See Figures 1, 2 and 3
The electronic distributor ignition system is found on all 2.5L, 3.9L, 5.2L, 5.9L Light Duty Cycle (LDC) and 5.9L Heavy Duty Cycle (HDC) engines. Therefore, unless otherwise noted, the procedures in this section pertain to all engines. The only engine which does not make use of the electronic distributor ignition system, the V10 HDC engine, is covered under "Distributorless Ignition Systems.''
The engine controller is an on-board digital computer which automatically advances or retards the spark to fire the spark plugs according to different engine operating conditions. It has a built in microprocessor which continually receives input from various engine sensors. Input data is fed to the computer by a the sensors located in the engine compartment. These sensors monitor timing, water temperature, air temperature, idle/off-idle operation and intake manifold vacuum. The program schedule module of the engine controller receives the information from the sensors, processes it, then directs the ignition control module to advance or retard the timing as necessary. This whole process goes on continuously as the engine is running, taking only milliseconds to complete a circuit from sensor to distributor.
Although, for the models covered by this guide, the engine controller performs essentially the same function, it has over the years been called a few different names. In 1989 models, the manufacturer called it the Single Module Engine Controller (SMEC); for 1990-91 models it was called the Single Board Engine Controller (SBEC); for 1992 it was simply known as the Engine Controller, and since 1993 it is called the Powertrain Control Module (PCM). The unit is found in the engine compartment mounted to the left or right side inner fender or the firewall.