relay shouldn't send power back.

Don't really see what blue wire now Might be that I was wrong.

On my alternator I have + to battery, then the sensing wire through the dashboard bulb to ignition.

is it the same cable ? if so if a diode in the alternator is shagged it could be that battery + is going through the wire to ignition, but then battery light should light up a bit with contact off / engine running.... Should see what wire and where coming from before I comment further, i might just make things more complicated then they really are now