I have been asked to assist a friend to add a split charger to charge a dedicated bow thruster battery. He already had a two battery system, so we swapped the two way split charge unit for a Sure Power three way unit. The Alternator is a Delco 219/290?? rated at 70A. It has four connections: ground (black), charge (red), field (purple) and one other (red/light blue). The purple and the red/light blue are connected via a 2 pole water-tight connector. I had assumed that the red/light blue was the sense wire, but I only get 14V at the batteries, even with revs on. The alternator puts out 14.8V. So: can anyone tell me whether this red/light blue wire really is the sense and, if not, then what is it for? I have traced it to a 4 way connector marked "multi engine cable disconnect". It becomes live as soon as the battery main breaker is closed. The purple (field) becomes life when the ignition is turned on. All wiring diagrams I have found show the fourth wire as red/purple and describe it as 'sense'. Any help would be appreciated. Thanks; Mart (Ardmory).