Glow Plug System: Description and Operation

Glow Plug System Components:

Glow Plug Relay:

The 7.3L diesel engine uses a glow plug system that preheats air in the combustion chamber to improve cold engine starting.

The system consists of:
^ intake manifold glow plugs.
^ glow plug relay.
^ Powertrain Control Module (PCM).
^ Engine Oil Temperature (EOT) sensor.
^ Barometric Pressure Sensor (BARO sensor).

The glow plug system is electronically controlled by the powertrain control module. If the temperature is below 55°C (131°F) the powertrain control module 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 powertrain control module determines how long the glow plugs will be on.

Glow Plugs - PCM-Controlled
The intake manifold glow plugs are located in the cylinder heads, under the valve covers.

The glow plugs are self-regulating. If the engine oil temperature is above 55°C (131°F), the powertrain control module will bulb-check the WAIT TO START lamp but not energize the glow plug relay.

The powertrain control module protects the glow plugs by energizing them for short durations if the battery voltage is abnormally high.

The glow plug ON time varies from 0-120 seconds depending on battery voltage, engine oil temperature and barometric pressure.

On California vehicles the glow plug monitor system is part of the On-Board Diagnostics II (OBD II) System.

The Glow Plug Monitor (GPM) system is designed to find failed glow plugs or failed wiring in the glow plug system.

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).