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 or electronic control module, 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, where the ECM only controlled certain parts of the engine.
Some of older automobile models had two computers, the ECM and the TCM. The ECM 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.