Hi. It is strange that the dash battery lamp came on after a minute or two of the engine running......sounds about the time it would take for an interconnect relay between coach and chassis batteries to be turned on by a BIRD or BIM battery control system (give or take). Since your coach and chassis batteries are seeing the same charging voltage (clearly at a charging level of 13.3 vdc; BIRD, BIM are enabled by alternator above 13.1 vdc approximately if I recall correctly). So is the closing of the interconnect relay causing the battery lamp to light? I don’t see why it would be since you are not on shore and gen is also off. I wonder if both battery strings (coach and chassis) would test good on a load test? It also sounds like the battery light is staying on once it comes on.
The other issue to me is that the charging voltage of 13.3vdc is very low for a “working” alternator especially feeding healthy lightly loaded batteries;....should be 14vdc minimum I would expect (14.5 ish). So my WAG is a bad regulator in the alternator or bad diode(s) perhaps......assuming good batteries; all charging system cables/connectors are clean and tight. I am just guessing here and don’t want you to spring some $$ for a new alternator (and related “class A” work required to change it) if that is not the problem or there is some easily repairable connection being missed.
__________________
|