Chapter 7 Computer Control Fundamentals 121
Troubleshooting and service procedures are the same
for vehicles using CAN systems and older buses. Older scan
tools, however, will not work on CAN systems without the
use of a special adapter. Newer scan tools are designed to
work with CAN systems. One unusual problem that some-
times occurs with CAN systems is a failed ECM creating
enough electrical interference to confuse the other ECMs,
causing false codes and erratic operation. Sometimes the
only way to diagnose this problem is to disconnect ECMs
one at a time to isolate the problem to a particular one.
This may also be necessary when internal communications
seems to have failed.
ECM External Communications
There are three types of external serial communication
used in automotive computer systems. Most vehicles use a
system called universal asynchronous receive and transmit
(UART). This signal is used for communication between the
ECM, off-board diagnostic equipment, and other control
modules. UART is a data line that varies voltage between
0–5 volts at a fixed-pulse-width rate. Vehicles may use
UART, but may also depend on the use of Class 2 and CAN
serial communication. Class 2 data is transferred by tog-
gling the line voltage from 0–7 volts, with 0 being the rest
voltage, and by varying the pulse width. The variable pulse
width and higher voltage allows Class 2 data communica-
tion to better utilize the data lines.
ECM Internal Components
There are several main circuits in the ECM. These
circuits are:
p
Input processor.
p
Memory.
p
Central processing unit (CPU).
p
Output processor.
These circuits are not individually serviced. They are all
replaced when the ECM is replaced. However, it is important
to have some idea of how each circuit works. The following
sections explain these circuits in detail.
Input Processor
The input processor takes the analog voltage from the
input sensors and converts it into digital pulses that can be
used by the computer circuits. Most sensors can send infor-
mation at a faster rate than the ECM can process. The input
processor reduces the amount of incoming information to
a manageable level by averaging a series of analog voltage
pulses into a single digital pulse.
The input processor also serves as a protective device.
Early ECMs were easily damaged if a sensor or wire became
shorted or grounded. Newer input processors limit the
amount of current entering the ECM, preventing shorts and
voltage surges from damaging the ECM.
Memory
The memory contains the preset values for a properly
running engine. There are two kinds of memory in the com-
puter: fixed and volatile. The fixed memory circuits in most
ECMs cannot be changed. However, the fixed memory in
some ECMs is replaceable, while others can be modified
with the ECM in the vehicle. The volatile memory is able to
receive and store information from the sensors and output
devices.
Fixed Memory. Fixed memory is used to retain the
operating instructions for the ECM and the vehicle’s base
engine or system information. There are four types of fixed
memory.
Read-only memory (ROM) contains the basic instruc-
tions that tell the ECM how to operate. If power to the ECM
is disconnected, the information is not lost. ROM is installed
at the factory and, if defective, requires ECM replacement.
Programmable, read-only memory (PROM) contains
the operating parameters for the engine, drive train, or
system. This information is used by the ECM as a reference
for proper input sensor, output device, engine, drive train,
and vehicle operation. If power to the ECM is removed,
the information in the PROM is not lost. In some cases,
the PROM can be removed if it is defective or an updated
PROM is needed to correct a driveability problem. However,
in ECMs, the PROM is permanently affixed to the circuit
board. Most computer-controlled vehicles use PROMs in
their ECMs.
Some ECMs use erasable, programmable, read-only
memory (EPROM). These are affixed to the ECM circuit
board and can be reprogrammed by exposing the ECM to an
ultraviolet light to clear the EPROM and then programming
it with the new information. Unfortunately, this can only
be done by the ECM manufacturer, which makes it neces-
sary to replace any ECM that has a defective EPROM or if
updated programming is needed to correct a problem.
Many of the newest ECMs use electronically erasable,
programmable, read-only memory (EEPROM) or flash-
erasable, programmable, read-only memory (FEPROM).
These types of memory, while affixed to the ECM, can be
easily programmed or updated using computerized equip-
ment. All or part of a vehicle’s operating information is
downloaded or “burned into” the EEPROM or FEPROM.
Some EEPROM- and FEPROM-equipped ECMs can be pro-
grammed away from the vehicle.
Volatile Memory. The information in volatile memory
changes with vehicle operation and indicates ongoing
changes in the control system or other engine systems.
Volatile memory is used to store diagnostic codes and tem-
porary information on vehicle operating conditions from
the input sensors. The incoming information can be used
by other computer circuits as part of the ECM’s learning
function. If power to the ECM is disconnected, all informa-
tion stored in volatile memory is lost.
All ECMs have random-access memory (RAM). While
the vehicle is operating, information from the sensors, such
as minimum intake airflow rate and diagnostic trouble