Glow Plug: Description and Operation

Fig. 27 Glow plug installation:

During cold starts, diesel compression heat is dissipated rapidly through the cold engine so that a pre-heating provision is necessary to ensure compression ignition. The glow plugs are threaded into the cylinder head so they project into each combustion chamber, Fig. 27. A heating element in each plug gets red hot whenever current is applied to the plug terminals.
Current is supplied to the glow plugs directly from the battery by a relay which is controlled by the ignition switch. A temperature sensor connected to a time circuit in the relay controls pre-heating time. The colder the temperature, the longer the pre-heating time. The glow plug light is on when the plugs are being heated and goes off when the engine is ready to start.
There are two types of factory glow plug systems installed. The "fast glow", which was factory installed on non-turbocharged engines, and the "slow glow" which was installed on turbocharged engines.