Q: What Is an ECU and What Does It Do?

The electronic control unit (ECU) used in today’s cars and trucks is used to control the engine and other components’ functions. An ECU is a computer with internal pre-programmed and programmable computer chips that is not much different from a home computer or laptop. The vehicle’s engine computer 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). These units can be an advantage by having more modules in one location but may be a disadvantage by adding longer wires to reach the component it operates.

Most newer vehicles have started using a communication line between different modules on a vehicle so they can share information and redundant sensors do not have to be used. For instance, a speed sensor at a wheel detects the wheel speed and will be 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.

The use of sharing input sensors throughout the vehicle using only two data lines between ECUs has cut the amount of wiring used in the vehicles. Sharing information between modules also means they need a common language between them so they can operate as a group. 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. This reduces the amount of wire needed and you do not need to go to each ECU when wanting to test them.