Computers and Control Systems: Description and Operation

EEC Electrical Schematic:

GENERAL SYSTEM OPERATION
This computerized fuel, ignition and emission control system is referred to as EEC-IV (Electronic Engine Control). The central component of this system is the Electronic Control Assembly (ECA), a microprocessor containing specific calibrations designed to optimize fuel economy and driveability while maintaining low emissions. Based on information received from a number of sensors and other electronic components (switches, relays, etc.) and values programmed into its memory, the ECA generates output signals to control various relays, solenoids and other actuators. The ECA is located behind the right side kick panel.

The ECA judges air/fuel ratio as rich or lean according to a voltage signal generated by a heated oxygen sensor. Based on oxygen sensor and other input data, the ECA generates a controlling signal to the fuel injectors that regulates "ON" time (injectors are normally closed). Because fuel pressure remains constant, the ECU controls air/fuel ratio adjustments by varying injector "ON" time.

DIAGNOSTIC SELF TEST
The diagnostic self test is divided into three separate tests: KEY ON ENGINE OFF self test, ENGINE RUNNING self test, and CONTINUOUS self test. Self test is not a conclusive 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-IV 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 (KOEO) and Engine Running (ER) self tests are functional tests which only detect faults present at the time of the self test. Continuous self test is an ongoing test that stores fault information in Keep Alive Memory (KAM) for later retrieval.

FAILURE MODES EFFECT MANAGEMENT
To provide a margin of safety by maintaining driveability in the event of certain system failures, Failure Mode Effects Management (FMEM) is initiated by the ECA should one or more sensor inputs fail. When a sensor is perceived to be out of specified limits by the ECA, it will substitute a fixed sensor value and continue to monitor the faulty input. If the sensor returns to programmed operational parameters, the ECA will revert to normal control. ECA memory will set trouble code 98 when FMEM is in effect and the "CHECK ENGINE" light will remain "ON." "CHECK ENGINE" light is also referred to as the Malfunction Indicator Lamp (MIL).

INPUTS AND OUTPUTS
Components can be grouped as ECA input and output devices. Input devices control or produce voltage signals monitored by the ECA. Output devices are components controlled by the ECA.

Input Devices
^ Coil tach signal (IDM)
^ EEC power relay
^ Engine coolant temperature sensor (ECT)
^ Air charge temperature sensor (ACT)
^ Heated exhaust gas oxygen sensor (HEGO)
^ Barometric absolute pressure sensor (BAP)
^ Clutch interlock three function switch (M/T)
^ Brake on/off switch (BOO)
^ Mass air flow sensor (MAF)
^ Neutral drive switch (A/T)
^ Neutral sensing switch (M/T)
^ Throttle position sensor (TP)
^ Vehicle speed sensor (VSS)

Output Devices
^ A/C cutout
^ Canister purge solenoid (CANP)
^ "CHECK ENGINE" light
^ Fuel pump relay (FP)
^ Idle speed control (bypass air)
^ Injectors bank #1
^ Injectors bank #2
^ Spark angle word (SAW)
^ Converter clutch and shift solenoids (A/T)