Am I right the the sensor is effectively a pulse generator and the head contains a counter ??
I'm assuming your K100 is stock. The speed sensor is located on the top of the final drive. It is a variable reluctance (VR) sensor that detects the gear rotating inside the drive. It outputs a low voltage (~0.5V) sine wave that has six peaks per rear wheel revolution. The signal is fed into the instrument cluster where the circuitry inside amplifies the signal and then counts the pulses to indicate the speed.
There is a connector under the right battery cover, at the back of the cluster and within the cluster.Typically, poor connections somewhere in the path causes the signal to degrade in quality and the speedo to respond erratically. Your investigation should concentrate on finding the poor connection and eliminating it, by making sure all connections are clean and secure.
- remove the speed sensor and make sure it is clean (sometime iron filings gather on it)
- locate the connection under the right battery cover and clean/reseat it
- reseat the instrument cluster connector a few times to help remove any corrosion
- if these actions fail to clear up the problem, you will need to open the gauge cluster to locate the amplifier card and clean its contacts
Before I go digging I also want to have a look at the fuel lights, of which the red one stays on (I know you all think i am too fussy).
I was going to start to delve into checking probes for resistance, but some research in advance tells me my bike should have a float switch.
Your 86 has a float sensor inside the tank that has a variable resistor to indicate fuel level and a switch to turn on the low fuel light. If the light is always on, then somehow that circuit is shorting to ground. The low fuel light circuit has a connection at the tank, and at the instrument cluster. You will need to empty the tank of gas to service the sensor because it will need to be removed.
- separate the tank connector and test resistance between the white wire and brown wire on the tank-side connector. It should be open circuit with the float raised, and closed circuit with the float depressed. If it always shows closed circuit, the problem lies in the sensor.
- Remove the connector from the rear of the instrument cluster. Test for resistance between the white wire on the harness side of the tank connector and ground. If it always shows closed circuit, the problem lies in the wiring harness somewhere.
