Ford 1999 Explorer Owner's Manual page 32

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,
(refer to the figures for approximate
zone coverage areas). As you move
closer to the obstacle, the rate of
the tone increases. When the
distance to the obstacle is less than
25.0 cm (10 in.), the tone will sound
continuously. If the system detects a
stationary or receding object further
than 25.0 cm (10 in.) from the side
of the vehicle, the tone will sound
for only 3 second. Once the system
detects an object approaching, the
tone will sound again.
A reverse sensing control allows the
OFF
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. Press the control to disable or enable the system.
The indicator will remain illuminated to 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.
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