Battery? How old is the one you have now? If its the OG battery it might be that under load your alternator is giving good voltage compensating for a weak or dying battery. May not be the issue but I am trying to give you some hope that it might be some thing simple.

I know that when my battery drained and I jumped it, it went into trans fail safe mode as I started to drive and stayed until I replaced the battery.

GL

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2002 X5 4.6 Sold
Dinan CAI, TB, Mass Air, Stage 4 engine & tranny software, X-pipe
2004 745Li
1997 Ford Mustang Cobra Vert
1985 Mazda RX-7
13B-REW single turbo 60-1, Haltech, Meth injection, MSD Ignition, and much more
2008 Mazda Speed3 Hypertech Tune