Early speedo misbehaving.

[Searider]

Evening all. I’ve got a May 93 430 with the LT77 gearbox and the speedo that starts at 1 o clock.
My speedo sometimes works fine but for much of the time the needle is either pinned at max or bouncing around randomly.
I’ve been reading lots of (old) posts about the 4 and 8 pulse sensors and the speedycables option to supply a look alike speedo.

However, what I can’t find is how to determine if it’s the sensor or the head unit that’s faulty.
If it’s the head unit then that’s one way to go, if it’s the sensor then the solution might be a bit more involved.

How do I determine what the cause of the issue is?

Thanks

Duncan