System Descriptions; Multiplex Communication; Wake-Up And Sleep; Fail-Safe - Honda Civic 2002-2003 Service Manual

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System Descriptions
The Multiplex Control System has four internal
functions:
• Multiplexing (send multiple signals over shared
wires)
• Wake up/sleep (runs at full power only on demand to
reduce battery draw)
Fail-safe (fixes or ignores faulty signals)
Self-diagnosis (Mode 1 for the system, Mode 2 for
input lines)
The system controls the function of these circuits:
• Entry light controllignition key light and ceiling light)
• Wiper/washer intermittent wipe and park functions
Keyless/power door lock
Meter assembly, temperature gauge, and indicator
lights
HVAC (compressor and fan control)
Key-in reminder
Headlight reminder
Seatbelt reminder
Multiplex Communication
To reduce the number of wire harnesses, digital signals
are sent via shared multiplex communication lines
rather than sending normal electrical signals through
individual wires.
• The input signals from each switch are converted to
digital signals at the central processing unit (CPU).
• The digital signals are sent from the transmitting unit
to the receiving unit as serial signals.
• The transmitted signal is converted to a switch signal
at the receiving unit, and
it
operates the related
component or monitors a switch.
• There are exclusive communication lines between
the ECM, the gauge assembly, and the under-dash
fuse/relay box.
Wake-up and Sleep
The multiplex control system has "wake-up" and
"sleep" functions to decrease parasitic draw on the
battery when the ignition switch is OFF.
In the sleep mode, the multiplex control unit stops
functioning (communication and CPU control) when it
is not necessary for the system to operate.
As soon as any operation is requested (for example, a
door is unlocked), the related control unit in the sleep
mode immediately wakes up and begins to function.
• When the ignition switch is turned OFF, and the
driver's or front passenger's door is opened, then
closed, there is about a 10 second delay before the
control unit goes from the wake-up mode to the sleep
mode.
If any door is open, the sleep mode will not function.
If a key is in the ignition switch, the sleep mode will
not function.
• When in sleep mode, the draw is reduced from
70-80 rnA to less than 10 rnA.
Fail-safe
To prevent improper operation, the multiplex control
system has a fail-safe function. In the fail-safe mode,
the output signal is fixed when any part of the system
malfunctions (for example a faulty control unit or
communication line).
Each control unit has a hardware fail-safe function that
fixes the output signal when there is any CPU
malfunction, and a software fail-safe function that
ignores the signal from the malfunctioning control unit
and allows the system to operate normally .
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