Diesel engine preheating system (BMW 5 Series E39)

When the engine is cold, it is not possible to create a high temperature in the combustion chamber that would allow the fuel to ignite spontaneously by compression alone, so the engine must be preheated.

With this circuit, a pin glow plug is installed in each combustion chamber. The glow plug consists mainly of a housing and a pin heating element pressed into it. When the electric current is turned on, the winding in the pin heating element heats up to a temperature of +850°C within a few seconds, due to which the duration of the engine warm-up before starting rarely exceeds 5 seconds. As soon as the preheating system indicator on the instrument panel goes out, the engine can be started. Duration of preheating. as well as automatic heating after starting the engine (up to about 20 seconds) is regulated by the DDE control unit using the preheating system relay. The relay must be switched on in accordance with the electrical diagram.