The needle on the Coolant temperature gauge on my newly acquired K1100LT [1997] jumps off the scale when the ignition is switched on even though the engine is stone cold.  It will return when tapped after ignition switched off.   How can I determine where the fault is - sensor or gauge or in between?  Any helpful comment appreciated by this new member.

Have a shot at testing the sensor unit resistance. The resistance should decrease as the temperature rises. This can be measured by pulling the black wire off the sensor unit at the coolant drain location and measuring the resistance from the middle of the sending unit to the engine block. Normal reading at operating temperature should be around 40 ohms. If it's a lot higher than this, you might need a new sensor unit but not too expensive and can be changed pretty easily without losing too much coolant.CheersGarry