Computers and Control Systems: Description and Operation
OPERATIONElectronic Engine Control (EFI)
The Electronic Engine Control System (EEC IV) includes a Powertrain Control Module (PCM) that receives inputs from various sensors. The PCM uses this information to control fuel flow, Exhaust Gas Recirculation (EGR) and ignition. These three systems and the PCM work together to provide improved fuel economy and performance, and lower exhaust emissions.
SUBSYSTEMS AND COMPONENTS
Fuel Flow
The Electronic Fuel Injection System (EFI) is classified as a multi-point, pulse time, mass air flow fuel injection system. A metered amount of fuel is injected into the intake air stream, in accordance with engine demand, through four fuel injectors mounted on the intake manifold. The PCM determines the required fuel flow rate from various engine sensors.
The Electronic Fuel Pump
The electronic 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 the 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 that 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.
Exhaust Gas Recirculation (EGR)
(Manual Transmission Only)
The EGR Vacuum Regulator (EVR) controls the EGR valve movement. The PCM receives data from various sensors and also checks the existing valve position through the EGR Valve Position (EVP) Sensor. The PCM calculates if the present EGR flow should be increased, maintained, or decreased and determines how to operate the EGR Valve to control emissions.
Ignition
The EEC system has a special ignition control system that has no magnetic pickup or vacuum advance mechanisms. Instead, all ignition timing is controlled by the PCM.
PCM Power Relay
The PCM power relay supplies power to the PCM and EEC system-related components. When the ignition switch is turned to "RUN" or "START", voltage is applied to the PCM power relay coil, and the relay's contacts close. This allows voltage to be applied from the EEC power fuse in the engine compartment/fuse relay box, through the PCM power relay contacts to the PCM, fuel pump relay, WOT cutout relay, fuel injectors, idle air control valve, EGR Vacuum Regulator (EVR) (manual transmission only), A4LD transmission (if equipped) and Mass Air Flow (MAF) Sensor.
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 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 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. The sensor output is a DC voltage 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 for regulating the air/fuel ratio to the PCM by sensing the oxygen content of the exhaust gases. Too much oxygen indicates a lean mixture, while too little oxygen indicates a rich mixture.
Power Steering Pressure (PSP) Switch
The Power Steering Pressure (PSP) Switch is used to indicate increased engine load to the PCM. The switch will signal increased engine load to the PCM when power steering fluid pressure increases (while parking, for example). The PCM will then increase engine idle speed to prevent engine stall.
Engine Coolant Temperature (ECT) Sensor
The Engine Coolant Temperature (ECT) Sensor 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 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 calculate fuel delivery, spark timing and EGR control.
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 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.