Computers and Control Systems: Description and Operation

GENERAL SYSTEM OPERATION

The EEC-IV system provides accurate, instantaneous fuel metering control of fuel injection timing and duration. A sensor in the intake manifold measures the temperature of incoming air and transmits a corresponding electrical signal. Another temperature sensor inserted into the engine cooling system gives information on engine temperature. A switch senses throttle plate position and produces, then transmits an electrical signal to tell the control unit when the throttle is in the CLOSED or WIDE-OPEN position. An oxygen sensor in the exhaust manifold measures the amount of free oxygen remaining in the exhaust gas, which is an indication of cylinder combustion efficiency and sends a corresponding signal to the control unit. Crankshaft position information is transmitted by a crankshaft position sensor, located either on the crankshaft or in the distributor.
The Powertrain Control Module (PCM) processes the input data from these sensors and produces output control signals to the fuel injectors, regulating precise fuel discharge through the injector nozzles. The microcomputer circuit also produces output signals that adjust ignition spark timing to provide the best balance between driveability and fuel economy.
The EEC-IV 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-IV systems and related components are essentially the same. However, in adapting the EEC-IV system to different engines, different combinations of components are required.


DIAGNOSTIC SELF TEST

The diagnostic self test is divided into three separate tests. They are the KEY ON ENGINE OFF self test, the KEY ON ENGINE RUNNING self test, and the CONTINUOUS self test. The 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 Key On Engine Running (KOER) 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 retrieval at a later time.

FAILURE MODES EFFECT MANAGEMENT

Failure Modes Effect Management (FMEM) is a back-up strategy in the Powertrain Control Module (PCM) designed to maintain vehicle operation if one or more inputs fail. When a sensor input is perceived to be out of limits by the PCM, an alternative strategy will be initiated.
The PCM will substitute a fixed in limit sensor value and will continue to monitor the faulty sensor input. If the faulty sensor begins to again operate within limits, the PCM will return to normal engine running strategy.
When the vehicle is operating in FMEM, the CHECK ENGINE LIGHT will be ON and a code 98 or 998 will be displayed during self test.

HARDWARE LIMITED OPERATION STRATEGY

IF the EEC-IV system faults are too extreme for the FMEM mode to handle it will go into Hardware Limited Operation Strategy (HLOS). In HLOS mode the software operations have stopped and the PCM is running on hardware control only. The default strategy for this mode has a minimal calibration strictly to allow the vehicle to operate until it can be serviced.

NOTE: In HLOS mode the self test Diagnostic Trouble Codes (DTCs) will not be output.