Glow Plug System: Description and Operation

Fig. 1 Diesel engine pre-heating system schematic:

The pre-heat control system consists of glow plugs for each cylinder, a control module, coolant temperature sensor and indicator lamps on the instrument panel, Fig. 1. Voltage is applied to the control module when the ignition switch is in start and run positions, and the control module controls current flow to the glow plugs. The control module is protected by a fuse in the fuse panel, and glow plugs are protected by a fuse in the control module.
When the ignition switch is turned to the run position and coolant temperature is less than 140°F, voltage is applied to the glow plugs and yellow ``wait-to-start'' indicator is illuminated. The glow plugs will continue heating as long as the yellow lamp is on, even if the engine is started. After 2 - 10 seconds of heating time, depending upon coolant temperature, the yellow lamp is turned off and a green indicator is illuminated. A built-in timer in the control module allows the glow plugs to continue heating for 8 - 10 seconds after the green lamp is turned on. In addition, the glow plugs are energized whenever the ignition switch is in the start position, regardless of engine coolant temperature or control module timer operation. When the ignition switch is returned to the run position after starting, the glow plugs are de-energized after a short delay.