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Appendix A: Vehicle Computer Basics
Basics of Computer-Controlled Vehicles
This section explains the engine computer control system, the types of sensors and
how the computer controls engine fuel delivery, idle speed and timing. Additional
information may be found in technical support books at your local library or auto parts
store. The more you know about the computer system, the better you can diagnose
vehicle computer problems.
Computer controls were originally installed on vehicles to meet federal government
regulations for lower emissions levels and improved fuel economy. This began in the
early 1980s when basic mechanical systems were no longer able to accurately
control key engine parameters. A computer could be programmed to control the
engine under various operating conditions, making the engine more reliable. While
these early systems were very limited in the scope of their control, providing only 10-
14 trouble codes, they did help guide the vehicle repair process.
Today, computer controls have made cars and trucks faster, cleaner, and more
efficient than ever before.
What the computer controls:
The main control areas of the vehicle computer are fuel delivery, idle speed, spark
advance, and emissions controls. Some on-board computers may also control the
transmission, brakes, and suspension systems as well.
What has not changed?
A computer-controlled engine is very similar to the older, non-computerized engine. It
is still an internal combustion engine with pistons, spark plugs, valves, and
camshaft(s). The ignition, charging, starting, and exhaust systems are very similar as
well. You test and repair these systems just as before. The technical manuals for
these components show you how to perform the tests. Additionally, compression
gauges, vacuum pumps, engine analyzers, and timing lights will continue to be used.
The Engine Computer Control system
The vehicles on-board computer, or Powertrain Control Module (PCM), is the heart
of the system. It is sealed in a metal box and connected to the rest of the engine by a
wiring harness. The PCM is located, in most cases, in the passenger compartment,
behind the dashboard or in the kick panel position, although some manufacturers
locate the computer control module in the engine compartment area. Most PCMs can
withstand a lot of vibration and are built to live in a rugged environment.
The PCM is programmed by the factory. The program is a complex list of lookup
tables and instructions telling the computer how to control the engine based on
various driving conditions. To do its job, the computer uses sensors to know what is
happening and then provide instructions back to a network of switches and actuators
throughout the vehicle.
Sensors give the computer information
Sensors are devices which measure operating conditions and translate them into
signals the computer can understand. Some examples of sensors: thermistors (for
temperature readings), potentiometers (like a throttle position sensor), and signal
generators (such as an 02 sensor).