Engine Control Module: Description and Operation

DESCRIPTION
The Engine Control Module (ECM) will be referred to as a control module. The control module is designed to maintain exhaust emission levels to Federal or California standards, while providing excellent driveability and fuel efficiency. Review the component sections and the wiring diagrams in order to determine which systems are controlled by each specific control module. The control module monitors numerous engine and vehicle functions and controls the following operations:
^ The Fuel Control
^ The Ignition Control (IC)
^ The Knock Sensor (KS) system
^ The Automatic Transaxle shift functions
^ The Cruise Control Enable
^ The Generator
^ The Evaporative Emission (EVAP) Purge
^ The A/C Clutch Control
^ The Cooling Fan Control

OPERATION
The ECM can supply 5 volts or 12 volts to the various sensors or switches. This is done through pull-up resistors to the regulated power supplies within the ECM. The resistance is 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 the resistance is too low. Therefore, a digital voltmeter (J 39200) with at least 10 megohms input impedance is required in order to ensure accurate voltage readings.
The ECM controls the output circuits such as the injectors, the relays, the IAC, etc. by controlling the ground or the power feed circuit through the transistors or a device called an Output Driver Module.

EEPROM
The Electronically Erasable Programmable Read Only Memory (EEPROM) is a permanent memory that is physically part of the ECM. The EEPROM is not a removable component. The EEPROM contains program and calibration information that the ECM needs in order to control the powertrain operation. Unlike the PROM used in the past applications, the EEPROM is not replaceable. If the ECM is replaced, make sure that the new ECM's software/calibration is correct for the vehicle and is the most recent version. Special equipment, as well as the correct program and calibration for the vehicle, is required in order to re-program the ECM.