What does a vehicle ECU do?

Electronic control units (ECUs) in modern vehicles may total more than 70 for various subsystems.

Historically, the engine control unit has been the most significant processor. Others are used for Autonomous Driving, Advanced Driver Assistance Systems (ADAS), transmissions, airbags, antilock braking/ABS, cruise control, electric power steering, audio systems, power windows, doors, and mirror adjustment, and battery and recharging systems for hybrid/electric vehicles, among other things.

While some of these are self-contained subsystems, communication between them is critical. For example, a subsystem may require the control of actuators or the receipt of data from sensors.

The CAN standard was created to address this gap. One significant advantage is that the interconnection of various vehicle systems enables the implementation of a wide variety of safety, economy, and convenience features solely through software - functionality that would require additional cost and complexity if such features were "hard wired" using traditional automotive electrics.

Several examples include the following:

Auto start/stop

Various sensor inputs from around the car (speed sensors, steering angle, air conditioning on/off, engine temperature) are combined via the CAN bus to decide whether the engine can be switched off when the vehicle is not moving to optimise fuel economy and pollutants.

Electric park brakes

The "hill hold" functionality utilises data from the vehicle's tilt sensor (which is also used by the burglar alarm) and road speed sensors (which are also used by the engine control, ABS and traction control) via the CAN bus to determine whether the vehicle is stopped on an incline. Similarly, the CAN bus receives inputs from seat belt sensors (which are part of the airbag controls) to identify whether the seat belts are secured, allowing the parking brake to automatically release when the vehicle is moved.

Parking assist systems

When the driver shifts into reverse gear, the transmission control unit can transmit a signal through the CAN bus activating both the parking sensor system and the door control module, causing the passenger side door mirror to tilt downward to indicate the location of the curb. Additionally, the CAN bus accepts inputs from the rain sensor, which is used to activate the back windscreen wiper when reversing.

Auto lane assist/collision avoidance systems

The CAN bus also uses the inputs from the parking sensors to feed outside proximity data to driver assist systems such as Lane Departure warning, and more recently, these signals are used to actuate the brakes by wire in active collision avoidance systems.

Auto wiping

The rain sensor (typically used for automatic windscreen wipers) communicates with the ABS module over the CAN bus to initiate an undetectable application of the brakes while driving to clear moisture from the brake rotors. This feature is available on particular high-performance Audi and BMW cars.

Engineers can put sensors in the most desired location, and many ECUs can use their data. For instance, external temperature sensors (traditionally located in the front) can be integrated into the outside mirrors, avoiding engine heating and data collection for the engine, climate management, and driver display.

The automotive industry recently established the LIN bus (Local Interconnect Network) standard to supplement CAN in non-essential subsystems like air conditioning and infotainment. As a result, data transfer speed and reliability are less critical.