K75 Starting Problem  

By Rob Lentini

     Modern motorcycle “brains”, the electronic control unit, are pretty smart.  For example a temperature sensor located in a strategic location on the engine will send a signal to the brain telling it if the engine is cold or at running temperature.  The brain will then adjust the fuel injectors to enrichen or lean the fuel mixture accordingly.  A poor running motorcycle just may have a faulty temperature sensor, and as Rob Lentini, BMW Guru, so eloquently puts it “The [BMW] fuel injection is programmed for coolant temp, and the engine will run like crap if it doesn't get a proper signal.”  Using a K75 as the mule here is how Rob recommends you troubleshoot a coolant temperature sensor.

     The sensor is located on the top front of the engine, just aft of the cam chain cover. It's mounted on a standpipe, which accepts the radiator hose coming from the left side of the radiator to the engine. There is a three-wire connector attached to the sensor. The sensor itself is a negative temperature coefficient design, meaning that as coolant temperature increases, resistance of the sensor decreases.

     This particular sensor has two elements - one inputs to the fuel injection computer, the other to the coolant over-temp light and fan. Both elements have the same temperature/resistance curve.

     First check for continuity from the temp sensor connector to the fuel injection computer connector.  You will need a wiring diagram to find the correct pin at the computer. DO NOT attempt to measure through the computer; you could destroy it.  If continuity is good, measure resistance through the sensor. With the engine cold you should have several K ohms through both circuits in the sensor. If you don't, the sensor is probably defective.

     Drain the coolant, remove the sensor, and recheck it at various temperatures by heating the sensor in water on the family stove and by freezing it in the freezer.  At high temps you should measure only a couple of hundred ohms, at freezing temps several K ohms.  Ask your local dealer for a copy of their temperature/resistance graph to compare your readings; Iron Horse made a copy for me.

     Be sure the sensor is bad; new ones cost about $40! If it is bad, it should be reading low resistance at cold temperatures. This would tell the computer that the engine is warm, and needs a leaner fuel/air mix--just the opposite of actual requirements.

     The next time your motorcycle begins to run rough and your gas mileage swings wildly, might be a good idea to check that coolant temperature sensor.  A sweet running bike is such a joy to ride. 

     Thanks to Rob Lentini for letting SEAT publish his temperature sensor troubleshooting article.  More can be found at the Internet BMW Riders web site (www.ibmwr.org).


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