Mercury 1999 Mountaineer Owner's Manual page 33

Controls and features
The reverse sensing system detects
obstacles within approximately 1.8
meters (5.9 ft.) of the rear bumper
with a decreased coverage area at
the outer corners of the bumper. As
you move closer to the obstacle, the
rate of the tone increases. When the
distance to the obstacle is less than
20.0 cm (8 in.), the tone will sound
continuously.
A reverse sensing control allows the
driver to enable and disable the RSS
when the ignition is ON. The
reverse sensing control indicator
momentarily illuminates when the
ignition is turned ON. The OFF
indicator remains illuminated when
the system is disabled. The system
defaults to ON every time the
reverse gear is selected. The
indicator will remain illuminated to
OFF
indicate a failure of the reverse
sensing system.
Always keep the sensors (located on the rear bumper/fascia) free
from dirt, snow and ice (do not clean the sensors with sharp
objects). These elements may cause the system to operate
inaccurately.
If the vehicle sustains damage to the rear bumper/fascia, leaving it
misaligned or bent, the sensing zone may be altered causing inaccurate
measurement of obstacles.
33