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What is the Engine Control Unit (ECU) and how does it work?

What is an Engine Control Unit (ECU)?

What is an ECU?

The use of the term ECU in the automotive industry refers to an Engine Control Unit (ECU) or an Engine Control Module (ECM), which is a component of any automotive mechatronic system, and not just for the control of an engine.

It can also be called the Powertrain Control Module (PCM), if it controls both an engine and a transmission.

For the purposes of this discussion, we shall focus on the Engine Control Unit.

What does an ECU do?

Fundamentally, the ECU controls the injection of the fuel and, in petrol engines, the timing of the spark to ignite it. It determines the position of the engine’s internals using a Crankshaft Position Sensor so that the injectors and ignition system are activated at precisely the correct time. While this sounds like something that can be done mechanically (and was in the past), there’s now a bit more to it than that.

An internal combustion engine is essentially a big air pump that powers itself using fuel. As the air is sucked in, enough fuel has to be provided to create power to sustain the engine’s operation while having a useful amount left over to propel the car when required. This combination of air and fuel is called a ‘mixture’. When there is too much mixture, the engine will be full throttle, when its too little, the engine will not be able to power itself or the car. Not only is the amount of mixture important, but the ratio of that mixture has to be correct. With too much fuel and too little oxygen, the combustion is dirty and wasteful. While with too little fuel and too much oxygen makes the combustion slow and weak.

Vehicle engines used to have the mixture quantity and ratio controlled by an entirely mechanical metering device called a carburetor, which was little more than a collection of fixed diameter holes (jets) through which the engine ‘sucked’ the fuel. With the demands of modern vehicles focusing on fuel efficiency and lower emissions, the mixture must be more tightly controlled. The only way to meet these strict requirements is to hand over control of the engine to an Engine Control Unit (ECU). The ECU has the job of controlling the fuel injection, ignition, and ancillaries of the engine using digitally stored equations and numeric tables rather than by analogue means.

  • Check Engine Light is illuminated.
  • Engine misfires
  •  Lower engine performance 
  • Car fails to start

Before replacing the ECM, extensive diagnostics should be conducted to determine it as the root cause.

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