The ECU controls the relay that powers the fuel pump. This relay is also responsible for powering the injectors and spark coils. The relay is energized when the ignition is turned and also when the ECU determines that the engine is rotating. If the no engine rotation is detected the relay is de-energized after about two seconds.
The relay coil has about 100 ohm resistance so the required current to drive it is not great. The circuit is used to drive the relay is the left hand portion of the first diagram. The transistor is directly controlled by the microcontroller via the 39k resistor. The collector of the transistor connects to the relay coil. The other side of the relay coil connects to +12V. The diode is to protect the transistor from transients voltages when turning the relay off.
There are two parts to this circuit. The main part of this circuit is shown in the second figure. 12 volts is supplied from the ECU power relay. Voltage Vdd is used to control the gates of the FET output transistors. The microcontroller is supplied +5 volts by the voltage regulator 7805. A small inductor is used to generate a decoupled +5AV that is used for the analog portions of the circuit. This includes power for the throttle position sensor and the air pressure sensor.
The other part of this circuit is the right hand side of the first diagram. This is the circuit that drives the ECU power relay. The base of the transistor is a keyed 12 volt source. The collector is connected to the one end of the ECU power relay coil, with the other end being connected to +12V. When the key is turned on the transistor conducts, energizing the relay and supplying power to the ECU. The WMECU would also drive this circuit and keep itself powered for 10 seconds after turning the key off. My16M does not have this requirement. The zener diode is to protect the transistor from transients voltages when turning the relay off.. A diode like the other transistor could not be used as the leakage current through the coil was enough to power the ECU.
