Engine Control Module: Description and Operation

ENGINE CONTROL MODULE (ECM)
The Engine Control Module (ECM), located under the instrument panel, is the control center of the fuel injection system. If constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The ECM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the "Check Engine" malfunction indicator lamp, and store a diagnostic trouble code or diagnostic trouble codes which identify the problem areas to aid the technician in making repairs.
For service, this ECM is replaced as a complete unit. There are no serviceable parts in the ECM. The calibrations are stored in the ECM in the Electronic Erasable Programmable Read Only Memory (EEPROM).

ENGINE CONTROL MODULE (ECM) FUNCTION
This ECM supplies either 5 to 12 volts to power various sensors or switches. This is done through resistances in the ECM which are so high in value that a test light will not light when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low.

Therefore, a 10 megohms input impedance digital voltmeter is required to assure accurate voltage readings.

The Engine Control Module (ECM) controls output circuits such as the fuel injectors, idle air control valve, air conditioning clutch relay, etc. by controlling the ground circuit through transistors or a device called a "quad-driver".