Speedometer not working

[Pmor]

I have a analogue speedo with four wires powering it, it's recently started to behave erratically or not work at all. This is on a 2litre pinto kit.
I presume the unit is powered by some sort of sensor?
Any ideas as to what I should be testing for to check the input to the unit, otherwise I suspect the speedo itself is kaput.
Many thanks. Pete

[Fury1630]

Can't help with your particular unit, but the four wires will be live, earth signal & panel light.

Red & black are likely to be live & earth, white is likely to be light. Any idea where the sensor is? Options are one one of the hubs pointing at a rotating bolt head, on the prop, pointing at some small magnets, or on the gearbox right side at the front end of the tail shaft if it's Ford.

[Pmor]

Hi Fury.
Thanks for getting back so quickly and identifying the wires.
Not sure whereabouts the sensor is, it is an old Ford Sierra lump and gearbox.
I have seen an Isspro speedo that looks identical but that has an additional purple wire, so not sure what that's for?
Pete