Neil, i understand the battery light is controlled by the ECU - i unplugged the excitor and the light came on which proved the ECU could see the coil and was most likely not at fault. In testing, i used a test lamp to provide some resistance through the excitor. I didn't want to ground it directly. Without the equipment to view a pulse, a multimeter gives an indication of the pulse width. As it was, the measured voltage was low indicating the ECU was pulsing on more than off. Anyway, i replaced the alternator with a used unit and it worked. I outlined the fault finding above to assist anyone else with alternator issues.