The electronic control unit (ECU) used in today’s cars and trucks is used to control the functions of the engine and other car components. An ECU is a computer with internal pre-programmed and programmable computer chips that is not much different from any other computer or a laptop.
The vehicle’s engine computer unit ECU is used to operate the engine by using input sensors and output components to control all engine functions.
The ECU needs inputs from vehicle sensors like the crankshaft sensor and camshaft sensors to compute the information using a program that has been stored in the ECU on a programmable memory chip. The ECU program will use the inputted sensor information to compute the needed output like the amount of fuel injected and when to spark the coil in order to start the engine.
There are different ECUs used for different systems on the vehicle. The different ECUs used can be for the transmission, traction control or ABS, AC, body functions and lighting control, engine, air bags, or any other system a vehicle may have. Some vehicles may incorporate more than one ECU into a single unit called a powertrain control module (PCM).
Most newer vehicles have started using a communication line between different modules on a vehicl. For instance, a speed sensor at a wheel detects the wheel speed and will send an input to the anti-lock brake module (ABS) ECU. Instead of sending many wires from the one sensor to other ECUs, the ABS ECU will share the information on the network communication lines to all the ECUs that use the information, like a transmission for its shifting of gears, the speedometer to show the speed of the vehicle, or the suspension system to control the suspension as needed.
When one computer goes down or does not share information due to an error, then it may affect other modules if they need the sensor input from the failed module.
The engine ECU in most vehicles is connected to the onboard diagnostic connector and will relay all diagnostic information on this line to all the other modules or ECUs.