FORD 2008 Explorer v.3 - Manual (Page 285)

Certain add-on devices such as large trailer hitches, bike or
surfboard racks and any device that may block the normal
detection zone of the reverse sensing system may create false beeps.
The system will assist the driver in detecting certain objects while the
vehicle moves in reverse at speeds less than 6 mph (10 km/h). The
system is not effective at speeds greater than 6 mph (10 km/h) and may
not detect certain angular or moving objects.
The system detects obstacles within approximately 16 feet (five meters),
at speeds above 2.5 mph (4 km/h) behind the rear bumper with a
decreased coverage area at the outer corners of the bumper. If vehicle
speed is below 2.5 mph (4 km/h), the system detects obstacles within
approximately 9 feet (2.6 meters) behind 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 approximately less than 18 inches (45.0 cm),
the tone will sound continuously. If the system detects an object that is
approaching the vehicle at such a rate that rapid braking is required, a
very high rate tone will sound. If this tone is heard while reversing, the
driver is advised to slow down immediately until the tone either changes
to a slower rate or stops.
While receiving a warning the radio volume will be reduced to a
predetermined level. After the warning goes away, the radio will return
to the previous value.
Driving
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