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.