What is a car brain box & functions of ECU and PCM


Posted by: Joshua-Philip Okeafor

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What exactly does a brain box do in your vehicle? And does it matter whether it is ECU or PCM? What do they mean? Check all your answers here!


Cars brain boxes (ecu or pcm) date back to a period as old as the invention of cars. They are automated electronic devices that combine microprocessors for the monitoring and control of various functions in a vehicle.

The brain boxes in most cases are specialized devices focusing on controlling multiple functions in a vehicle e.g. cruise control, automatic gear shifting, climate control, engine running, fuel injection/emission control, collision avoidance, powered mirror, seat & rear mirror controls, etc.

The brain boxes, be it an Engine Control Unit (ECU) otherwise known as an Engine Control Module or Powertrain Control Module (PCM) are usually custom built by different car manufacturers, with usually multi-purpose abilities. Let's further check them out:


The brain box is correctly named as it controls all electronic systems in the car 

Overview of the Engine Control Unit (ECU)

The Engine Control Unit (ECU) otherwise called the ECM is a processer that electrically takes incoming sensor data and uses that information to alter the function of electronic systems for better performance.

ECUs can also be said to be embedded systems in automotive electronics that controls one or more of a vehicle’s electrical systems or sub-systems.

ECUs were first invented in the early 1980s and could only control fuel injection. As electronics and engine design keeps improving, its function has spread to every aspect of operation in cars.

In recent builds of engines, the ECU gathers information from the intake, exhaust, cooling system and several internal components to oversee the running condition of the engine. Hence, it can decide the situation of the camshafts, the throttle, ignition timing, fuel injection timing and wastegate pressure.


ECU or engine control unit oversees the running of engine components  

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Overview of the Powertrain Control Module (PCM)

The powertrain control module (PCM) is also an automotive component and control unit, used on motor vehicles. It generally combines controls of engine control unit (ECU) and the transmission control unit (TCU). The PCM is the main computer in the vehicle that control all its functions; for example, when the PCM decides to change gears, it can back off the throttle to make the shift smoother.

The introduction of the PCM dates back to the late 1970s, but its official usage was recognized in the 1980s in conjunction with electronic controlled carburetors and lockup torque converters.

The PCM senses and monitors various of the following factors in a vehicle:

  • It monitors the vehicle’s performance by using sensors such as temperature, oil and fluid level, air intake, emission level, camshaft angle etc. In failing to restore them to ideal levels, the engine warning light appears.
  • By controlling ignition timing, the PCM directs the speed of firing spark plugs, helping the vehicle’s engine deliver an optimum amount of power per RPM.
  • It adjusts the air-to-gas ratio being pumped into the vehicle’s cylinders which automatically saves fuel;
  • It ensures that the vehicle’s idle speed is not raced up to an unnecessary RPM, thereby managing any ancillary activities such as air conditioning.


The PCM is the car's principal computer, and integrates all systems in the vehicle

What relates the ECU and PCM?

The ECU and PCM tends to confuse owing to these factors:

  • Be it ECU or PCM, the vehicle manufacturers stand them out through their mode of configuration.
  • Be it ECU or PCM, it can be ascertained through modern or older vehicles. Older automobile models had two computers, the ECM and the TCM. The ECM controlled the engine function and the TCM controlled the transmission. For cars with a manual transmission, they only had an ECM because there was no need for a computer to operate the transmission functions.
  • Be it ECU or PCM, the term Electronic control unit is often used as a common term for any built-in system that controls multiple electrical features in a car. This again depends on the vehicle’s manufacturer.

Functions of Car Brain Boxes (ECU or PCM)

The brain box (be it ECU or PCM) performs the following functions;

  • It determines what position the gas pedal is at while idling;
  • It controls the car’s transmission, telling it when to switch gears;
  • It controls all the accessories in your vehicle (windows, door-locks, sunroof etc);
  • The major function of a car’s brain box is the monitoring of a series of actions to register errors with how it is operating thereby noticing the problem and generating the trouble code, and subsequently causing the check engine light to come on.

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Many systems are activated or deactivated through impulses from the brain box, either pcm or ecu

Repairing/ Replacing the ECM or PCM

When your vehicle starts developing the underlisted problems and its root mechanical causes seem hard to find, it’s possible the brain box (ECU or PCM) needs repair or replacement in most cases.

  • Check the engine light when it goes on and off and doesn’t seem to correspond to an actual mechanical problem.
  • Stalling, lose of power, and misfiring of engine may occur without any pattern to frequency or severity.
  • Check for engine performance issues when you notice a reduction in fuel efficiency, power, and acceleration.
  • When the vehicle does not start or is difficult to start. The engine may still crank but won’t be able to start without vital inputs from the computer.

Always make sure that all mechanical reasons are eliminated for your car’s performance issues before deciding to repair or replace your brain box (ECM or PCM).

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Joshua-Philip Okeafor

Joshua-Philip Okeafor

Car buying & selling

Joshua, or KK as friends call him, is a Filmmaker, Writer and Director. A Christian, Joshua is a product of Nigeria’s foremost film school, the National Film Institute, Jos, where he majored in Writing/Directing. Joshua began his writing career at age 18 when an older brother gave him a four page outline of a children novel. Joshua intends to keep writing and directing. His screen name is sometimes Joshua Kalu Ephraim (Writing), and sometimes Joshua KK (directing).

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