Engine Control Module: Description and Operation

BASIC OPERATION
The Electronic Control Module (ECM) controls the fuel injection system, ignition timing, and the idle speed control system. The ECM also interrupts the operation of the air conditioning and Exhaust Gas Recirculation (EGR) systems, and controls power to the fuel pump (through the control relay). The ECM consists of an 8-bit microprocessor, random access memory (RAM), read only memory (ROM), and an input/output interface.

Based on information from the input sensors (engine coolant temperature, barometric pressure, air flow, etc.), the ECM determines optimum settings for the output actuators (injection, idle speed, ignition timing, etc.).

SELF-DIAGNOSIS
The ECM constantly monitors input and output signals for correct operating range. If an irregularity occurs, the ECM stores a trouble code which can be accessed via the self-diagnosis output terminal. Diagnostic results can be read with a voltmeter or multi-use tester.

ECM trouble code memory is maintained by direct battery voltage so diagnostic results are held in memory even if the ignition key is turned off (except for the oxygen sensor code). However, trouble codes will be erased if the battery terminal or the engine control unit connector is disconnected.