Alternator turning off after a few mins

[Hooter]

Ive been reading other threads here about changing settings to turn on alternator etc...
My problem is slightly different, as its in a conversion and E38 L77 and 6L80 came from a 2014 VF Holden Calais
I have connected as per factory, and on startup shows approx 12v for a few seconds and then climbs to 13.5 ish....
After about 5 mins it drop back down to 12v and then slowly drops from there, which if left too long will flatten battery......
If you shut down engine and restart, it repeats the same process... good for approx 5 mins and then gone.
Is this something in settings or another problem.

When I initially installed this I wired the 2 field wires the old way, that is through the older vehicles charge light, and interestingly enough the charge light didnt work, but it did charge the battery.
So my reckoning said, if charge light doesnt work, then connect to ecm, as it may work better. I actually got similar readings on volt meter, but never noticed the drop until battery started showing low...
anyone experienced similar problems????

[Dannn]

If its wired similar to the U.S. cars with the gen L and F terminal alts, you should be able to check the L terminal at the alternator for a 0-5v signal. Usually with the L terminal off it should be around 1v, with the L terminal commanded on you should see about 4.5v. The PCM outputs a 5v Pulse width modulated signal based on battery voltage to charge it. lower the PWM, lower the voltage. You should be able to scan and see commanded L terminal percentage.