SIP speedo temp readout problem

[Tractorman]

My SIP speedo temp readout started varying from normal to higher readings then back. I checked the connections, all OK so I changed the sensor and it was fine for around 20 miles but now same fault. Any ideas anyone. The sensor is mounted on the plug and been fine for thousands of miles.

[Fast n Furious]

If the readout remains stable on a cold engine then it is unlikely to be a faulty clock or faulty thermocouple. Most likely then, vibration from a running engine is contributing somehow, either infecting the clock, or the wiring from the thermocouple sensor to the clock.

The best way I've found to generate a safe constant hight temperature to the thermo sensor is to attach the business end of a soldering iron to the sensor using a wrap of copper wire temporarily. This will give you a fixed high temperture for the clock. If all seems stable, start and rev the engine a bit and turn the headset from side to side regularly, then see if the reading becomes erratic. If so, I would suspect the integrity of the wiring from the sensor to the clock. Check for chaffing along the frame. The action of steering can cause wiring continuity issues in the headset as this where most wiring movement occurs.
Hope this helps.

[Tractorman]

Thanks for your comprehensive reply but I sorted it out some time ago. It was the sensor wire, it chafed through by the headset movement and was shorting out to earth.