C. 40-120 normally used for coolingwater and 50-150 for oil. Normally the BMW sensor gives 50 Ohm at 90 degrees, which corresponds to 90 degrees indication on MotoMeter and VDO gauges (with the 40-120 range).

I used a VDO gauge with the BMW sensor and the indication is spot-on.

Thank you all for your suggestions. I'll get to work on it and let you all know what I find. Knowledge is wealth and hopefully what I find will help the next person that runs into this.

How time flys. I stripped this baby while tracing wires and cleaned up the connections as i went. At the end of my travels i found a "second" temperature sensor. I had changed the sensor on the block behind the radiator but it made no difference. The guage still pegged hot when i started the machine and stayed there. The Haynes manual didn't show a second temp sensor but mentioned one on the cooling fan (or close by.) I have the original manual for the bike and the wiring diagram didn't show another sensor as well. Well to shorten this up, i traced the black wire all around the bike to the sensor on the right side of the bike (facing), right next to the oil sensor. It looked right so i put a wrench on it and it sheered off at the block. (A real good sign it was not well.) For $22 which is amazingly cheap for BMW, i installed it and problem solved. I'm sure if i hadn't been taking all the pain and cancer drugs i may have caught it sooner, but the manuals and wiring diagrams were useless. Thank you all for pointing me in the right direction.

good work .....yay!!!!

Well done an excellent result and good on you for perservering.
Regards,
K-BIKE