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ECM's and PCM's are similar. An ECM is an Engine Control Module, a PCM is a Powertrain Control Module. Earlier models are sometimes called ECU's (Electronic Control Unit) or CCC (Computer Command Control). The main difference is an ECM controls just the engine functions, whereas a PCM controls the transmission as well. 1980 was the first year of an ECM. 1980-1993 models use a removal PROM (chip) inside the ECM, which contains the programming parameters. 1994 and newer models use a flash memory (PCM is programmed via serial cable), which contains vital information like transmission type, gear ratio, whether the vehicle has traction control, etc. With 1993 and older ECM's, you can simply replace your ECM with one of the same service # (or compatible service #), and install your existing PROM and your vehicle will function as it should. With 1994 and newer models, not only do you need to replace the PCM with one of the same service #, but you must also replace it with the same broadcast/programming code. The programming code (typically 4 characters), contains the identity of the programming. If you replace a PCM with one with a different programming code, your transmission may not function, speedometer may not work, etc, and will require re-flashing by a qualified technician.