GTV6 Speedo

[Fylnn]

Dear  Brains Trust,

I am trying to diagnose why the speedo in my GTV6 does not work.  The previous owner had 'rewired' the back of it for some reason, so my first suspicion is that it is the speedo itself.  But equally not sure the sender is working.  I am after some ideas as to what I am looking for.  I imagine the sender reads pulses and sends a signal of some description to the speedo.  Does anyone know what the signal should be and how I check it?  Is is a voltage or something?  The wiring to the back of the speedo has 4 wires apart from the light wire.  2 seem to be a 12 V input, I assume this is a reference voltage or just the power, and then the 2 wires from the sender. 

Any ideas gratefully accepted.

[colcol]

Re speedo, i think that GTV 6's and Alfa 33 had the same or similar setup, it has been the biggest problem i have had with my car, for ease of production, Alfa had it so it had a push on plug at the back of the speedo, this works fine until the terminals age and get a bad connection and then no speedo, what a lot of auto electricians was hard wire the speedo so the power went straight into the speedo and not through a dodgy plug as i beleive they don't operate on 12 volts but about 5 volts, so there is even less oomph to push the volts through, but sometimes it can be the sender plug that comes out of the gearbox, and once again this can be also hardwired to avoid dodgy connections, also it could also be the sender in the gearbox, from my memory, a good one the red and white wires should have about 4 ohms with a multimeter, i myself have always had a spare 'control' sendor that i know works, and when the speedo stops working, it is easier to change the sensor than to remove the speedo, Colin.