NBB
Well-Known Member
Have to see a wiring diagram - and someone can correct me - but seems to me the two batteries have to be connected in some way. If so, then charging and checking voltage would be non-trivial unless completely removed or isolated from vehicle. I understand the smaller aux is basically there to keep the electronics powered during starting. That your uConnect is going blank during the attempted start seems like a clue to me. Maybe that aux battery is not in the condition you think it is in - maybe a few days of work killed it - permanently. My personal experience on CAN bus vehicles is that poking and prodding "wakes up" many individual pieces of the system, which will kill the battery in just a day or two. Charger may make you think it's good, but it really needs to be load tested while isolated - you're really looking at voltage from the larger battery. If it were me and I had no complete wiring diagram, I'd remove and test the battery, then put a scope directly on the terminals and look for voltage drop during start process.
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