Engine Control Module: Description and Operation

Engine Control Module (ECM):

DESCRIPTION
The Engine Control Module (ECM) is a compact microprocessor that is the control center of the fuel injection system. It constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The ECM performs the diagnostic function check of the system.

It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (Service Engine Soon), and store a DTC or DTCs which identify the problem areas to aid the technician in making repairs. Refer to System Diagnosis for more information on using the diagnostic function of the ECM.

COMPONENTS
For service, this ECM only consists of two parts: a controller (the ECM without a PROM) and an assembly called a PROM.

PROM
To allow one model of ECM to be used for many different vehicles, a device called a calibrator or Programmable Read Only Memory (PROM) is used.

The PROM is located inside the ECM and has information on the vehicle's weight, engine, transmission, axle ratio, and other items specific to each vehicle. While one ECM part number can be used by many vehicle lines, a PROM is very specific and must be used for the right vehicle. For this reason, it is very important to check the latest parts book and service bulletin information for the correct part number when replacing a PROM.

An ECM used for service (called a controller) comes without a PROM. The PROM from the old BCM must be carefully removed and installed in the new ECM (see Service and Repair).

OPERATION
The ECM supplies 5 or 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, the use of a 10 megohm input impedance digital voltmeter (J 39200) is required to assure accurate voltage readings.

MEMORY
There are three types of memory storage within the ECM: ROM, RAM and PROM.

ROM
Read Only Memory (ROM) is a permanent memory that is physically soldered to the circuit boards within the ECM. The ROM contains the overall control algorithms. Once the ROM is programmed, it cannot be changed. The ROM memory is non-volatile, and does not need power to be retained.

RAM
Random Access Memory (RAM) is the microprocessor "scratch pad." The processor can write into, or read from this memory as needed. This memory is volatile and needs a constant supply of voltage to be retained. If the voltage is lost, the memory is lost.

PROM
Programmable Read Only Memory (PROM) is the portion of the ECM that contains the different engine calibration information that is specific to year, model and emissions. The PROM is a non-volatile memory that is read only by the ECM.

The PROM should be retained with the vehicle following ECM replacement.