Computers and Control Systems: Description and Operation

Electronic Engine Control (EEC) System:

NOTE: Image is for illustration only. Specific components and systems will vary depending on vehicle and model.


GENERAL SYSTEM OPERATION
The Electronic Control Assembly (ECA) processes sensor input data and produces output control signals to regulate fuel injection quantity. ECA also produces output signals that adjust ignition timing to provide optimum driveability and fuel economy. Engine temperature information is provided by a sensor in the cooling system. The throttle switch senses throttle plate position and informs the ECA of CLOSED or WIDE-OPEN THROTTLE conditions. The oxygen sensor measures free oxygen remaining in the exhaust stream as an indication of lean or rich combustion conditions. Crankshaft position comes from a sensor 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.

DIAGNOSTIC SELF TEST
The diagnostic self test is divided into four separate tests: KEY ON ENGINE OFF (KOEO), KEY ON ENGINE RUNNING (KOER), SWITCH MONITOR test, and CONTINUOUS self test. The self test is not a final test by itself, but 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 memory integrity and processing capability, verifying various sensors and actuators are connected and operating properly. The KEY ON ENGINE OFF and KEY ON ENGINE RUNNING self tests are functional tests only, detecting faults present at the time of the self test. The SWITCH MONITOR test checks input signals sent from individual sensors and switches to the ECA. The CONTINUOUS self test is an ongoing test storing fault information in Keep Alive Memory (KAM) for retrieval at a later time.

LIMITED OPERATING STRATEGY
Limited Operating Strategy (LOS) is the ECA back-up operating mode. It maintains vehicle operation if one or more inputs fail. When sensor input is out of limits, the ECA initiates its alternate strategy. The ECA substitutes a fixed in-limit sensor value and continues to monitor the faulty sensor input. If a faulty sensor begins to operate within limits, the ECA returns to normal engine running strategy. The CHECK ENGINE light is ON when operating in LOS.