Electronic Speedo
 

I have a Smiths/Telemitics speedo by Caerbont.
At SVA the speedo stopped at 63mph for no aparent reason, so i needed to find a solution quickly.
The speedo wiring was basicaly correct so other factors were the problem.
I wired the speedo direct and earth the sender and just got a pass.
Today we re visited the issue but how do you test your solution if you have to run the wheels at 70 mph without a rolling road?
I did not feel safe putting the car on axle stands with the wheels doing 70+ but Gavin had a great idea how to test the speedo sensor.

We removed the sensor and rewired the sensor using shielded 2 core, it may be beter to use 2 single core shielded cables but it worked for us. we did not secure the wireing so we could easily change the connections if we needed, so it was wired as we would want it in the car but not fixed, we could hold the reed sensor ( Gavins excellent idea was to put the magnets on a drill chuck) and spin up the magnets using a cordless drill holding the sensor close to the magnets.
We got readings but had to keep recalibrating the speedo until we could spin the speedo up to 130 mph.
The speedo works to its full range and is fully shielded.

Hope this helps if you have similar problems