Description of operation

Fig 1: Identifying Switch Description Of Operation

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When the switch (503) for the electrically heated rear seat is pressed, B+ is fed to pin 9 on TWICE. TWICE grounds the relay (511) for the electrically heated rear seat via pin 52.

The relay trips and supplies B+ via fuse 33 to the heating pads in the rear seat and to the indicator in the switch.

The heating time depends on the outdoor temperature. TWICE receives information on the outdoor temperature from SID via the bus.

When the outdoor temperature exceeds -5°C, the heating is connected for 5 minutes. If the outdoor temperature is lower than -5°C, the activation time is longer. At -14 °C, the activation time has reached a maximum. The time is programmable 3- 18 minutes, but is normally preset at 8 minutes.

Fig 2: Temperature Graph

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Fig 3: Button And Control Lighting Circuit Diagram

Courtesy of SAAB-SCANIA OF AMERICA, INC.

So that the battery is not drained unnecessarily when the starter motor is cranking (+15 ON, +54 OFF), TWICE turns off the heating. When the ignition key then returns from ST to ON, TWICE turns on the heating again.

For safety reasons, the disconnection time is maximized at 20 seconds so that the heating works even if +54 is missing in the event of a fault.

TWICE receives information on the ignition +54 from DICE via the bus. Ignition +15 is connected to pin 16 of TWICE.