Glow Plug Control Switch: Description and Operation

This system improves cold engine starting performance. When ignition switch is turned On, a ``Wait-to-Start'' light illuminates on dash panel next to cold start knob. As this happens, relay No. 1 closes and full system voltage is applied to glow plugs. If engine coolant temperature is less than 86° F, relay No. 2 also closes. The control module extinguishes ``Wait-to-Start'' light after 3 seconds. If ignition is left on for approximately 3 more seconds without cranking engine, No. 1 relay opens, shutting off current to glow plugs to prevent overheating. However, if engine coolant temperature is below 86° F, relay No. 2 remains closed to apply limited voltage to plugs through glow plug resistor until ignition is turned Off.
During engine cranking, relay No. 1 is cycled by control module to provide alternate voltage to glow plugs between 4 and 12 volts with relay No. 2 closed, or between 0 and 12 volts with relay No. 2 open.
With engine running, alternator output signals cause control module to prevent No. 1 relay from cycling, and afterglow function takes over. If engine coolant temperature is below 86° F, limited voltage is applied to glow plugs through relay No. 2 and glow plug resistor. When clutch and neutral switches are closed or coolant temperature exceeds 86° F, relay No. 2 opens and all current to glow plugs is cut off.