Manual

Onboard Diagnostics
COMPUTER ENGINE CONTROLS
16 OBD2
The Basic Engine Computer Control System
The on-board computer is the heart of the Computer
Control System. The computer contains several programs
with preset reference values for air/fuel ratio, spark or
ignition timing, injector pulse width, engine speed, etc.
Separate values are provided for various driving conditions,
such as idle, low speed driving, high-speed driving, low load,
or high load. The preset reference values represent the ideal
air/fuel mixture, spark timing, transmission gear selection,
etc., for any driving condition. These values are programmed
by the vehicle manufacturer, and are specific to each vehicle model.
Most on-board computers are located inside the vehicle behind the dashboard,
under the passenger’s or driver’s seat, or behind the right kick panel. However,
some manufacturers may still position it in the engine compartment.
Vehicle sensors, switches, and actuators are located throughout the
engine, and are connected by electrical wiring to the on-board computer.
These devices include oxygen sensors, coolant temperature sensors,
throttle position sensors, fuel injectors, etc. Sensors and switches are
input devices. They provide signals representing current engine
operating conditions to the computer. Actuators are output devices. They
perform actions in response to commands received from the computer.
The on-board computer receives information inputs from sensors and
switches located throughout the engine. These devices monitor critical
engine conditions such as coolant temperature, engine speed, engine
load, throttle position, air/fuel ratio etc.
The computer compares the values received from these sensors with its
preset reference values, and makes corrective actions as needed so
that the sensor values always match the preset reference values for the
current driving condition. The computer makes adjustments by
commanding other devices such as the fuel injectors, idle air control,
EGR valve or Ignition Module to perform these actions.
The Computer Control System consists of an on-board
computer and several related control devices (sensors,
switches, and actuators).
OUTPUT DEVICES
Fuel Injectors
Idle Air Control
EGR Valve
Ignition Module
On-Board
Computer
INPUT DEVICES
Coolant Temperature Sensor
Throttle Position Sensor
Fuel Injectors
INPUT DEVICES
Oxygen Sensors
TYPICAL COMPUTER
CONTROL SYSTEM