Fuel Delivery and Air Induction: Description and Operation
Powertrain Control Module:
GENERAL DESCRIPTION
The Computer Command Control System (C3) for vehicles with port fuel injection is a computer controlled multiport electronic fuel injection, ignition and emission control system. The heart of the system is a computer referred to as a "Powertrain Control Module" or PCM (in previous years referred to as Electronic Control Module - ECM.) The PCM monitors operating conditions like engine speed/load and air/fuel ratio through various engine sensors, including an exhaust gas oxygen sensor, and controls the following:
^ Fuel Delivery (pump) and Fuel Injection System
^ Ignition / Electronic Spark Timing (EST)
^ Electronic Spark Control (ESC - knock sensor)
^ Air Conditioning Compressor Clutch
^ Engine Cooling Fan(s)
^ Secondary Air Injection
^ Exhaust Gas Recirculation (digital EGR solenoid valves)
^ Idle Speed Control
^ Evaporative Emission Control System (purge control)
^ Torque Converter Clutch. (TCC)
^ Electronically Controlled Automatic Transmission (shift control)
The PCM has two serviceable parts. A controller, called a PCM, and a MEM-CAL (Memory Calibration unit), which also contains the fail-safe programming to operate the vehicle if the PCM or other vital system fails.
ON-BOARD SELF DIAGNOSTICS - FAULT RECOGNITION
The computer can recognize problems with sensors, output devices and circuits, and alert the driver by turning on the "SERVICE ENGINE SOON" light on the instrument panel. Problems in specific devices or circuits are identified by "trouble codes", which are stored in the computers memory when a problem is detected in a computer controlled device or circuit. Trouble codes can be read using a "SCAN" tool or by entering the PCM into diagnostic mode and counting the flashes of the SES light. See DIAGNOSIS, TESTING AND INSPECTION PROCEDURES/TESTING PROCEDURES/ON-BOARD DIAGNOSTICS for display procedures and trouble code descriptions.
ADAPTABILITY
The system has a learning ability which allows it to make corrections for minor variations in the fuel system to improve driveability. The computer stores actual sensor signal values in its memory during normal operation and uses them as the default values when the engine is started again. This ensures good performance and driveability each time the vehicle is driven, even if conditions are changing. If the battery is disconnected to clear trouble codes or for repair, the learning process has to begin all over again, and a change may be noticed in the driving performance of the vehicle. To reset the vehicles learning ability, make sure the engine is at operating temperature and operate the vehicle at part throttle, moderate acceleration, and idle conditions, until normal performance returns.
MODES OF OPERATION
The ECM monitors information from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called "modes." All the modes are controlled by the ECM and are as follows.
Starting Mode:
When the ignition is first turned "ON", the Electronic Control Module (ECM) turns "ON" the fuel pump relay for two seconds, and fuel pump builds up pressure. The ECM also checks the coolant temperature sensor and the throttle position sensor, and determines the proper air/fuel ratio for starting. This ranges from approximately 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) engine coolant temperature. The ECM controls the amount of fuel delivered by changing how long the injectors are turned "ON" and "OFF". This is done by pulsing the injectors for very short times.
Clear Flood Mode:
If the engine floods, it may be cleared by pushing the accelerator pedal down all the way. When the throttle position is greater than 80% during cranking, the ECM completely turns OFF the fuel. No fuel is delivered from the injector as long as the throttle position is greater than 80% and the engine rpm is below 600. If the throttle is released to less than 80%, the ECM returns to starting mode.
Run Mode:
The run mode is either "Open Loop" or "Closed Loop". When the engine is first started and the rpm is above 400, the system goes into "Open Loop" operation. In "Open Loop," the ECM ignores the signal from the O2 sensor, and calculates the air/fuel ratio based on inputs from the Coolant Temperature Sensor (CTS) and the Intake Air Temperature (IAT) sensor, formerly called the Manifold Air Temperature (MAT) sensor. The system remains in "Open Loop" until the following conditions are met:
1. The O2 sensor has a varying output, showing that it is hot enough to operate properly.
2. The Coolant Temperature Sensor (CTS) is above a specified temperature.
3. A specified amount of time has elapsed after starting the engine.
The specific values for the above conditions vary with different engines, and are stored in the memory calibrator (MEM-CAL) portion of the ECM. When these values are met, the system goes into "Closed Loop" operation. In "Closed Loop," the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the O2 sensor. This allows the air/fuel ratio to stay very close to 14.7:1.
Acceleration Mode:
When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent possible hesitation, the ECM increases the pulse width to the injectors to provide extra fuel during acceleration. The amount of fuel required is based on throttle position, manifold air pressure and engine speed.
Fuel Cutoff Mode:
During normal operating temperature, to prevent possible engine damage from over-speed, the ECM cuts off fuel from the injectors when the engine speed exceeds a specified speed, to prevent tire damage.
^ With A/T - 6500 rpm in any forward gear, or in reverse. (3000 rpm in neutral.)
^ With M/T - 7000 rpm in any forward gear, or in reverse. (No cutoff in neutral.)
^ With touring tread tires - 115 mph.
^ With high performance tires - 124 mph.
During cold weather operation, on vehicles equipped with automatic transaxle, the shift points are altered to prevent engine over-speed. Fuel is also cutoff during rapid deceleration.
Deceleration Mode:
When the driver releases the accelerator pedal, air flow into the engine is reduced. The corresponding changes in throttle position and manifold air pressure are relayed to the ECM, which reduces the injector pulse width, to reduce fuel flow. If decel is very rapid, or for long periods (such as long closed throttle coast-down), the ECM shuts fuel completely OFF, to protect the catalytic converter.
Converter Protection Mode:
The ECM constantly monitors engine operation, and estimates conditions that could result in high converter temperatures. If the ECM determines the converter may overheat, it causes the system to return to "Open Loop" operation and enriches the fuel mixture.
Battery Voltage Correction Mode:
When battery voltage is low, the ECM can compensate for the weak spark by:
^ Increasing the amount of fuel delivered.
^ Increasing idle rpm.
^ Increasing ignition dwell time.
FAIL-SAFE OPERATION
The control module also has a built in back-up system contained in its Memory Calibration (MEM-CAL) unit. This provides near normal driving conditions during system malfunction. When problems are detected in certain systems, the PCM will revert to its fail-safe mode so the vehicle can be driven until repaired.