Glow Plug System Operation

Relay Operation

Glow plug ON time is dependent in oil temperature and altitude. The glow plug relay/Glow Plug Control Module (GPCM) comes on between 1 to 120 seconds and does not come on at all if oil temp is above 131°F (55°C).
Check continuous and KOEO DTCs. If DTCs are present, see DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 6.0L DIESEL article.
Verify B+ voltage is supplied to the GPCM.
Using the scan tool, access GPCTM and EOT PIDs. Verify sufficient glow plug ON time and voltage (Dependent on oil temperature and altitude).
Turn ignition to ON position, measure voltage (ON time).

NOTE: Wait To Start indicator ON time (1-10 seconds) is independent from glow plug relay ON time.

Glow Plug Resistance

Disconnect the 4-pin connector at the front of the valve cover.
Measure glow plug resistance to battery ground. Resistance should be 0.1-2.0 ohms.
Measure harness resistance to GPCM. Resistance Should be 0-1 ohm.