Computers and Control Systems: Description and Operation
OPERATIONElectronic Engine Control (EFI)
The Electronic Engine Control System (EEC IV) includes the Powertrain Control Module (PCM) that receives inputs from various sensors. The PCM uses this information to control fuel flow and ignition. These two systems and the PCM work together to provide improved fuel economy and performance, and lower exhaust emissions.
SUBSYSTEMS AND COMPONENTS
PCM Power Relay
The PCM power relay supplies power to the PCM and EEC system-related components. When the ignition switch is turned to "START" or "RUN", voltage is applied to the PCM power relay coil, and the relay's contacts close. This allows voltage to be applied from EEC power fuse in the engine compartment fuse/relay box, through the relay's contacts to the PCM, fuel pump relay, WOT cutout relay, fuel injectors, idle air control valve, canister purge solenoid, A4LD transmission (if equipped), Mass Air Flow (MAF) Sensor, and canister purge solenoid.
Fuel Flow
The Electronic Fuel Injection System (EFI) is classified as a multi-point, pulse-time fuel injection system. A metered amount of fuel is injected into the intake air stream in accordance with engine demand through six fuel injectors mounted in the lower manifold near the intake ports. The PCM determines the required fuel flow rate from various engine sensors.
The electric fuel pump supplies fuel under pressure to the fuel injectors. When the ignition switch is turned to "START" or "RUN", voltage is applied from the PCM power relay to the fuel pump relay coil. The coil is grounded by the PCM, and the relay's contacts close. Voltage is now applied from fuel pump fuse in the engine compartment fuse/relay box, through the relay's contacts and the inertia fuel shutoff switch to the fuel pump. The inertia fuel shut-off switch is a safety device which cuts off voltage to the fuel pump in the event of a collision. Once the inertia fuel shut-off switch opens, it must be reset manually.
Ignition
The ignition system contains an ignition control module which is mounted on the side of the distributor. Two signal circuits, 395 (GY/O) and 929 (PK), and a ground, 259 (0), are connected between the Ignition Control Module (ICM) and the PCM.
Mass Air Flow (MAF) Sensor
The Mass Air Flow (MAF) Sensor directly measures the mass of the air flowing into the engine. The sensor output is used by the PCM to calculate the injector pulse width for proper air/fuel ratio.
A4LD Transmission (if Equipped)
The A4LD transmission is an electronically controlled four speed automatic transmission. The PCM uses inputs from various sensors to control the operation of the A4LD transmission.
The A4LD transmission has self-test capabilities much like those in other electronic control systems.
The PCM power relay supplies power to the PCM and PCM System-related components. When the ignition switch is turned to "RUN" or "START", voltage is applied to the PCM power relay coil, closing the relay's contacts. Voltage is supplied to the shift solenoid and the Torque Converter Clutch (TCC) Solenoid, through the PCM power relay.
The Throttle Position (TP) Sensor is a potentiometer. The sensor output is a DC voltage that varies with throttle angle. By monitoring the TP sensor output and other sensors, the PCM calculates the proper transmission line pressure, shift scheduling, and torque converter clutch.
The Profile Ignition Pickup (PIP) signal is produced by Electronic Ignition (EI). It sends RPM and crankshaft position information to the PCM to determine shift scheduling and torque converter operation.
The shift solenoid provides gear selection of third and fourth gears by controlling the pressure to the shift valves.
The Torque Converter Clutch (TCC) Solenoid provides the torque converter clutch control by shifting the converter clutch control valve to apply or release the torque converter clutch.
The Vehicle Speed Sensor (VSS) generates an AC signal that is proportional to vehicle speed.
The PCM uses this speed signal and other inputs to determine the shift scheduling and converter clutch control.
The Brake On/Off (BOO) Switch is used to prevent converter clutch operation when the brake has been depressed.
This input is ignored if the Throttle Position Sensor (TPS) indicates more than one third throttle position.
The Engine Coolant Temperature Sensor (ECT) is a thermistor in which resistance decreases as engine coolant temperature increases. The PCM measures the voltage drop across the engine coolant temperature sensor and uses this information to help calculate fuel delivery, spark timing and EGR control and clutch converter operation.
Idle Air Control (IAC) Valve
The Idle Air Control (IAC) Valve controls engine idle speed by regulating the amount of air allowed to pass around the throttle plates. This permits the PCM to make idle speed corrections to prevent engine stall during cold engine warm-ups as engine load changes.
Throttle Position (TP) Sensor
The Throttle Position (TP) Sensor is a potentiometer with a DC voltage output that varies with throttle plate angle. By monitoring the Throttle Position (TP) Sensor output, the PCM calculates fuel delivery requirements based on driver demand.
Heated Oxygen Sensor (HO2S)
The Heated Oxygen Sensor (HO2S) provides a voltage signal to the PCM for regulating the air/fuel ratio by sensing the oxygen content of the exhaust gases. Too much oxygen indicates a lean mixture, while too little oxygen indicates a rich mixture.
Engine Coolant Temperature (ECT) Sensor
The Engine Coolant Temperature Sensor (ECT) is a thermistor whose resistance decreases as engine coolant temperature increases and increases as engine coolant temperature decreases, Negative Temperature Coefficient (NTC). The PCM measures the voltage drop across the ECT and uses this information to calculate fuel delivery. spark timing, and EGR control.
Intake Air Temperature (IAT) Sensor
The Intake Air Charge Temperature (IAT) Sensor is a thermistor whose resistance decreases as manifold air temperature increases and increases as manifold air temperature decreases, Negative Temperature Coefficient (NTC). The PCM measures the voltage drop across the IAT and uses this information to let the PCM know what conditions must be met.
Vehicle Speed Sensor (VSS)
The Vehicle Speed Sensor (VSS) generates an AC signal that is proportional to vehicle speed. The AC signal is sent to the PCM and is used for engine management and speed control (if equipped).
Data Link Connector (DLC) And Self-test Input (STI) Connector
These two self-test connectors are used for diagnostic procedures pertaining to the PCM. The PCM stores a self-test program in its permanent memory. When activated, it checks the EEC System by testing the PCM and verifies that various sensors and actuators are connected and operating properly.
Canister Purge
The carbon canister collects fuel vapors from the fuel tank and other components to be burned later in the engine. The PCM controls the canister purge solenoid and grounds the solenoid, releasing fuel vapors to be burned in the engine.
Pressure Feedback EGR (PFE) Sensor
The Pressure Feedback EGR Sensor is a transducer that monitors the pressure drop across an orifice that EGR flows through. The PCM uses this feedback signal from the transducer to control EGR flow.
EGR Vacuum Regulator (EVR) Solenoid
The PCM controls exhaust gas recirculation by varying the voltage applied to the EGR Vacuum Regulator (EVR) Solenoid. The EVR then regulates the vacuum level present at the EGR Valve to control exhaust gas flow into the intake manifold.