Engine Control Module: Description and Operation

The ECU is a computer that monitors various engine sensors, including an exhaust gas oxygen sensor, and then uses this information compared with information stored in its memory, to control fuel injection, ignition timing, and certain emission control devices.

FUEL INJECTION
Fuel is supplied to the fuel rail and injectors at high pressure by the fuel pump in the tank. A fuel pressure regulator varies fuel pressure slightly depending on manifold vacuum. When fuel demand is high as indicated by low manifold vacuum, the pressure is increased slightly. A pulsation damper helps to reduce fuel pressure oscillations from the pump. The injectors are electric solenoid valves that get power when the engine is cranking or running and turn "ON" when the computer (Engine Control Unit - ECU) completes their circuits to ground. Injectors spray fuel into the intake port when they are turned "ON". The amount of fuel sprayed depends on the injector "pulse width" (length of time the injectors are turned "ON" during each injection cycle). Injectors are operated sequentially, which means injectors turn "ON" in the same sequence as the engine firing order, so that injection occurs at each cylinder while the intake valves for that cylinder are open.

STARTING AND WARM-UP
During cranking when the engine is cold, a cold start injector sprays additional fuel into the intake manifold dynamic chamber to provide a richer mixture for easy starting. When the engine is running cold, the fuel injection system operates in what is known as "OPEN LOOP" mode. In this mode, the ECU determines the amount of fuel needed depending on the engine coolant temperature and the amount of air flow as measured by the air flow meter. A specific pulse width is assigned for certain air flow rates at various temperatures. These are the "base" pulse width values stored in the computer memory which provide a slightly rich air/fuel ratio for good driveability while the engine is cold and during warm-up.

NORMAL WARM ENGINE OPERATION
When the engine warms up, the fuel injection system switches to "CLOSED LOOP" operation. In this mode, the computer monitors several sensors in addition to the air flow meter and temperature sensor, including an exhaust gas oxygen sensor. Signals from the oxygen sensor tell the ECU if the air/fuel ratio is rich or lean. The ECU then modifies the base pulse width accordingly to increase or decrease the amount of fuel being injected, depending on the need at that moment. Injector pulse width is adjusted several times each second to maintain the air/fuel ratio as close as possible to the ideal ratio.

IDLE SPEED CONTROL
The idle speed is controlled by the Bypass Air Control (BAC) valve. The BAC valve regulates how much measured air is allowed to bypass the throttle valves, depending on engine temperature and signals from the ECU. The BAC valve has two parts, the "Air Valve" which is open when cold to provide cold start fast idle, and the Idle Speed Control (ISC) valve which is electrically operated by the ECU. In the air valve, a temperature sensitive wax pellet is heated by engine coolant. As the engine warms up, the pellet expands against the air valve, closing it. When the engine is warm, idle speed is controlled by the ECU through duty cycle signals to the ISC valve (the ratio of "ON" time vs."OFF" time).

DECELERATION FUEL CUT-OFF
The ECU recognizes when the engine is decelerating by monitoring the engine speed and an idle switch that tells the ECU when the throttle is closed. When the throttle is closed and the engine rpm is above a certain speed the ECU will leave the injectors turned "OFF" and no fuel in injected. This reduces emissions, prolongs catalytic converter life, and improves fuel economy.

INTAKE AIR SYSTEM - VARIABLE INERTIA CHARGING SYSTEM (VICS)

Variable Inertia Charging System (VICS):

The intake manifold has two paths that air can take on the way to the cylinders, a long and a short path. The two paths are separated by a shutter valve, like a throttle valve in each intake runner. When the shutter valves are closed, air must take the long path. When the valves are open, air can take a more direct short path to the engine. The ECU opens and closes the shutter valves at different engine speeds to take advantage of harmonic oscillations in the intake manifold runners. This helps "pack" more fuel and air into the cylinders during each cylinder cycle (improved volumetric efficiency), increasing torque and horsepower over a broader rpm range.

IGNITION TIMING CONTROL
Ignition timing is also controlled by the computer. The ECU monitors signals from sensors in the distributor. From these signals, the ECU can calculate crankshaft position and engine speed. The ECU uses this information, compared with information from other engine sensors, to determine the correct spark advance for the engine speed and load at any given time. The ignition coil "fires" when the ECU interrupts the low voltage signal to a power switching transistor (igniter). The igniter turns the primary coil circuit "OFF" causing the ignition coil to discharge. For more information on the ignition coil, igniter, and other secondary ignition components, refer to IGNITION SYSTEM


SELF-DIAGNOSTICS
The EGI system has self-diagnostic capability. When a fault is detected in a monitored component and/or circuit, the ECU will store a numbered code in its memory (malfunction code) and the Malfunction Indicator Lamp (MIL) will light and stay on while the engine is running, alerting the driver to seek service. The technician can then access the ECU fault memory and display trouble codes identifying the faulty component(s) or circuit(s), to aid in troubleshooting and repair.

NOTE: The ECU constantly monitors input devices for malfunctions. However, it only checks for output device faults during a three second period after the ignition is switched "ON" and test connector is grounded.

FAIL-SAFE MODE
To provide a margin of safety and maintain driveability in the event of certain system failures, the ECU has a fail-safe mode. In this mode, the ECU will substitute a fixed "in range" signal for that of the failed sensor. This allows the vehicle to be operated until repaired, although driveability will probably be affected. The ECU automatically switches to fail-safe mode if a fault is detected in any of these inputs:

^ Airflow meter
^ Atmospheric pressure sensor
^ EGR position sensor
^ Feedback system-left and right sides
^ Intake air thermosensor
-in airflow meter
-in dynamic chamber
^ Oxygen sensor-left and right sides
^ Throttle sensor (A/T)
^ Water thermosensor