DESCRIPTION SERVICE MANUAL NUMBER 22
Page 5A-16 90-860074--1 FEBRUARY 2002
ECM FUNCTION
The ECM supplies 4.9 volts (generally rounded to 5 volts, in conversation) 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 is
required to assure accurate voltage readings.
MEMORY
There are three types of memory storage within the ECM: ROM, RAM and EEPROM.
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 programs. Once the ROM is
programmed, it cannot be changed. The ROM memory is non-erasable, 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 erasable and needs a
constant supply of voltage to be retained.
EEPROM
Electronic Erasable Programmable Read Only Memory (EEPROM) is the portion of the
ECM that contains the different engine calibration information that is specific to each marine
application.
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