Glow Plug System: Description and Operation

1984
The glow plug system features a self-limiting feature that regulates maximum temperature, while the glow plugs are programmed to shut off automatically should the vehicle not be started within the specified time period.

Fig. 5 Glow plug control module. 1984:

System Components
The glow plug control module, Fig. 5, is an integral assembly that includes the timer functions, lamp switch and glow plug relay. The control module serves the following functions:
1. Controls wait lamp operation, which varies according to system voltage and/or ambient temperature.
2. Controls system shutdown timing depending on voltage and ambient temperature.
3. An overvoltage function that protects the glow plugs from failure, should higher than normal voltages be incurred.
4. A thermal cutout function that disengages the glow plug system when module temperatures are greater than 113°F.
5. A power relay function that switches the voltage applied to the glow plugs.
6. A quick reset function that permits the module to recycle quickly, after initial shutdown time.
This control module can only be used with glow plugs that regulate their own temperature. The new glow plugs used have positive temperature coefficient properties, which mean they have low resistance values at low temperatures and high resistance values at high temperatures. The new plugs offer a fast temperature rise similar to past fixed resistance plugs, plus improved and simpler glow plug control.

Fig. 6 Glow plug control circuit schematic. 1984:

System Operation
The glow plug control circuit, Fig. 6, operates the glow plug system in three steps: pre-glow, after-glow and off. During pre-glow, the circuit activates the wait lamp and heats the glow plugs until they are sufficiently warm to start the engine. During after-glow, the circuit deactivates the wait lamp, but continues to apply power to the glow plugs. During the Off cycle, the circuit removes power from the glow plugs and keeps it off until the engine is restarted.
As stated previously, the glow plug control module controls all circuit functions. The thermal controls open the pre-glow and after-glow switches to end the respective cycles, and are responsive to engine temperature. When the system is energized with the engine cold, both switches are in the ``Cold'' position. As time passes, current flow heats the thermal controls, moving both switches toward their ``Hot'' position. The time needed for each switch to reach its ``Hot'' position is dependent upon how cold the engine and control module were when the system was first energized. As the pre-glow switch reaches the ``Hot'' position, the wait lamp deactivates and the engine may now be started. The control module continues to operate, whether the engine is started or not, since the after-glow switch has not yet reached its ``Hot'' position. Current flow continues to heat the thermal control. When the thermal control reaches full temperature, the after-glow switch moves to ``Hot,'' opening the path to ground from the coil of the glow plug relay. This allows current flow to bypass the coil of the reset relay. With the path to ground now open, current must flow to bypass the coil of the reset relay. With the path to ground now open, current must flow to ground through the reset relay coil. The glow plug relay de-energizes, removing power to the glow plugs. The reset relay now energizes, opening the contact of the relay and locking off the thermal controls. When the ignition switch is turned off, the reset relay contact closes, and the glow plug module is ready to repeat the cycle. If the engine is above 140°F when restarted, the thermal controls will be ``Hot,'' energizing the reset relay and preventing glow plug operation.
The over-voltage protector protects the glow plugs should battery voltage rise above 14 volts. When the protector senses over 14 volts, it opens the circuit to the glow plug relay coil and prevents current from flowing to the glow plugs. After a short time, the protector closes the circuit. If battery voltage is still above 14 volts, it will reopen the circuit again. The protector continues to cycle in this way as long as the over-voltage condition exists and as long as glow plug operation is needed.