GMC Sierra 1999, Owner'S Manual

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The compass can be calibrated in one of two ways:
D Drive the vehicle in circles at 5 mph (8 km/h) or less
until the display reads a direction, or
D Drive the vehicle on your everyday routine and after
several turns the compass will become calibrated and
will display a direction.
Compass Variance
The compass is set in zone eight upon leaving the
factory. It will be necessary to adjust the compass to
compensate for compass variance if you live outside
zone eight. Under certain circumstances, as during a
long distance cross
-
country trip, it will be necessary to
adjust for compass variance. Compass variance is the
difference between earth’s magnetic north and true
geographic north. If not adjusted to account for compass
variance, your compass could give false readings.
To adjust for compass variance:
1. Press and hold the COMP button, located at the
bottom of the mirror, for three seconds until a zone
number appears in the display.
2. Find your current location and variance zone number
on the following zone map.
3. Press the COMP button on the bottom of the mirror
until the new zone number appears in the display.
After you stop pressing the button in, the display will
show a compass direction within a few seconds.