After researching this article, I'm still not entirely clear on the difference between an ECM (engine control module) and ECU (engine control unit). Of the two terms, ECU seems to be the most popular. Google verifies its preference for ECU by delivering more detailed search results, whereas some other sources seem to treat the ECM as a subcategory of ECUs.
In practical terms, I'm not sure if it really matters. That's the conclusion I draw when reputable sources (such as automotive repair manuals and long-established technology publications) use the terms interchangeably. One of my favorite authorities on explaining automotive intricacies to the average American, Mike Allen of Popular Mechanics, has, on occasion, skirted the issue by referring to the system as the "engine management computer." One such reference was in an article discussing the backlash against Toyota, when, in 2010, a handful of its cars were found to accelerate seemingly on their own -- in other words, he was breaking down a complicated issue for people who were curious about it, but needed the situation explained at the most basic levels. (Remember that? A bunch of Toyota owners, claimed their cars could accelerate on their own without warning and all efforts to stop them failed. The problem was blamed on faulty automatic throttle control units sending bad signals to faulty ECUs -- or something like that -- but never ultimately concluded.)
So if you found this article by searching for "engine control unit" or any other combination of related terms, I hope this clears up the confusion. And for what it's worth, Mike Allen also managed to explain, in simple terms, why the ECU on a bunch of Camrys, Corollas and Avalons simply couldn't have failed at the rate reported by the media.
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- Allen, Mike. "10 Car Mysteries Solved: Expert Car Care Clinic Q&A." Popular Mechanics. Sept. 8, 2009. (April 18, 2012) http://www.popularmechanics.com/cars/how-to/repair-questions/4330138
- Allen, Mike. "Anatomy of Toyota's Problem Pedal: Mechanic's Diary." Popular Mechanics. March 3, 2010. (April 14, 2012) http://www.popularmechanics.com/_mobile/cars/news/industry/4347704
- Allen, Mike. "Toyota Sudden Acceleration Backlash." Popular Mechanics. March 8, 2010. (April 22, 2012) http://www.popularmechanics.com/cars/news/industry/toyotarecalelectronics2010
- Delphi. "Delphi MT80 Engine Control Module." (April 14, 2012) http://delphi.com/manufacturers/auto/powertrain/gas/ecm/mt80/
- ECM--ToGo. "ECM Tech Info." 2012. (April 24, 2012) http://www.ecmtogo.com/ECMTips2.htm
- ECM--ToGo. "Removing the ECM ECU TCU electronic control module transmission control module." 2012. (April 24, 2012) http://www.ecmtogo.com/removing_ecm.htm