Hy britcon, the only things I would add to what baldeagle said, is to check the voltage at the battery once the engine is running. Also is the alternator grounded to the engine, other than through its mounting hardware, and do you have ground wires between the cab and engine, cab and frame, cab and fenders, box and frame, it is impossible to have too many ground wires. Do you have an ammeter or a voltmeter?
