Civic Si L4-2.0L (2004)
Information Bus: Description and Operation
System Description
System Description
The Multiplex Control System has four internal functions:
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Multiplexing (send multiple signals over shared wires)
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Wake up/sleep (runs at full power only on demand to reduce battery draw)
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Fail-safe (fixes or ignores faulty signals)
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Self-diagnosis (Mode 1 for the system. Mode 2 for input lines)
The system controls the function of these circuits:
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Entry light control (ignition key light and ceiling light)
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Wiper/washer intermittent wipe and park functions
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Keyless/power door lock
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Meter assembly, temperature gauge, and indicator lights
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HVAC (compressor and fan control)
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Key-in reminder
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Headlight reminder
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Seat belt 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.
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The input signals from each switch are converted to digital signals at the central processing unit (CPU).
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The digital signals are sent from the transmitting unit to the receiving unit as serial signals.
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The transmitted signal is converted to a switch signal at the receiving unit, and it operates the related component or monitors a switch.
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There are exclusive communication lines between the ECM, the gauge assembly, keyless receiver unit, 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.
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In the sleep mode, the multiplex control unit stops functioning (communication and CPU control) when it is not necessary for the system to
operate.
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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.
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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.
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If any door is open, the sleep mode will not function.
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If a key is in the ignition switch, the sleep mode will not function.
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When in sleep mode, the draw is reduced from 70 - 80 mA to less than 10 mA.
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.
