B3.9 and B5.9 Series Engines
Section 13 - Electrical Equipment - Group 13
If the lamp goes out, there is a short in the switch.
Switch On, Lamp Off, Engine Stopped
This condition can be caused by an open circuit. To de-
termine where an open circuit exists, check for a blown
fuse, a burned-out bulb, defective bulb socket, or an open
circuit in No. 1 or "D+" lead circuit between alternator and
Switch On, Lamp On, Engine Running
Check the drive belt and alternator pulley to be sure the
alternator is rotating.
Disconnect the lead to terminals No. 1 and No. 2 (Delco)
or D+ (Bosch K1). If the lamp stays on, there is a short
to ground on the alternator side of the lamp.