Generally, ECM, PCM, and ECU refer to the same thing - an Engine Computer. However, there is a fine line between these terms. In most occasions the term ECU applies to Asian car manufacturers, whereas ECM and PCM usually represent to Chrysler computers. Some General Motors sub-brands use the term "PCM" on their products as well. From an engineering standpoint, these terms denote different things as well.
ECM/ECU
ECM or Electronic Control Module (sometimes referred to as an “ECU” - Engine Control Unit) receives incoming data from various sensors to alter the function of electronic systems. Most of the times, it is dedicated to an engine electronics only - all its' electronics and sensors. In this case, the vehicle would have additional computers responsible for other operations in it (e.g. AC, power windows, gearbox).
The first ECMs were introduced in the early 80's. The only thing they were capable of controlling back in the day, was the fuel injection system. But as car electronics have learned, the function of the Engine Control Module has expanded to almost every phase of operation. Modern engines are equipped with ECM modules that gather information from the intake, exhaust, cooling system and some other internal components to estimate the running condition of the engine. Based on collected data, it sets the position of the camshafts, throttle; it sets up the ignition/injection timing.
TCM
Some vehicles are also equipped with a TCM (Transmission Control Module). This unit determines if the vehicle's transmission needs to shift. Over the years, these devices have had their functionality vastly improved as well. These days, not only does it track the input from the transmission, but also provides information about the cruise control, throttle, and traction control. Thus, it can decrease wheel spinning and gear hunting. Automatic transmissions and even dual clutch manual transmissions are coming with TCM modules installed, whereas the traditional manual gearbox does not. Manual transmissions have electric functions controlled by the ECU.
PCM = ECM + TCM
PCM (Powertrain Control Module) on the other hand controls several different aspects of the vehicle. Usually, it's Engine & Transmission. Sometimes, it may even be the steering and emissions systems, too. By allocating control of all the systems in one unit, the PCM can better regulate their functions (ex. when the PCM commands to change gears, it can pull back the throttle to get the shift smoother). The result is better fuel economy and power delivery.
Even though a PCM may seem like a single assembly from the outside, most of them combine a separate ECU and TCM, each of them has a dedicated processor and programming memory. They operate independently throughout the unit operations, sharing data in case a certain function expects both systems to work in tandem. It also has a great benefit for tuners, because it means they can program one system and leave another unaffected.
Conclusion
PCM and ECM happen to be the most common terms for the actual engine computer. ECU is a general term that can mean anything driven by a computer since it stands for "Electronic Control Unit". In the automotive electronics industry, ECU is used as a generic term for any built-in system that controls multiple electrical features in a car. It usually includes:
- Body Control Module (BCM),
- Brake Control Module (BCM or EBCM),
- Central Control Module (CCM),
- Central Timing Module (CTM),
- Electronic/Engine Control Module (ECM),
- General Electronic Module (GEM),
- Powertrain Control Module (PCM),
- Suspension Control Module (SCM),
- Transmission Control Module (TCM), etc.
Sometimes one assembly consolidates several different systems from this module list. Most modern cars have up to 80 different ECU Units.
