06GMK 6250 All Wheel Steering
06GMK 6250 All Wheel Steering
06GMK 6250 All Wheel Steering
Function description:
Under normal highway steering conditions axles 1,2,3,5, & 6 are all steering axles.
All the highway steering axles are interconnected by a mechanical linkage to
synchronise the axles. A mechanical locking mechanism (steering idler assembly) is
installed to enable axle 5 & 6 to be unlocked from the synchronised position and steer
independently from the front axle group. However, once unlocked from the front axle
group, axles 5 & 6 remain synchronised by a mechanical linkage. Axle 4 has steering
capability, but in highway mode it remains locked in the straight-ahead position by an
internal lock within the steering cylinder. There is no mechanical connection to the
other steering axles. Axle 3 has a single steering cylinder for normal highway steering
and a hydraulic cylinder locked by an internal lock, in mid stroke, which acts as a
drag link between the normal steering cylinder and the wheels.
The all wheel steering system allows all six axles to be steered in crab or co-ordinated
modes on the jobsite. Steering geometry, which is suitable for highway travel does
not normally allow correct steering alignment in the all wheel steering mode. The
problem of overcoming the Ackermann principles, which are applicable to the
steering geometry during highway travel, is achieved with a unique
electronic/hydraulic control steering system.
When operational, the steering system allows the front three axles, 1-3 to be steered
with the steering wheel. The third axle has an additional independent hydraulic
control for the drag link steering cylinder, dictated by the relative steering angle
between axles 1-2 & 5-6 respectively. This allows the third axle to be steered by
combined or individual control of the two cylinders connected to it. Three control
modes are possible for axle 3. Mode 1: control from the conventional steering
cylinder when the steering wheel is turned. Mode 2: with additional series control to
the drag link cylinder, thus combining the cylinders and giving additional steering
lock. Mode 3: with control only from the drag link cylinder when the steering wheel is
not turned from the straight-ahead position. Axles 5 & 6 are steered with a rocker
switch from the driver’s seat. Axle 4 is steered by independent cylinders with control
dictated by the relative steering angle of axles 1-2 & 5-6 respectively.
The system consists of an electronic control module (ESX). The module is given
input/command values from a potentiometer mounted on the mechanical steering lock
assembly, which gives the steering position reference (command value) between axle
1-2 & 5-6. Axles 3 and 4 each have individual potentiometers to measure their actual
angles in relation to the command values. The electronic module compares the
difference and via individual proportional control valves and individual steering
cylinders, brings axles 3 and 4 to correspond with the steering angles of the axles 1-2
& 5-6 respectively. Axle 3 overcomes the problem of standard steering geometry by
using the additional drag link cylinder to provide the steering lock required for correct
wheel tracking in the all wheel steering mode. As axle 4 is steered by independent
cylinders, it can also meet the wheel tracking requirements of all wheel steering.
GMK 6250 Training Information. May 98. All wheel steering. Page 1
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• The third axle lockable drag link cylinder, fourth axle lockable steering cylinder
and the steering idler assembly are all unlocked on selection of all wheel steering.
• Control of the front axle steering is from the steering wheel and the rear axles are
steered by a rocker switch located on the driver’s seat.
• Once unlocked, the steering idler potentiometer gives a command value to ESX
related to the relative position of the front and rear axles.
• ESX control via proportional solenoids will control the steering cylinders until the
actual position stroke potentiometers correspond with the command value. This
control matches the third & fourth axle wheel tracking to the selected steering
configuration dictated by the front and rear axles, which produces the command
value from the relative angle potentiometer in the rear steering idler assembly.
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System functions:
Selection of independent rear steering is by a switch -S4 in the driver’s cab. A relay
interlock -K1 is controlled by the Allison ECU to inhibit power to the switch until the
gear selector is placed in neutral or a low gear not higher than second. Subject to
correct gear selection, the ESX module receives a 24v unlock signal.
The ESX module then provides power to the pneumatic solenoids -Y1 & Y12, which
will then unlock the lockable drag link cylinders of axles 3 & 4 and unlock the rear
steering idler assembly. Switches in the cylinders and the steering idler provide
locked/unlocked status to the ESX and when unlocked give an up-shift lock signal to
the Allison ECU/unlocked warning via light -H6 in the driver’s cab.
Solenoid valves -Y21, -Y22, -Y31 & -Y32 are energised by ESX to disengage the
hydraulic connection between axle pair 5 & 6 from steering axles 1, 2 & 3.
Additionally, the solenoid valves now connect the steering cylinders of axles 5 & 6 to
the direction valves -Y2 left lock & -Y3 right lock, which give independent steering
control via the driver’s seat switch to axles 5 & 6.
