Posting mainly for Search function, its seems that using an aftermarket alternator will cause the engine light to come on, i tried two different aftermarket alternators and both would not talk to the computer;
the first 1 from cooldrive been jayisec or something alike would charge all the time with engine light (which it isnt suppose to do, should only charge when it needs to if battery is fully charged)

the 2nd alternator i tried was al delco which would charge intermittently which it should but also did not talk to the computer as it lit up the engine light.

So in the end i just got a 2nd hand one from Jap Wholesale.

Hope this helps someone else later down the track!
Paul