Glow Plug System: Description and Operation

Fig. 7 Glow plug controller test connections. 1985:

1985
A glow plug relay in the glow plug controller regulates the hearing of the glow plugs, Fig. 7. The relay is operated by the thermal controls in the controller. These thermal controls respond to engine temperature as well as the heating coil inside the thermal control unit.
When the ignition switch is turned to the Run position, battery voltage is applied to the ``Wait'' lamp. The ``Wait'' lamp switched is closed when cold and open when hot. When the lamp is on, battery voltage is applied to the glow plugs and they begin to heat. After approximately six seconds, when the glow plugs and engine are sufficiently heated, the ``Wait'' lamp goes out and the engine can be started. At this time, the glow plug relay remains closed continue heating the glow plugs to reduce exhaust smoke and improve cold driveability.
After approximately one minute of engine operation, the glow plug relay is opened and heating of the glow plugs is stopped. If the engine is restarted when hot, the thermal controller keeps both the lamp switch and relay open to prevent any further plug heating.
When the engine cools, the ``Wait'' lamp switch returns to the closed position and the glow plug relay remains open until battery voltage is applied to the thermal controls.
The thermal controls provide protection from overvoltage conditions by automatically de-energizing and energizing the glow plug relay. If the overvoltage surge remains present, voltage is cycled on and off to the glow plugs without damaging them.