Engine Control Module: Description and Operation

The Electronic Control Unit (ECU) consists of an 8-bit microprocessor, random access memory (RAM) and Read Only Memory (ROM). The ECU actively controls the operation of the fuel injection system, the ignition timing, and has full control of the idle speed control system. The ECU also interrupts operation of the air conditioning and EGR systems under some conditions, and controls the power to the fuel pump (through the engine control relay). Based on information from an array of input sensors (engine coolant, barometric pressure, air flow etc.), the ECU determines the optimum control and drives the output actuators accordingly. In addition to this the ECU monitors its input and output circuits. When the ECU detects an irregularity in a monitored circuit, the ECU sets a fault code in memory, and makes it available at the self-diagnosis output terminal. The fault codes can be read with a voltmeter or multi-use tester. Moreover, since fault code memory is powered by the battery, the fault codes remain in memory even when the ignition key is turned OFF, (except for the oxygen sensor code). The fault codes will, however, be erased when the battery or the ECU is disconnected.