[PSIG]

Yes, from that diagram, the alternator should use the internal regulator as-is, assuming it's good, the Meter Fuse is good and the bulb isn't burned-out.

[JHolland]

I have checked out that topic and as far as I am aware, since my MX5 is a 1989 model, its as early as it gets in terms of wiring, so in theory the alternator shouldn't be controlled by the ECU.
From what I've found on other forums the alternator should just self govern.

Yes, that is correct, my current MX5 is a 92 1.6 and that self-governs.

[hgtorin]

To anyone whos following or has found this thread, I've found a couple of things.
Firstly, the reason the car wasn't starting was a whole other issue - broken ground cable on the CAS.
Secondly, putting the stock ECU back in, the car starts up straight away and the alternator is charging the battery. Off voltage was 12.8, and when the car was running, it went to 13.45, so I can pretty safely say it isn't a failed alternator.
In the next couple of weeks I may try and re-fit the Speeduino with the help of a mate who knows a lot more about this stuff than I do and will make another reply on this thread if we are able to solve the issue!
Thanks for everyones help thus far!

[JHolland]

What part number is the stock ECU?