The first stage of ECU operation is in fact power management. This is where various voltages are regulated and the power-up of the ECU is handled. Most ECUs have sophisticated power management due to the variety of components inside, accurately regulating 1.8V, 2.6V, 3.3V, 5V, 30V and upto 250V all from the car’s 10-15V supply. The power management system also allows the ECU to have full control over when it powers itself down – i.e. not necessarily when you turn off the ignition switch.
Once the correct voltages are supplied, the microprocessors can begin to boot up. Here the main microprocessor reads software from the memory and performs a self-check. It then reads data from the numerous sensors on the engine and converts them into useful information. This information is often transmitted over the CANbus – your car’s internal computer network – to other electronic modules.
Once the main microprocessor has interpreted this information, it refers to the numeric tables or formulae within the software and activates outputs as required.
Example. Should the Crankshaft Position Sensor show the engine is about to reach maximum compression on one of the cylinders, it will activate a transistor for the relevant ignition coil. The aforementioned formula and tables within the software will cause the activation of this transistor to be delayed or advanced based on throttle position, coolant temperature, air temperature, EGR opening, mixture ratio and previous measurements showing incorrect combustion.
The operation of the main processor inside the ECU and the activation of many outputs is overseen by a monitoring microprocessor – essentially a second computer that makes sure the main computer is doing everything correctly. If the monitoring microprocessor is not happy with any aspect of the ECU, it has the power to reset the whole system or shut it down completely. The use of the monitoring processor became imperative with the application of drive-by-wire throttle control due to safety concerns should the main microprocessor develop a fault.