Operation of the all wheel rear steering via the seat switch provides a direction-input
signal to ESX and energises solenoid valve -Y9 via relay K30 (suspension interlock)
for operating pressure. This valve will re-route P1 supply from steering circuit 1 to
the proportional control valve block at port P. The pump volume is also increased to
55 litres per min via the 5.5-mm flow control orifice, which is in the diverter valve
block. An additional valve –Y11 beside the diverter valve also energised by ESX
now connects a 150 bar relief valve to the load sense line of P1. This will reduce the
actual rear steering pressure to approximately 165 bar.
Oil entry into the proportional valve block is controlled by solenoid valve -Y10,
which is energised by the ESX module. Although the porting arrangement looks
strange, this is only to achieve maximum flow into the valve by using both ports.
Once oil enters the block, it supplies both proportional valves via a 2.5-mm restrictor
for each valve. The proportional valve functions -Y5 = Left lock axle 3, -Y6 = right
lock axle 3, -Y7 = left lock axle 4 & -Y8 right lock axle 4 are all controlled by a
(mA) current output from ESX. A single 1.5 litre accumulator is connected to the oil
supply downstream of the solenoid valve -Y10 to make up any flow requirement.
There are two relief valves integrated into the valve block. One is set to 180 bar and
functions as a maximum system pressure safety valve, the other is set to 130 bar and
functions as a sequence valve to give control priority to axles 3 & 4. Oil flow exits
the block via a 2.0-mm restrictor from port A3, onward bound to the direction control
block for axles 5 & 6.
Oil enters the rear steering direction control block at port P5 and gains access to the
direction valve through solenoid valve-Y4 de-energised, or is routed through a 2 mm
restrictor when energised. The solenoid is energised by ESX to give a slow motion
function to reduce the speed of axles 5 & 6 when locking/unlocking the rear steering
idler assembly. The direction valve -Y2 left lock & -Y3 right lock routes oil to the
steering cylinders from the direction control block via the front/rear blocking
solenoids on ports P1 – P4. 20 bar in line relief valves are connected to these ports to
generate back-pressure in the steer cylinders to reduce reaction during highway travel.
GMK 6250 Training Information. May 98. All wheel steering. Page 3
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When locked for highway operation, the rear steering idler assembly rotates only as a
fixed idler to connect the front and rear steering groups. As the steering levers remain
locked at 1800 there is no change to the output (command value) of the relative angle
potentiometer. However, when the rear steering idler is unlocked the steering levers
are separated and are free to rotate in any direction, irrespective of the other lever’s
position. The difference from the 1800 position, or relative angle of the two levers is
used to drive the potentiometer and produce the output (command value) necessary to
control the all wheel steering function.
Command value is
Command value
produced relative to
remains at zero
lever positions
value
The power supply to the relative angle potentiometer is 8.5 volts. As long as the
levers remain at 1800 to each other, the command value will remain at zero value.
The zero value = half the voltage span. The command value voltage will rise or fall
depending on the lever positions, which are dictated by the selected steering
configuration. This rising/falling command value is used to control the proportional
valves via the ESX module and thus select the correct direction and degree of steering
lock required for axles 3 & 4. A rising command value from zero will select a left
steering lock and a falling command value from zero will select a right steering lock.
GMK 6250 Training Information. May 98. All wheel steering. Page 4
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To rear
To front axles 5 - 6
axles 1 - 3
Air pressure from solenoid valve –Y12 will provide air to overcome the internal spring force in the
cylinder (similar to maxibrake) and lift the outer body to unlock/disengage axles 5 & 6 from the
front axles. This allows both levers to rotate freely, irrespective of the other lever’s position.
Potentiometer
The following page, showing possible steering configurations illustrates the relative
angles of the steering idler levers and the resulting command value outputs. Although
compatible command values can be found, these are produced by different
configurations, that still requires the same steering configuration of axles 3 & 4.
This symbol will be used to represent the relative steering angles of the
rear steering idler levers and the corresponding command value.
GMK 6250 Training Information. May 98. All wheel steering. Page 5
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b)
X= < 0
c)
X= 0
d)
X= > 0
e)
X= Max
f)
X= > 0
g)
X= 0
h)
X= < 0
j)
X= Min
GMK 6250 Training Information. May 98. All wheel steering. Page 6
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The key-operated switch is turned on. For this purpose, the driving
speed must be <15 km/h and the gear signal has to be ≤ 2nd. The
driving speed and the gear signal are not checked by the ESX.
Current is supplied to the ESX after having turned on the key-operated
switch (UE and UB) and therefore, the ESX is completely out of action
during highway travel. The separate steering system is activated by
means of a toggle switch, which is connected to D+ of the ESX.
