Bongo Fury

My 2003 Bongo's still in the garage in Glasgow, and the mechanics are still getting nowhere cos they've no wiring diagrams. All we know is that the speedo is going crazy (leaping all over the dial with the vehicle parked) and the hold light is flashing.

Now oddly, they disconnected all the sensors from the back of the transfer box, and yet the speedo still goes nuts as soon as the engine starts; the mile-ometer also spins round even though the vehicle isn't moving!

What could be sending a signal to the speedo? Could a malfunctioning ECU be sending a signal back to it?

We're all confused!

You need to let people know what engine you have, as the petrol and diesels have different ECUs.

Sorry, it's 2003 Turbodiesel automatic 2.5 litre.
We've found that the speedo sensor on the back of the transmission unit (that feeds the speedo on older Bongos) has a blanking plate on it, and no evidence from the wiring loom that it ever was connected.
I've heard that modern cars take the speedo input from the ABS system, so maybe this is the case here.

We're all confused, and Dunnings Automatics, who are experts who are kindly talking to us, are also confused