The glow plug system on the 7.3L Power Stroke Diesel is a pretty simple system. There are two types of systems, one is the Glow Plug Relay System which is common on the federal trucks and the Glow Plug Monitoring System which is found on California trucks as well as all Excursions. The main difference in the two systems is the federal systems use a Glow Plug Relay while the Glow Plug Monitoring System uses a Glow Plug Control Module (GPCM) or a Low Resistance Shunt.
The glow plug system is electronically controlled by the Powertrain Control Module (PCM). If the temperature is below 131°F the PCM
will energize the glow plugs immediately after the key is placed in the ON
position. Then, depending on the readings from the engine oil temperature (EOT)
sensor and the barometric pressure (BARO) sensor, the PCM
determines how long the glow plugs will be on.
The Wait To Start (WTS) light or Glow Plug Indicator on the dash is controlled separately from the glow plugs. If the engine oil temperature is above 131°F, the PCM will bulb-check the WAIT TO START lamp but not energize the glow plug relay. The Glow Plug Indicator (GPL) is located in the instrument cluster and is used to prompt the operator when the engine is ready to be cranked and started. When the light goes out, the vehicle is ready to start. On-time normally varies between 1 and 10 seconds. Wait to Start indication is independent of glow plug relay on-time as the glow plug system can stay on after the indicator goes off.
The Glow Plug Monitoring (GPM) System monitors the glow plugs. This provides feedback to the PCM for single failures. The GPM system uses a low resistance shunt to conduct current to the left and right bank glow plugs. Three sensing wires measure the voltage drops across the shunt when the glow plugs are operating. A failure in the glow plug system will set a diagnostic trouble code (DTC).
The California glow plug system is composed of solid state Glow Plug Control Module (GPCM), glow plugs and the associated wiring harness. The glow plug on-time is controlled by the Powertrain Control Module (PCM) and is a function of oil temperature, barometric pressure and battery voltage. The PCM enables the GPCM which drives the individual glow plugs. Glow plug on-time normally varies between 1 to 120 seconds. In addition to PCM control, the GPCM internally limits the glow plug operation to 180 seconds regardless of PCM commanded on-time. The power to the glow plugs is provided through the GPCM solid state drivers directly from the vehicle battery. The GPCM monitors and detects individual glow plug functionality, and the control and communication links to the PCM. The failures detected by the GPCM are passed to the PCM using a serial communication signal on the diagnostic line.
Note: Wait to start lamp on-time is independent from GPCM on-time.
Glow Plug Relay Differences
Glow Plug Diagrams
Here is a video on how the Glow Plug Relay can be tested.
Here is a video on how the glow plugs work.
Glow Plug Testing