Maybe you have a suggestion:

Situation:

A Cat with 2 engines and 2 alternators

"Yes, it is an external regulator, a NextStep. Which works just fine
with both engines going, it is only when the port engine is running by

itself that it overcharges. There must be a problem with the wiring
somewhere that I just can't find. Also, we have 5 gel cell batteries
and they all seem to be fine."

I believe the Parallel Solenoid has been replaced.

This is not my boat but is an issues with someone elses, any
suggestions, the only info. that I have is the above, so a Q and A
does not apply.....

TIA