Computers and Control Systems: Description and Operation

Computer System:

PURPOSE
The Computer Command Control System is a computerized fuel, ignition and emission control system designed to maintain fuel economy, good driveability and reduced vehicle emissions.

OPERATION
Feedback Control
An Electronic Control Module (ECM) monitors data from various sensors and controls functions such as air/fuel ratio, emission control devices and on-board diagnostics.

The ECM is a computer that judges air/fuel ratio as rich or lean according to oxygen sensor output voltage. Based on oxygen sensor and other input data, the ECM generates a controlling signal to the throttle body injectors that regulates "ON" time. Because fuel pressure remains constant, the ECM controls air/fuel ratio adjustments by varying injector "ON" time.

When operating in "Open Loop," the ECM defaults to preprogrammed values for air/fuel ratio control. The system is in "Open Loop" before the oxygen sensor has warmed up to operating temperature. When operating in "Closed Loop," the ECM continually adjusts air/fuel ratio according to oxygen sensor input and operating information received from various other sensors.

Self-Diagnostics
This system incorporates self-diagnostic capability that is monitored via a dash mounted "Check Engine" (MIL) light. When the system detects a fault in a monitored component and/or circuit, the light will come on and stay on (while the engine is running), alerting the driver to seek service. With intermittent problems, the light may come on and then go out, but a trouble code will be stored in ECM memory if correct parameters are met. The technician can then extract trouble codes from the ECM that will aid in the performance of efficient troubleshooting and repair.

Components can be grouped as ECM input and output devices. Input devices control or produce voltage signals monitored by the ECM. Output devices are components controlled by the ECM.

Ignition System / Ignition Control (IC)
This function of the ECM is to determine the correct spark timing based on such things as engine temperature, engine rpm and vehicle load.

Electronic Spark Timing (EST) is part of the Computer Command Control System. The ECM monitors information from various sensors, computes the desirable spark timing and signals the distributor when timing changes are required. The distributor does not contain centrifugal advance weights, springs or a vacuum advance unit. Base timing is adjustable. If it is not correctly set, the entire spark advance curve will be off.

Spark knock (detonation) is controlled by an Electronic Spark Control (ESC) system that can retard ignition timing up to 20°. This allows the engine to use maximum spark advance to improve driveability and fuel economy.

The system consists of an ESC module and knock sensor. When the sensor detects knock, it produces a voltage signal that is sent to the ESC module. As long as the module receives no signal from the sensor, it sends a signal voltage (8-10V) to the ECM and the ECM calculates normal spark advance.

When the module receives a knock signal, it changes signal voltage to the ECM to 0V and the ECM retards timing to reduce knock.

Knock Sensor (KS)
This system uses an spark knock sensor and an additional control module to inform the ECU that detonation is occuring. The three components work together to provide optimum ignition timing under all conditions.