Interesting one this,

Car was presented with reports of ABS/DSC lights coming on & limp home cming in as well as various other lights coming on, basically scan of most systems revealed loads of codes relating to voltage being to high, now I never experianced the problems first hand so to date it has had new genuine battery(registered to car) and a hella alternator.

Fault still ocurring. The fault is not present the whole time,the car needs to be driven for about half hr,switched off for ten mins or so then once restarted and driven off the DSC/ABS comes on followed by gearbox going mental and everything else going mad. On live data the second befoe ABS light cuts in the voltage can be seen to shoot to 17.4 volts up to a max of 19v, now limped back to workshop left to idle voltage going up and down wildly (horrible).

Alternator has a single signal wire coming from it and if unplugged charge rate returns to stable 14.2v. So it seems we have a control problem , I'm just a bit stuck as where to go now as AD has no wiring diagram. I have cleaned earths,main positives, have disconnected cooling fans and problem still happens. If something like the cooling fans had a bad connection could they cause the ECU the request max output and generally cause mayhem . I'm thinking this could be an ECU fault or something that consumes a lot of power getting high resistance when hot.Wire from ECU to alt appears to be fine.

It's a nightmare to even get it to the state where it misbehaves, sorry for long boring post just struggling to make sense of this one so any help or suggestions gratefully received.

Thanks
Matt

Sorry just read the new hella alternator bit.
Was it the same fault before the alternator was fitted.

Because still think the alternator will have its own regulater.