My 89 K100RS has been in a no start, poor enrich running condition. Upon cleaning up the water temperature sensor, the engine now is in a hard starting condition and the engine runs poorly in an enrich running position.

My question are these:

Can the water temperature sensor start going bad but still allow the engine to be started? Can a bad water temperature sensor cause a poor, enrich running, engine? When I disconnect the water temperature sensor harness while the engine is running the engine straiten up and runs like it should. I do not really understand the relationship of the temperature sensor to the Fuel Injection Control Module except that it passes a ground to Pin #10 on the Fuel Injection Control Module. Can I have a good water temperature sensor and a bad a Fuel Injection Control Module? The engine will not start if the sensor wiring harness is disconnected from sensor.

I attempted to ohm the sensor but did not understand the ohm results. I have a new sensor on order but I am wondering if I can have other problems causing my hard starting, poor, enrich running engine on my K100RS.

Any comments, suggestions, advice or recommendations are welcome and appreciated.

Bill Hutchens
San Antonio, Texas
03/16/2018

Yes it surely can. I have seen it on many cars and bikes. They don't actually stop but just give the ECU bad info.

often the sensor connections (wiring side as well ) can be corroded  get some deoxit ???

also the threaded relief into the water stub can become corroded so that a higher than normal resistance between the sensor body and the water stub threaded section .


measure from the gearbox connection point (to battery ) and the body of the temperature sensor  meter on low ohms , it should measure  below 1 ohm  if not  remove the sensor and clean it and the threads up

I installed an additional washer (copper , soldered  so that is presents a lead- tin surface ) between the sensor and the water stub body , I figured the aluminium washer that I removed was so corroded that some type of electrolysis had set up with the dissimilar metals and location


good luck

The temperature sensor provides the enrichmt function that used to be done by the choke.

When the engine is cold it has a resistance of around  2500 ohms.  At normal operating temperature it drops to a round 250 ohms.

A dirty connection will make the engine run like the choke is stuck.