Control Units

One of the most important developments in the automotive service industry is the use of computerized engine controls. These systems constantly adjust fuel, ignition and emission systems, resulting in better fuel economy, increased performance and reduced emissions. Another result of the computer systems is an ever-increasing load of wiring, sensors and solenoids in the engine compartment. The aim of this section is to help untangle both the computer systems and the fear with which most servicemen approach them.

The traditional "tune-up" is performed to help improve engine operating efficiency. In vehicles with computerized engine controls, the computer is constantly tuning the engine. However, the tune-up abilities of computer systems vary greatly. Some systems only affect air/fuel ratios. Others can also control things like air injection and EGR operation. On the most sophisticated models, the computer controls fuel injection, air/fuel ratio, ignition dwell and timing, air injection, EGR, canister purge and other functions.

This introduction will cover the major components of computer systems, some basic servicing tips and list some necessary tools.

The heart of any computer system is the control unit, or control module. These computers are usually sealed in a metal box and are linked to the system by a wiring harness with multi-pin connector. The computers on most Ford and Chrysler Corp. vehicles are in the engine compartment, while General Motors, AMC, and Ford EEC-III computers are located in the passenger compartment (for protection from temperature and vibration).

Some units have a replaceable programming section, referred to as a PROM (GM) or Calibration Assembly (Ford). These plug into the main computer and "tailor" the programming to fit specific emission requirements or vehicle equipment.

The function of the computer is to coordinate engine equipment and operation. During certain operating conditions, the computer operates in what is called "Closed Loop". This indicates that the computer is actually "running" the engine, based on information received from the various sensors. At other times, the computer operates in "Open Loop", where preprogrammed information in the computer is used for governing engine operation.