1. 1993, F 350, 7.3 IDI, E4OD, 4x4. The glow plug indicator light works sometimes. Replaced the glow plugs and still no glow plug panel light when I go to start up. Engines always starts right up, but I'm uncertain how to check this indicator light circuit. The relay seems to be working but not the glow plug indicator light. Need ways to check this circuit, please help.

 

https://www.ford-trucks.com/forums/1...-tutorial.html

wait to start light lamp diagnoses

if the WTSL remains ON
unplug the connector at control unit.
turn ignition switch ON.
if the WTSL goes OFF,disconnect the ground cables to batteries and replace the control unit.
if the WTSL remains on,trace out and repair the wire to the indicator light.

if the WTSL doesn't turn on
unplug the connector at control unit.
connect a jumper wire from the harness side to ground.
turn ignition switch ON.
if the WTSL remains OFF,replace the bulb or trace out and repair the wire.

if the wait to start light flashes rapidly
happens when two or more GP's are burned out
test for continuity (or via the test light method as noted by Niles above) and replace as needed.