Temp gauge suddenly in red engine not running

[Allan NZ P5b]

1971 p5b saloon fitted with coupe gauges 10 or more years ago by me.
Just last week on last outing, noticed temperature suddenly fully in the red. I watch it carefully.
Stopped car and checked temperature fine, car fitted with a temperature sensor for electric fan and fan had not cut in, checked with temp gun and only normal temperature.
When going to restart car after goes straight to full right ( HOT) ENGINE NOT STARTED. If I disconnect lead goes back to cold.
the suggestions are

1 bad earth? not sure what this means engine earth, battery earth as all other electrics seem good from wiring diagram appears to earth though sensor body
2 bad sensor?

I can do more experimenting by earthing sensor wire and seeing what happens or is there an earth inside the gauge pod i cant remember?

Fuel gauge reads accurately ( well as accurately as always)

bit on so would like direction so as to solve quickly

Thanks as always in advance

[johnwp5bcoupe]

Hi Alan if you have 10v? from the regulator and the petrol gauge is ok it sound like your temperature sensor is faulty as basically the circuit is a volt meter.
Good luck

[Allan NZ P5b]

Thanks John. yes had already checked regulator as had an issue with it a few years ago and replaced.
Bought a new Tridon sensor TTS029 2 minute job thank you. I knew car was not actually overheating as fan was not cutting in, but still it was disconcerting.