Civic L4-1668cc 1.7L SOHC MFI (2003)
Body Control Systems: Description and Operation
System Descriptions
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 (including intermittent wipe and park functions)
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Interlock system
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Keyless/power Door Lock
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Key-in reminder
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Meter assembly, temperature gauge, and indicator lights
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HVAC (compressor and fan control)
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/PCM, 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.
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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.
