you can check the alt by disconecting the wire from the dash lamp then take a feed from the battery with 5W BULB in line connected to the warning light terminal on the alt and a lead from D+ straight to the battery then start the engine , the light should go out ,with a meter check the volts to the battery it should be around 14.5v on start up falling to around 13.8v,if this is ok leave the wire to d+ on and reconnect the warning light wire and recheck if ok its on your split charge . hope this helps you trace the point of the fault.