Computers and Control Systems: Description and Operation

DESCRIPTION
The Electronic Engine Controls (EEC-IV) system uses input sensors to assess engine parameters, driver demand and environmental conditions. The Powertrain Control Module (PCM) compares this input information with preprogrammed operating strategies to provides accurate, optimum control of fuel injection timing and duration.

The EEC-IV system also uses the inputs and operating stratagies to control:
^ Idle speed.
^ A/C compressor cutoff at wide open throttle.
^ Knock control (to eliminate detonation and ping).
^ Fuel pump operation.
^ Transmission control.
^ Emission related components;
- Exhaust Gas Recirculation (EGR).
- Evaporative canister purge.

INPUTS
- Heated Oxygen Sensor (HO2S) - Provides oxygen content of exhaust for fuel control feedback
- Engine Coolant Temperature (ECT) - Provides engine operating temperature data.
- Intake Air Temperature (IAT) - Provides air temperature information for proper fuel mixture.
- Mass Air Flow (MAF) - Determines engine load.
- Throttle Position (TP) - Determines driver demand.
- Camshaft Position (CMP) - Provides engine speed and fuel injection timing data
- Cylinder Identification (CID) Provides cylinder sequencing information for fuel injection timing.
- EGR Valve Position (EVP) - Determines the amount of EGR valve opening for EGR feedback control, ignition timing, and corrections to the MAP signal.
- Brake On/Off (BOO) - Provides engine load and driver demand information for control of idle speed.
- Clutch Pedal Position (CPP) - Provides engine load and driver demand information for control of idle speed.

NOTE: For a more complete description of these inputs see component level descriptions.

OPERATION
The PCM operates in a continuous cycle in which it:

Senses Inputs
The PCM continuously monitors the sensor inputs to determine the operating conditions and driver demand.

Makes Decision
The PCM then compares this information to current operating strategy (stored within the permanent memory of PCM). From this comparison the PCM adjusts the fuel injection pulse width, spark advance, EGR flow, and idle speed.

Checks Results
The PCM uses the HO2S (fuel control), Knock sensor (timing advance), EVP (EGR flow), and camshaft positions sensor (engine speed) to determine the effectiveness of its previous adjustment. The cycle then repeats itself.

ALTERNATIVE OPERATING STRATEGIES

Adaptive Strategy
The EEC-IV system constantly monitors and adjusts the PCM calibration to correct for normal wear and aging of components. The PCM uses these calibration adjustments to shift fuel delivery calculations and idle speed specifications to compensate for the aging of the vehicle.

Failure Modes Effect Management
Failure Modes Effect Management (FMEM) is a back-up strategy 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 Malfunction Indicator Light (MIL) will be "ON" and a code 998 will be displayed after the KOEO 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 sensing, comparison, and decision process has stopped and the PCM is running on pre-programmed control only. The default strategy for this mode has a minimal calibration, and used strictly to allow the vehicle to operate until it can be serviced.

NOTE: In HLOS mode the self-test trouble codes will not be output.

TESTING
The EEC-IV diagnostic system is divided into three separate Self-tests.

Continuous Self-Test
This self-test operates continuously while the key is "ON". This self-test checks sensor inputs for proper specifications and checks outputs for proper voltages. If any parameters are out of specification it stores a trouble code. It will keep this trouble code for 40-80 key cycles after the condition is cleared.

Key On Engine Off (KOEO) Self-Test
This test must be initiated by the technician. The KOEO self-test is similar to the Continuous self-test but this test also cycles output actuators and solenoids and at the conclusion of the test it outputs all On Demand (hard) trouble codes and all trouble codes stored in memory.

Key On Engine Running (KOER) Self-Test
This test must be initiated by the technician. With the engine running the PCM checks all operating parameters and its ability to regulate them. At the conclusion of this test the PCM outputs all On Demand (hard) trouble codes. The KOER self-test does not output trouble codes stored in memory.

NOTE: For further information on these self-tests see Testing and Inspection / Procedures / Quick Test Description.