Engine Control Module: Description and Operation
ECM Inputs And Outputs:
ECM And Connector:
ECM Location/Installation:
COMPUTER INPUTS/OUTPUTS
The "Control Module" (referred to as the ECM - Electronic Control Module or Engine Control Module) is located in the left rear of the engine compartment and is the control center of the Computer Command Control system. The ECM constantly monitors various sensors, compares this information with programmed information stored in its memory, and operates output devices to control fuel delivery, idle speed, ignition timing, and certain emission control systems.
The ECM has two serviceable parts. A controller, called the ECM, and a MEM-CAL (Memory Calibration unit), which contains the programming memory and performs "fail-safe" functions to operate the vehicle if the ECM or other vital system fails.
The ECM controls output devices through the use electronic switches called "Drivers". Most output devices, such as solenoids and relays, get power from the ignition when the key is turned "ON" and become energized when the driver in the ECM completes the circuit to ground. When the ECM is commanding a device "ON," the voltage potential of the output is "LOW" or near zero volts (driver circuit grounded.) When the ECM is commanding a device or component "OFF" (driver circuit "open") the voltage potential of the circuit will be "HIGH," or near 12 volts. The exception to this is the primary fuel pump relay drive. In this case, one side of the relay circuit is permanently grounded, and the ECM provides battery voltage to energize the relay.
SELF DIAGNOSTIC FUNCTION
The computer can recognize problems with sensors, output devices and circuits, and alert the driver by turning on the "Malfunction Indicator Light" (MIL) 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 MIL 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 operating parameters in the computer memory, make sure the engine is at operating temperature and operate the vehicle at part throttle, moderate acceleration, and idle conditions, until normal performance returns.
FAIL-SAFE OPERATION
The ECM also has a built in back-up system to provide near normal driving conditions during system malfunction. These functions are performed by the MEM-CAL unit installed in the ECM. When problems are detected in certain systems, the PCM will revert to its fail-safe mode so the vehicle can be driven until repaired.
ASSEMBLY LINE DATA LINK (ALDL) CONNECTOR
A special diagnostic connector, called the Assembly Line Data Link or ALDL connector, is provided. This connector is used to enter the computer into diagnostic mode for displaying trouble codes. An electronic "SCAN" tool can also be plugged into the ALDL connector to display trouble codes, monitor engine sensors and switches, and in some cases, operate output devices for diagnosis and testing.
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 amount of fuel injected is controlled by varying how long the injectors are pulsed "ON". The ECM checks the coolant temperature sensor, manifold absolute pressure sensor, and the throttle position sensor, and determines the injector pulse width for the proper air/fuel ratio to start the engine. This ranges from approximately 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) engine coolant temperature.
Clear Flood Mode:
The ECM will enter "clear flood" mode when the throttle position is greater than 80% and the engine speed is below 600 rpm (cranking.) If the engine is flooded, holding the throttle wide open while cranking will cause the ECM to enter this mode. While in "clear flood" mode, injector pulse width is calculated to provide an air/fuel ratio of 20:1. Excess fuel will be purged from the cylinders as the engine is cranked over. 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 500, 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), Manifold Air Temperature (MAT) sensor, Manifold Absolute Pressure (MAP) Sensor, and Throttle Position Sensor (TPS.) The Camshaft Sensor signal is used to synchronize the injection pulses with the opening of the intake valves for each cylinder (sequential.) 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 are stored in the memory calibrator (MEM-CAL) portion of the ECM. When these values are met, the system goes into "Closed Loop" operation. During "Closed Loop" operation, the ECM monitors the signal from the oxygen sensor and modifies the base injector pulse width to maintain the air/fuel ratio very close to the ideal ratio of 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 primary injectors to provide extra fuel during acceleration. The amount of fuel required is determined by throttle position, manifold air pressure and engine speed.
Power Modes:
The driver can select from two power modes, "FULL" and "NORMAL", with a console mounted key switch. When the power mode switch is in the "NORMAL" position, the ECM disables secondary port throttle valves and secondary injectors. If the "FULL" power mode is selected the ECM opens the secondary port throttle valves and begins operating the secondary injectors, if there are no trouble codes stored, the oil temperature is within an acceptable range, and throttle position/engine speed indicate a demand for full power. Secondary injectors are pulsed simultaneously with the primary injectors and fuel delivery to each cylinder is divided equally between them. During wide open throttle in full power mode, the ECM continues monitoring the oxygen sensor signals. If a lean condition is detected for more than two seconds, secondary fuel/intake operation is disabled and a Code 55 is set.
Fuel Cutoff Mode:
To prevent possible engine damage from over-speed, the ECM cuts off fuel from the injectors when the engine speed exceeds 7,100 rpm. If a Code 61 is stored and the ECM detects vacuum at the vacuum sensor in normal run mode, secondary port throttle valve operation is disabled and fuel cut-off occurs at 3,000 rpm.
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.
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.
NOTE: Most sensor inputs are related to more than one output function. A failure in one input device, such as the oxygen sensor, can affect several of the systems controlled by the computer.