User's Manual

LAUNCH X-431Tool User’s Manual
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system or external scan tool.
The on board diagnostic system in different
vehicle may indicate the fault in different way. In
general, the following ways are popular:
1) Blinking of fault indicators on the instrument
panel
2) Swing of multimeter index
3) Blinking of red/green LED on ECU
4) Shown by a special device
The more intuitionistic way for showing the fault
is by external scan tool.
The diagnostic system of modern cars is
equipped with diagnostic socket or DLC (data
link connector). Fault code in vehicle ECU can
be read out by connecting a scan tool to the
diagnostic socket. In early time, each vehicle
system had its own type of diagnostic socket.
Since 1996, SAE has specified a uniform
system called OBDII on board diagnostic
system. Each vehicle that is made in
accordance with the system may have the
same diagnostic socket, same definition of pin
and same description of fault code. The control
units of every system are connected to bus, so
only one diagnostic socket is needed for
readout of fault code. OBD provides the
possibility of reading data stream and
displaying working status of switches and
actuators. Moreover, external scan tool can
send commands to activate the actuators.
X-431TOOL is such an external scan tool for
vehicle diagnosis. It not only has the diagnostic
function like reading fault code, reading data
stream, actuation test, displaying sensor
waveform, coding control unit, etc., but also has
PDA function.
Basic Knowledge of ECU
Composition of Vehicle ECU
Vehicle ECU consists of ALU, CPU, power
source and memory.
1. ALU
It converts the electronic signal from each
sensor into digital signal needed by CPU.
2. CPU
The digital signal is transferred to CPU for
computation and logical operation so that
correct air/fuel mixture can be obtained. CPU
can also compute ignition timing and idle speed,
and control exhaust, fuel and diagnostic
systems.
3. Power source
The reference output signal (5V) of ECU and
the 12V voltage for control module are from
vehicle battery. Memory is also dependent on
the power supplied by the battery.
4. Memory
ECU contains three types of memory: ROM,
RAM and PROM (or EEPROM).
1) ROM
ROM stores the programming information
that can only be read by ECU. The program
in ROM is not editable. Power cut will not
damage the data in ROM.
2) RAM
It is for temporarily storing the inputted data,
fault code and computed result. Power cut
will make all information in RAM erased.
3) PROM
Its program is made by manufacturer to suit
the application of transmission, engine,
exhaust control, driveability, rear axle ratio,
etc. PROM is can be detached from ECU
and power cut will not damage the data in
it.
4) EEPROM
Its function is similar to PROM. The
difference between EEPROM and PROM is
that EEPROM can be re-programmed by
manufacturer with a special device.
Precautions Related to ECU
The circuit inside the ECU is very accurate and
may be easily damaged by overvoltage, static
electricity and/or magnetic fields. Pay more
attention when diagnosing the devices that is