JLG 4394RT Service Manual - Manual (Page 168)

SECTION 4 - HYDRAULICS
4-2 – JLG Lift – 3121249
Cylinders With Single Counterbalance Valve
OPERATE ALL FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1. Using all applicable safety precautions, activate
hydraulic system.
2. Shut down hydraulic system and allow machine to
sit for 10-15 minutes. If machine is equipped with
bang-bang or proportional control valves, turn igni-
tion switch to on, move control switch or lever for
applicable cylinder in each direction, then turn igni-
tion switch to off. If machine is equipped with
hydraulic control valves, move control lever for
applicable cylinder in each direction. This is done to
relieve pressure in the hydraulic lines.
Carefully remove hydraulic hoses from appropriate
cylinder port block.
3. There will be initial weeping of hydraulic fluid, which
can be caught in a suitable container. After the initial
discharge, there should be no further leakage from
the ports. If leakage continues at a rate of 6-8 drops
per minute or more, the counterbalance valve is
defective and must be replaced.
4. To check piston seals, carefully remove the counter-
balance valve from the retract port. After initial dis-
charge, there should be no further leakage from the
ports. If leakage occurs at a rate of 6-8 drops per
minute or more, the piston seals are defective and
must be replaced.
5. If no repairs are necessary or when repairs have
been made, replace counterbalance valve and care-
fully connect hydraulic hoses to cylinder port block.
6. If used, remove lifting device from upright or remove
prop from below main boom, activate hydraulic sys-
tem and run cylinder through one complete cycle to
check for leaks.
Oscillating Axle Cylinder Test (Lockout)
LOCKOUT SYSTEM TEST MUST BE PERFORMED QUARTERLY,
ANY TIME A SYSTEM COMPONENT IS REPLACED, OR WHEN
IMPROPER SYSTEM OPERATION IS SUSPECTED.
NOTE: Ensure platform is fully lowered prior to beginning
lockout cylinder test.
1. Place a 6 inch (15.2 cm) high block with ascension
ramp in front of left front wheel.
2. From platform control station, select LOW drive
speed.
3. Place DRIVE control switch to FORWARD position
and carefully drive the machine up ascension ramp
until left front wheel is on top of block.
4. Raise machine platform approximately 9 ft (2.7 m)
on the 3394RT or 10 ft (3.1 m) on the 4394RT.
5. Place DRIVE control switch to REVERSE position
and carefully drive the machine off the block and
ramp.
6. Have an assistant check to see that the left front
wheel remains locked in position off the ground.
7. Lower the machine platform; the lockout cylinder
should then release and allow wheel to rest on the
ground. It may be necessary to activate DRIVE to
release cylinders.
8. Place the 6 inch (15.2 cm) high block with ascension
ramp in front of right front wheel.
9. From platform control station, select LOW drive
speed.
10. Place DRIVE control switch to FORWARD position
and carefully drive the machine up ascension ramp
until right front wheel is on top of block.
11. Raise machine platform approximately 9 ft (2.7 m)
on the 3394RT or 10 ft (3.1 m) on the 4394RT.
12. Place DRIVE control switch to REVERSE position
and carefully drive the machine off the block and
ramp.
13. Have an assistant check to see that the right front
wheel remains locked in position off the ground.
14. Lower the machine platform; the lockout cylinder
should then release and allow wheel to rest on the
ground. It may be necessary to activate DRIVE to
release cylinders.
15. If the lockout cylinders do not function properly,
have qualified personnel correct the malfunction
prior to any further operation.