LMM Duramax fluctuating Alternator voltage

Hi All,

Have been chasing a issue on a LMM duramax engine I've converted into a Nissan Patrol, that has me really stumped.

I had a fluctuating idle, about +/- 25rpm, that i thought was a fuel issue. I replaced FPR, checked all lines, filters etc and it didn't fix it. Noticed at the same time the voltage on my banks idash was fluctuating anywhere from ~13.8v to 15.4v, so i started looking at the alternator. I noticed after 10min of running at idle it was very hot to the touch and thought something must be up and associated possibly with the high and fluctuating voltage. So i disconnected the 2 pin plug on the back of the Alt and straightaway it pegged the voltage at 13.8v on the idash and the fluctuating idle was gone - one problem found atleast.

When i say the idle/voltage fluctuates, it basically goes in a cycle. It will fluctuate from 13.8 to 15.4v for about 10 to 15 seconds then stabilizes bang on about 14.0v for a few seconds, then off it goes again.

For reference it has a brand new ACDELCO 160a alternator on it.

Things ive tried,

Put the original bosch 125a alt back on it - same problem
voltage checks with a meter at Alt and Bat - reading almost identical
voltage drop checks of positive and negative cables to B+ post on alt to Battery and Alt case to Battery neg terminal and chassis- all within spec.
All ground and pos connections are clean with no corrosion
Brand new tensioner
Ran a dedicated Negative jumper cable from Alt case to neg bat post and chassis - same
Ran a new + cable from Alt to fuse box - same.
Hooked a 2nd battery up in parallel with jumper cables - same
Continuity checks between L and F pins on Alt plug to ECM pinouts - all confirmed wired correctly and minimal resistance.
All connections are tight.
Battery is brand new, holds charge well and starts engine straight away.
No charging related DTC codes

All i can think of now is the ECM or BCM is potentially the problem??

I've discovered after using efilive and tech2 over the last few days, that the conversion package i was supplied has 3 different vin's between the ECM, BCM and TCM. Could this be the problem at all??

A vin search shows the ECM and TCM are both from 4x4 LMM vehicles but the BCM is from a 4x2 LMM vehicle.

This has me stumped and I'm hoping someone has come across this before. With the 2 pin connector unplugged on the back all works perfectly fine albeit just a fixed voltage output @13.8v which as ive read is the set fail safe voltage for the Alt