No live data once alternator begins charging

Hi all ,
Got a wierd one on a ls1 conversion im doing. i can log live data not a problem , but as soon as the engine starts and the alternator cuts in logging stops.I have tried other scan tools get same result its like the ecu throws a voltage fit , but the voltage isnt high !!
If i unplug the alternator 2 pin plug i have data . have checked grounds powers etc. Its a new alternator and its charging at 14.2 volts when running,
if voltage was high id say it that is the issue, but its charging perfectly normal. Hopefully someone has come across it before....

Check both grounds at the DLC.
Al