With all wheel steering mode, the axles 5 and 6 are controlled by
rocker-switch (DigIn 8, DigIn10). The axles 5 and 6 are controlled by a
two-position valve (output 7, output 8). As regards the software, no
limitation of the limit stops has to be integrated. With all wheel steering
mode, axles 1 and 2 are still hydraulically controlled. Axles 3 and 4 are
controlled according to the steering angle between the front axle/rear
axle system. Designated outputs are digital output 10 and PWM-output
5). A control lamp (output 11) shows the all wheel steering mode. If the
control lamp does not light up, the all wheel steering mode is not
GMK 6250 Training Information. May 98. All wheel steering. Page 7
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When de-activating the all wheel steering mode please consider the
following two cases:
1st case: Toggle switch was set to highway travel and is now
turned back to separate steering mode, before the
steering cylinders of the axles 3 and 4 will be locked.
Reaction: Steering cylinders 3 and 4 are unlocked (digital
output 6 on HIGH). Separate steering mode remains
activated.
2nd case: Toggle switch was set to highway travel and is now
turned back to separate steering mode, after the steering
cylinders of the axles 3 or 4 have been locked.
Reaction: Axle 3 and (or) axle 4 (depending on which axle
is already locked) are controlled until both steering
cylinders & the steering idler are locked. Afterwards, the
separate steering mode will be reactivated.
GMK 6250 Training Information. May 98. All wheel steering. Page 8
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1. The operator moves the steering wheel carefully until the front
axle and rear axle steering lock match. The steering locks
automatically.
2. The operator actuates the rear axles steering and searches for
the front axle position. For this purpose, the steering speed
(slow motion) of the axles 5 and 6 (digital output Baby14 on
HIGH) is reduced, as soon as the front axle/rear axle system
ratio is within a certain tolerance value +/-100. Once the steering
speed is reduced, it remains the same until all the steering
cylinders and the steering idler assembly is locked.
1.3 Troubleshooting
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GMK 6250 Training Information. May 98. All wheel steering. Page 11
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Mask Description
Read error memory Mask for reading the error memory. The last 15
errors will be saved in a memory.
Set error memory Mask for setting back the error memory. After
energising the field “read”, the error memory will be
set back.
Actual current of axles Mask for displaying the control valve flow (I max).
After activating the mask, the control valve flow of
axle 3 & 4 will be displayed in mA.
Regulating circuit axle 3 Amplification factor and integration time constant of
positioning regulator axle 3
Regulating circuit axle 4 Amplification factor and integration time constant of
position regulator axle 4
Sensor calibration This mask contains:
GMK 6250 Training Information. May 98. All wheel steering. Page 12
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Mask Description
Tolerance speed reduction Mask for the determination of the area, from
which the axles 5 & 6 are controlled with
reduced speed in degrees / 10
Monitoring of the digital in- and outputs In this mask, the digital in- and outputs can be
activated and de-activated.
Monitoring of regulating circuit and control In this mask, the monitoring of the axles 3 &
valve 4 can be activated and de-activated. This
function is deactivated, as deviations from the
desired value are possible, due to the
“pressure requirements”.
For monitoring the steering valve, a certain
PWM – relation will be assigned to a certain
current. Therefore, the max. and min. ohmic
resistance of the steering valves are to be
provided.
Valve pulsation In this mask, the pulsation time and amplitude
of pulsation for axle 3 & 4 can be determined.
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GMK 6250 Training Information. May 98. All wheel steering. Page 15
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The values “set value sensor (current corrections)” and “set value sensor (zero offset
compensation)” will be determined during the calibration by means of the following
parameters:
GMK 6250 Training Information. May 98. All wheel steering. Page 16
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119 Axle 3 locked during moving Axle 3 switch locked during steering mode
121 Axle 4 locked during moving Axle 4 switch locked during steering mode
122 Axle 5, 6 locked during moving Axle 5, 6 switch locked during steering mode
123 EXT 8,5 V out of tol. 8,5 V power supply sensor out of Toleranz
shortcut to ground or shortcut to UB
Discription
KS UB shortcut to 24V plus supply
KS ground shortcut to 24V minus supply
KS ground LU shortcut to 24V minus supply or line disconnection
LU Line disconnection
Tol tolerance
H.Eilers 21.03.00 1
Error-Code- Liste Separate Lenkung.xls
H.Eilers 21.03.00 2
Error-Code- Liste Separate Lenkung.xls
H.Eilers 21.03.00 3
Error-Code- Liste Separate Lenkung.xls
119 Axle 3 locked during moving Axle 3 switch locked during steering mode
121 Axle 4 locked during moving Axle 4 switch locked during steering mode
122 Axle 5, 6 locked during moving Axle 5, 6 switch locked during steering mode
123 EXT 8,5 V out of tol. 8,5 V power supply sensor out of Toleranz
shortcut to ground or shortcut to UB
Discription
KS UB shortcut to 24V plus supply
KS ground shortcut to 24V minus supply
KS ground LU shortcut to 24V minus supply or line disconnection
LU Line disconnection
Tol tolerance
H.Eilers 21.03.00 4