Computers and Control Systems: Description and Operation

Electronic Engine Control (EEC) System:

GENERAL SYSTEM OPERATION

The Electronic Control Assembly (ECA) processes input data from sensors and produces output control signals to the fuel injectors, regulating precise fuel delivery through the injectors. The ECA also produces output signals that adjusts ignition spark timing to provide the best balance between driveability and fuel economy. A temperature sensor inserted into the engine coolant system gives information on engine temperature. A throttle switch senses throttle plate position and produces, then transmits an electrical signal to tell the ECA when the throttle is in the CLOSED or WIDE-OPEN position. An exhaust gas sensor in the exhaust manifold measures the amount of free oxygen remaining in the exhaust stream, indicating cylinder combustion efficiency. Crankshaft position information is transmitted by a crankshaft position sensor, located in the distributor. The EEC system also controls Exhaust Gas Recirculation (EGR), A/C compressor cutoff at wide open throttle, and knock control to eliminate detonation under adverse driving conditions. The operating principles of all EEC systems and related components are essentially the same.


DIAGNOSTIC SELF TEST

The diagnostic self test is divided into four separate tests. They are the KEY ON ENGINE OFF self test, KEY ON ENGINE RUNNING self test, SWITCH MONITOR test, and the CONTINUOUS self test. The self test is not a final test by itself, but is used as a part of the functional check. The processor stores the self test program in its permanent memory. When activated, it checks the EEC system by testing its memory integrity and processing capability, and verifies that various sensors and actuators are connected and operating properly. The KEY ON ENGINE OFF and KEY ON ENGINE RUNNING self tests are functional tests that only detect faults present at the time of the self test. The SWITCH MONITOR test checks the input signals sent from the engine control system's individual sensors and switches to the Electronic Control Assembly (ECA). The CONTINUOUS self test is an ongoing test that stores fault information in Keep Alive Memory (KAM) for retrieval at a later time.


LIMITED OPERATING STRATEGY

Limited Operating Strategy (LOS) is a back-up strategy in the Engine Control Assembly (ECA) designed to maintain vehicle operation if one or more inputs fail. When a sensor input is perceived to be out of limits by the ECA, an alternative strategy will be initiated. The ECA substitutes a fixed in-limit sensor value and continues to monitor the faulty sensor input. If the faulty sensor begins to operate within limits, the ECA will return to normal engine running strategy. When the vehicle is operating in LOS, the CHECK ENGINE light will be ON.