Mechanics

Computer PCM

This article explains how an automotive computer PCM (powertrain control module) works.

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.

PCM Powertrain Control Module
PCM Powertrain Control Module

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 inductor 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.


Helpful Information

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.

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-20)