Computers and Control Systems: Description and Operation

System Diagram:

This vehicle is equipped with a 1.6 liter, SOHC 4 cylinder gasoline engine, and a computer controlled electronic fuel injection system. The system maintains an air/fuel mixture as close as possible to the ideal stoichiometric ratio, for all operating conditions. During normal operation, the computer monitors various engine sensors, including an exhaust gas oxygen sensor, and operates the injectors for varying lengths of time depending on the amount of fuel required. Ignition timing is also computer controlled. The computer monitors engine sensors and calculates crankshaft position, engine speed and load, to determine the correct spark timing.

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. The injectors are electric solenoid valves that get power when the engine is cranking or running and turn "ON" when the Powertrain Control Module - Engine (PCME) 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 in pairs, so that injection occurs at each cylinder at about the same time the intake valves for those cylinders are open.

STARTING
During cranking, all the injectors energize at the same time once for each ignition pulse (twice for each revolution of the crankshaft). This provides a richer mixture for easy starting. Once the engine is started, injectors operate in pairs.

DE-CHOKE CONTROL
If the engine won't start or is hard to start due to excess fuel in the manifold or cylinders (spark plugs wet with fuel), the PCME will stop injection under the following conditions:
^ Ignition switch is in the "START" position (engine cranking signal to the PCME).
^ Engine speed is less than 500 rpm (tells the PCME the engine is not running).
^ Throttle sensor signal indicates throttle wide open.
If all these conditions exist at the same time, no fuel will be injected. This allows air to be pumped through the cylinders, purging excess fuel.

WARM-UP
When the engine is running cold, the fuel injection system operates in what is known as "OPEN LOOP" mode. In this mode, the PCME determines the amount of fuel needed depending on the engine coolant temperature, throttle position sensor signal, 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 and throttle openings. 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, including an exhaust gas oxygen sensor. Signals from the oxygen sensor tell the PCME if the air/fuel ratio is rich or lean. The PCME 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 ideal.

IDLE SPEED CONTROL
There are two valves that regulate idle speed. The Air valve and the ISC valve.

The Air valve is a temperature sensitive valve that is open when cold. This allows additional measured air into the engine, bypassing the throttle valve, for cold start fast idle. 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 PCME through signals to the ISC valve (see below).

When the engine is warm, idle speed is controlled by the Idle Speed Control (ISC) valve. The ISC valve regulates how much measured air is allowed to bypass the throttle valves, depending on signals from the PCME. The ISC valve is electrically operated by the PCME through duty cycle signals (the ratio of "ON" time vs."OFF" time).

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

IGNITION TIMING CONTROL
Ignition timing is also controlled by the computer. The PCME monitors signals from sensors in the distributor. From these signals, the PCME can calculate crankshaft position and engine speed. The PCME 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 PCME 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 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 PCME 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 PCME fault memory and display trouble codes identifying the faulty component(s) or circuit(s), to aid in troubleshooting and repair.

FAIL-SAFE MODE
To provide a margin of safety and maintain driveability in the event of certain system failures, the PCME has a fail-safe mode. In this mode, the PCME 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 PCME automatically switches to fail-safe mode if a fault is detected in any of these inputs:

Airflow meter
Atmospheric pressure sensor
Feedback system
Intake air thermosensors
Oxygen sensor
Throttle sensor
Water thermosensor

INPUTS/OUTPUTS
Individual components function as either input devices or output devices. Input devices modify or generate voltage signals that the PCME "reads". Output devices are components operated or controlled by the PCME. Not all of these devices are monitored by the on-board diagnostic system. Some input signals come from devices that are not specifically PCME sensor devices. For example, the A/C amplifier signals the PCME when the air conditioning is turned "ON", for idle speed control (to compensate for the additional load on the engine). In the case of many output devices (relays and solenoids), the PCME output function is simply to make or break the ground circuit to turn a device "ON" or "OFF". The following is a list of input and output devices and signals.

INPUTS

A/C switch
Airflow meter
Atmospheric pressure sensor (built into the PCME)
Crankshaft pulley sensor (crank angle and engine speed)
Distributor sensors
-G1 signal (crankshaft position)
-Ne1 signal (engine speed)
Electrical load inputs
Headlight switch
Rear window defroster switch
Heater - A/C blower switch
Idle switch
Ignition switch ("START" position - engine cranking)
Intake air thermosensor
Knock sensor
Oxygen sensor
-Left side
-Right side
Power steering pressure switch
Stop light switch
Test connector
Throttle sensor
Vehicle speed sensor
Water thermosensor (engine fan)
Water thermosensor (EGI)

OUTPUTS

A/C relay/cut-off
Idle speed control (ISC) valve
Fuel injectors/injection timing
Igniter
Malfunction indicator lamp (MIL)
Oxygen sensor heater
Solenoid valves
^ Pressure regulator control
^ Purge control