This article explains how an automotive computer PCM (powertrain control module)
A car computer or powertrain control module (PCM) is a designated computer that
was developed to manage the engine and driveline components. This PCM consists of
electronics which are designed onto a multi-layer circuit board. The PCM monitors
and adjusts the air/fuel mixture and utilizes a catalytic converter to minimize
the amount of pollution produced from the engine. There are two modes of computer
operation, open loop which is used when the engine is cold and operates on a preset
program, and closed loop, which means the computer is operating while using the
various sensors. Closed loop occurs when the engine is at operating temperature.
The PCM monitors the input and output signals produced by various sensors in
the system, the PCM then adjusts the system as necessary. Sensors include: oxygen,
coolant temperature, mass air flow, air intake temperature, crankshaft angle, throttle
position, camshaft angle and engine knock. While computing feedback information
the PCM adjusts and controls ignition timing, camshaft position, fuel injector input,
fuel pump, cooling fan, emission system controls, forced air induction controls,
traction controls and transmission gear selections.
The computer operating program consists of a series of predetermined information
cells, these cells hold the equation for proper vehicle operation. If the computer
detects that it can't control a particular system it will illuminate the MIL (malfunction
indicator lamp), aka check engine or service engine soon light. This means the computer
has stored a trouble code. Visit -
Scan for codes
Trouble code definitions 1996 to present. Visit -
OBD2 trouble codes
Trouble code definitions 1996 and earlier. Visit -
OBD1 trouble codes
After the MIL has illuminated the vehicle enters into "limp mode" this means
its running on a predetermined program that has been selected from the factory.
This mode is not as efficient as the regular operation of the system operating properly.
Codes are pulled by connecting to a ALDL connector which is usually located under
the dash on the drivers side. Most 1996 and older vehicles utilize a "D" style plug-in
connector that connects to the OBD1 code reader or retrieval method.
Communication standards were established in the OBD2 operating system development.
These communication standards used a CAN (controller area network) system which
can achieve communication speeds of more than 500 Kbps.
Advantages of using information buses for communication is that if a fault occurs
with any of the process modules, it can be reported separately to a diagnostic tool.
Wiring is simplified by a technique known as multiplexing, a kind of wiring system
which is assigned for each module. Sensor circuits operate on a 5 volt reference
which drive system monitoring and control.
Article first published 2016-02-03