What Are the Differences Between ECU & PCM?


Modern automobiles have computerized components that monitor all aspects of the engine functions to maintain peak operational efficiency. Car manufacturers have focused their efforts on their vehicles being fuel efficient and having the power needed to navigate the highways and city traffic. The essential components for this task are the engine control unit or the powertrain control module.

Engine Control Unit

  • The ECU is also commonly known as the engine control module. This computer device controls various characteristics of the engine, such as variable valve timing, ignition timing, turbocharger boost, fuel injection, emission controls and other aspect of engine functions. Engine knock -- the condition in which the combustion mixture is ignited prematurely -- is controlled by the ECU retarding the ignition timing. The earliest ECUs for American cars was General Motors' Computer Command Control, which was introduced in 1979. It was installed on engines with carburetors or throttle body fuel injection systems.

Computer Command Control

  • General Motors' Computer Command Control helped its cars control emissions and improve engine performance. This unit made it possible to increase the compression ratio from 8.0-to-1 to 8.45-to-1 for its 3.0 liter V-6 engine. The ECU included a precise spark control and electronic idle speed control. The ECU helped the 3.0 liter develop as much horsepower as the larger 3.8 liter at higher rpm levels. The 3.0 also had a higher horsepower-per-pound ratio.

    In the Electronic Fuel Injection system, the ECU reads signals about manifold vacuum, engine speed and temperature. The information is digested by the ECU, which then delivers the proper amount of fuel to the engine. The precisely adjusted amount of fuel helped to control engine run-on.

Powertrain Control Module

  • A PCM controls the engine functions, like the ECU, but it is a combined control unit which also controls the powertrain. Therefore it does the work of two modules; the ECU and the transmission control unit. In Chrysler Caravan models, the PCM is connected to the vehicle's communication serial bus network. The PCM sends signals to the body control module so the gauges, warning lights and audible signals can notify the driver about the vehicle's condition. The PCM can be a combination of three or more different computers.

Modern PCM Attributes

  • Unlike the the older ECUs, modern PCMs are designed to work with the latest automatic transmissions and multi-port fuel injection systems. New PCMs help the engine and transmission management systems reduce emissions and increase fuel economy. They also help to smooth vehicle acceleration, improve cold weather starts and extend engine life. The Delphi MT86 is one such unit. The MT86 digitally controls knock through digital signal processing technology.

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