The Engine Control Unit or ECU is a designated computer that was developed to manage the engine control system. The ECM consists of electronics which are mounted on a multi-layer circuit board. The ECM 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 operation, closed loop, which means the computer has completely taking over the operation system. And open loop which is used when the engine is cold and operates on a preset program. The engine must be at operating temperature before it can go into closed loop.
The ECM monitors the input and output signals produced by various sensors in the system. The ECM then adjusts the system as necessary. Sensors can include: oxygen sensor, coolant sensor, mass air flow sensor, air intake sensor, crankshaft angle sensor, throttle position sensor, camshaft angle sensor and knock sensor. The ECM operating program consists of information cells. These cells hold the code for proper engine operation, if information is outside the cell parameters a MIL (malfunction inductor lamp) or "check engine light".
Once a "check engine" or a "service engine soon" light in your instrument cluster has illuminated the ECM has stored a diagnostic trouble code. This means the computer system on your vehicle has detected a problem with the various systems it controls. Your car or truck has many different sensors that monitor the various vehicle systems. If the trouble code light has been illuminated your vehicle enters into "limp mode" this means it is running on a predetermined program that causes poor mileage and increased emissions. An engine trouble code reader is an easy to use tool. All 1996 and newer vehicles utilize a "D" style plug-in connector that connects to the code reader. On most vehicles the connector is located at the driver's compartment. Once the code reader is plugged in you can retrieve trouble codes the engine computer has stored in its memory. These codes are the same codes the dealer and repair shops use to replace sensors and clear codes.
The ECM outputs a 5 volt reference to most sensors to drive the monitoring circuits. The ECM also controls the radiator cooling fan, air pump controls, fuel pump, EVAP system and more depending on the vehicle. Communication standards have being established in the OBD2 operating system. Among the communication standards controller area networking or CAN has become very popular and can achieve communication speeds of more than 500 Kbps, which is faster than most communication standards. Advantages of using buses for communication is that if a fault occurs with any of the process modules, it can be reported separately to a diagnostic tool. Manufacturers have different trouble code faults although some are generic. Wiring is simplified by a technique known as multiplexing. In this kind of wiring system is assigned for each module, which consolidates the output and input for that module.
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