Lincoln Electric 2004 Town Car Owner's Manual page 190

Driving
The system detects obstacles within approximately five meters (16.4 ft.)
behind the rear bumper with a decreased coverage area at the outer
corners of the bumper. In the figure, the smaller objects behind the
vehicle represent the near-vehicle and bumper corner coverage area of
the park aid system and the large object represents the extended
coverage area. As you move closer to the obstacle, the rate of the tone
increases. When the distance to the obstacle is less than 45.0 cm (18 in.),
the tone will sound continuously. If the system detects an object that is
approaching the vehicle at such as 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 volume. The radio volume may be
overridden using the radio volume control.
The extended rear park aid is
automatically enabled when the gear
selector is placed in R (Reverse)
and the ignition is ON. A park aid
control allows the driver to disable
the extended rear park aid only
when the ignition is ON, and the
gear selector is in R (Reverse).
The OFF indicator remains
illuminated when the system is
disabled. The system defaults to ON
every time the ignition is turned on. Press the control to disable or
enable the system.
The indicator will remain illuminated to indicate a failure of the extended
rear park aid.
Always keep the sensors (located on the rear bumper/fascia) free
from snow, ice and large accumulations of dirt (do not clean the
sensors with sharp objects). These elements may cause the
system to operate inaccurately.
190
2004 Town Car (tow)
Owners Guide (post-2002-fmt)
USA English (fus)