JLG 1850SJ Service Manual - Manual (Page 45)

SECTION 3 - CHASSIS & TURNTABLE
3121619 – JLG Lift – 3-3
3.4 STEERING CONTROL SYSTEM
There are three different modes of steering selectable by the
position of the steer select switch on the platform control
panel: crab, coordinated, and conventional two wheel steer-
ing. These are shown below.
Each wheel has its own steer cylinder, wheel angle sensor, axle
extend linear sensor, and proportional valve, allowing the con-
trol system to position each wheel to the ideal angle for all
steering modes and all steering commands. This is done
whether the axle is retracted, extended, or somewhere in
between. Changes in steering modes while drive is engaged
causes the wheels to automatically adjust to the appropriate
angle for the selected steering mode based on the position of
the inside front wheel. If the steer select switch is changed
without the footswitch depressed or the EMS is off, the wheels
will not move until the footswitch is depressed and a steering
or drive command has been initiated. The steering angles are
limited to +/- 20 degrees anytime the axles are not fully
extended. See the Axle Extension System for interaction with
the axle extension system. If a wheel cannot achieve its com-
manded angle within a specified time, it is considered
jammed. When a wheel is considered jammed during steering,
a fault is reported and the remaining wheels will continue to
their commanded position. The fault is cleared when the foot-
switch is cycled. If a wheel is jammed making it significantly
out of position, with regard to the other wheels, the drive
motors are restricted to their maximum displacement (slow
speed). Wheel angle sensor failures will result in an approxi-
mated steering control logic that will allow the operator to
move the machine until it can be repaired. The wheel at the
failed sensor will be driven based on the information available
from the other sensors. This wheel will not track perfectly and
will become farther out of position over time. When the wheel
becomes prohibitively out of position, the wheels can be
resynchronized by fully steering against the mechanical stops.
Axle extend sensor failures will result in an approximated
steering control logic that will allow the operator to move the
machine until it can be repaired. The axle at the failed sensor
Figure 3-1. Crab Steer
Figure 3-2. Coordinated Steer
Figure 3-3. 2 Wheel Steer