What Is the Difference Between a PCM and ECM?

Many people, including professionals, get confused by all the acronyms used in automobile mechanics. A PCM or powertrain control module is different from an ECM or engine control module or electronic control module (sometimes referred to as an electronic control unit or engine control unit ECU), even though most people believe the two are the same. Most modern or newer models only have a PCM, which controls all the engine functions and diagnostics, where the ECM only controlled certain parts of the automotive engine.

ECM

Some of older automobile models had two computers, the ECM and the TCM. The engine computer controlled the engine function, but the transmission was controlled by a different computer system called the transmission control module, or TCM. The ECM was also used with vehicles that operated with a manual transmission because there was no need for a computer to operate the transmission functions.

PCM

The PCM is the main computer in the automobile that controls all the functions of the vehicle. Everything from the vehicle's ignition timing to the proper flow of fuel into the fuel injection system, to the emissions, to the cruise control, to the throttle position, to the air conditioning/cooling system is under the control of the PCM. Along with the engine functions, the PCM controls the functions of the automatic transmission, such as automatic shifting when vehicle is accelerated or downshifting when the vehicle slows down to maximize the engine performance, fuel efficiency and fuel economy. The PCM does not have as much control of a manual transmission and in that type of vehicle will not necessarily offer better performance.

Repairs

Most modern vehicles, vehicles built after 1996, contain computer modules like the PCM, and it is a very reliable engine component. When the PCM does fail, it can easily exceed $1,000 for replacement, depending on the type of automobile. ECMs do not cost as much because they only found in older models and their function is limited. If a problem is occurring in the automobile, it probably is not the TCU (transmission control unit), BCM (body control module), PCM or ECM. A bad battery, fuel filter, bad injectors or burnt ignition coils can be the culprit that causes the check engine light to turn on rather than damage to the PCM or ECM. Once the PCM or ECM is damaged, it must be replaced.

Failure Signs

The automobile will operate differently when the ECM or PCM has failed. Some of the first systems of failure are that the engine will cough, choke or stall, because the PCM is not regulating the fuel mixture going into the fuel injectors. Since the ignition spark and crankshaft position are controlled by the PCM, the vehicle will not start if the computer is not working to control these functions. Almost every warning light on the instrument panel will begin to turn off and on randomly. The computer's monitor systems, such as the ABS and Check Engine warnings, alert the operator. When the PCM or ECM fails, the system is not getting the correct information and this causes all the warning lights to flicker on and off.