Multiplex Control System Description

Multiplex Control System Description

The multiplex control system has four internal functions:

- Multiplexing (send multiple signals over shared wires)
- Wake up/sleep models (runs at full power only on demand to reduce battery draw)
- Fail-safe (fixes or ignores faulty signals)
- Self-diagnosis (Mode 1 for system DTCs, Mode 2 for input lines)

The system controls the function of these circuits:

- Entry light control (ignition key light and ceiling light)
- Windshield wiper/washer (intermittent wipe and park functions)
- Interlock system
- Power door lock
- Gauge control module (temperature gauge and indicators)
- HVAC (Compressor control)
- Key-in reminder
- Lights-on reminder
- Seat belt reminder
- Keyless entry

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 data signals.
- The transmitted signal is converted to a switch signal at the receiving unit, and it operates the related component or performs a function.
- There are exclusive communication lines between the ECM/PCM, the gauge control module, the 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 in LOCK (0).

- 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 to LOCK (0), and the driver's or front passenger's door is opened, then closed, there is a delay of about 40 seconds 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.

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.