The voltage supply of the engine control unit is structured as follows:
Terminal 15 ON is signalled to the control unit directly via a separate pin.
Consequently, the control unit activates the main relay. The main relay supplies voltage to other inputs of the control unit. In the same way, the main relay secures the voltage supply of other control units and components.
The engine control unit still requires a permanent voltage supply via terminal 30 for memory purposes.
The earth supply of the engine control unit is ensured by several pins which are, however, interconnected internally.
The engine control unit can be addressed with the tester via the diagnostic link. Terminal 15 is generally required for diagnosis purposes.