Compass: Description and Operation
Interior rear view mirror with compass (where fitted)
Certain vehicles have an interior rear view mirror that features an automatic photochromatic dimming function and an electronic compass with LED display.
The compass mirror is a self-contained unit and does not interface with any other vehicle system or electronic control unit. The mirror is fixed to a metal bracket attached to the windscreen.
A three pin connector provides the electrical connection to the mirror's internal circuit board. Pin-1 of the connector provides the 12 V power supply to the board via the auxiliary relay located in the engine compartment fusebox. When the ignition switch is turned to the 'II' position, a 12 V supply is provided to energise the coil of the auxiliary relay via fuse 26 in the passenger compartment fusebox. The auxiliary relay's contact supply voltage is provided from the vehicle battery through fusible links 1 & 7. When the relay's contacts close, a 12 V supply is fed to the compass mirror circuit board via fuse 15 in the passenger compartment fusebox. This is the supply voltage feed for the mirror's internal compass.
Pin-2 of the electrical connector provides the path to earth.
Pin-3 of the electrical connector is a 12 V ignition switched supply voltage (position 'II' of the ignition switch). The supply voltage is provided to the mirror's circuit board via fuse 25 in the passenger compartment fusebox, and the reverse lamp switch (normally closed) for vehicles with manual transmission or the starter inhibitor / reverse light switch (normally closed) for vehicles with automatic transmission. This is the supply voltage feed for the mirror's photochromatic dimming function.
Compass
The mirror's compass display is activated when the ignition switch is turned to position 'II'. If the compass has been previously calibrated, the current direction of the vehicle will be shown in the LED display at the upper RH side of the mirror's reflective surface. The display is able to indicate one of eight compass points (i.e. N, NE, E, SE, S, SW, W or NW).
The compass mirror circuit board contains a microchip which is able to determine changes in vehicle direction due to the changes in the earth's magnetic field. The circuit board is located in the stem of the mirror so that it follows the changes in the vehicles direction and is not affected by adjustment of the mirror itself.
Because of changes in the lines of magnetic influence, the compass is set for operation in one of fifteen different zones depending on the current location of the vehicle. If the vehicle is to be driven in a different zone, the compass will have to be reset for operation in that zone. To determine the correct zone for a particular location on the planet, refer to the maps provided in the Owner's Handbook. The mirror's default setting is zone 8 which is suitable for central European countries (Germany, Austria, Italy etc.).
The compass should be set to the correct zone using the following procedure:
1 Switch the ignition to position 'II'.
a. If the compass displays one of the eight compass point directions, proceed to step '3'.
b. If the compass displays 'C' then proceed to step '2'.
c. If the display does not show either of the above, then the calibration button on the underside of the mirror should be pressed and held for approximately 6 seconds, using a small probe (e.g. paper clip or ballpoint pen). The compass should now enter the set mode and 'C' should be shown in the display.
2 To calibrate the compass, the vehicle must be driven slowly (5 mph (8 km/h) or less) in a circle until the display shows a direction (usually 1 or 2 revolutions). The mirror can calibrate itself during normal driving, but this will take considerably longer.
3 Set the compass to the relevant zone of magnetic influence by pressing the calibration button on the underside of the mirror for approximately 3 seconds, using a small probe, until a number is shown in the display. If the zone is being entered for the first time, the default zone setting '8' should be displayed. If the zone is being reset from a previous entry, the previous zone setting number will be displayed.
The calibration button should now be pressed and released with single presses, using a small probe, until the desired zone number appears in the display.
After approximately 10 seconds, the compass display should return correctly set to the required zone.
NOTE: the degree of magnetic variation from one zone to another is only very slight. Recalibration of the compass is not normally necessary unless several zone changes are undertaken.
Automatic dimming
The mirror's automatic dimming function operates when the ignition switch is turned to the 'II' position.
A light sensor is contained at the upper centre of the mirror which detects the intensity of light from the headlights of following vehicles in dark or low light conditions. When the light intensity is sufficiently high, the mirror automatically adjusts the brightness of the reflected light by photochromatically changing the mirror's reflective properties and so preventing glare affecting the driver.
When reverse gear is selected, the power supply for the mirror's photochromatic function is opened so that the mirror defaults to its normal reflective properties while the reversing operation is in progress.
Another light sensor is located on the rear of the mirror so that the unit can detect the difference between strong ambient light and that attributable to following vehicle headlights. To check operation of the light dimming function of the mirror, use the following procedure:
1 Turn ignition switch to position 'II', and ensure reverse gear is not selected.
2 Cover the sensor on the back of the mirror.
3 Shine a bright light on the front (reflective side) of the mirror; the mirror should darken.