Electronic Control Units

Once upon a time, automobiles were simple mechanical constructs. Then computers started taking over. Now, there's a different electronic control unit (ECU) for just about every function in your vehicle.

There are a lot of things going on in your engine and around your car as you drive. ECUs are designed to receive this information, via a number of censors, process that information, and then perform an electrical function. Think of them as the brains of your vehicle. As automobiles, trucks, and SUVs become more complex and outfitted with more sensors and functions, the number of ECUs designed to deal with those complexities increase.

Some common ECUs include the Engine Control Module (ECM), Powertrain Control Module (PCM), Brake Control Module (BCM), and General Electric Module (GEM). They control all functions associated with those components of the car, and they look and act a lot like a computer hard drive, often consisting of an 8-bit microprocessor, random access memory (RAM), read only memory (ROM), and an input/output interface. 

ECUs may be upgraded by the manufacturer or by a third party. They are usually protected to prevent unwanted tampering, so if you have a mind to try and turn anything off or to change a function, you won't be able to do it.

Fuel management is the main function of the Engine Control Module (ECM). It does this by controlling the vehicle's fuel injection system, the ignition timing, and the idle speed control system. It also interrupts the operation of the air conditioning and EGR systems, and controls power to the fuel pump (through the control relay).

Based on information received from the input sensors on things like engine coolant temperature, barometric pressure, airflow, and outside temperature, the ECU determines the optimum settings for the output actuators for fuel injection, idle speed, ignition timing, etc. The computer determines how long the injectors stay open—anywhere from four to nine milliseconds, done 600 to 3000 times per minute—which controls the amount of fuel used. The computer also controls how much voltage is sent to the fuel pump, raising and lowering fuel pressure. Finally, this particular ECU controls engine timing, which is when the spark plugs fire.

There is also an ECU that controls the airbag system, one of the most important safety features on your vehicle. Once it receives signals from the crash sensors, it processes this data to decide which, if any, airbags should be triggered. In advanced airbag systems, there may be sensors that detect the weight of the occupants, where they are seated, and whether they are using a seatbelt. All of these factors help the ECU decide whether to deploy the frontal airbags. The ECU also performs regular diagnostic checks and lights a warning light if anything is amiss.

This particular ECU is usually positioned in the middle of the vehicle, or under the front seat. This position protects it, especially during a crash, when it is most needed.