Oddly enough, mine doesn't seem to bump the voltage in tow/haul mode. Possibly because I changed some settings.
Supposed to happen with headlights on as well but I don't notice it. I usually hit 14.8V after startup, then drop to 13.7V after awhile..
something about 'idle boost' in the wiring diagrams. Haven't every heard of anyone experiencing an idle bump.
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as I understand idle boost from the hpt research I did a while back, it's the ecm being able to raise the idle of the alt can't keep up with the amp load, it can request a higher idle to keep up. the example given was a rainy hot day, full ac running, lights, wipers and all sitting in traffic for long time.
my c6 e38 has it defined in and populated in hpt, but it's not enabled size) since it was a old school 4 pin with a fuse box ref wire, I had hoped enabling it would give me a pwm out out to control the truck one. Sadly it isn't complete, if I command L term in when switched to pwm I would put a pwm out, but not control it and wouldn't turn on without manually doing it. i asked them if a segment swap was possible to add it, but it fell on deaf ears. to old to support to that level.
14.7v for a while after start and then switch down to 13.8 sounds the most correct I've heard for a agm. you might have cracked it.