JLG 740A, Service And Maintenance Manual

Page: 312 / 408

SECTION 5 - HYDRAULICS
5-26 – JLG Lift – 3121160
HIGH INLET VACUUM CAUSES CAVITATION WHICH CAN DAMAGE
INTERNAL PUMP COMPONENTS.
Table 5-11. System Pressure Not Reaching Pressure Compensator Setting
Item Description Action
Check Pressure Compensator control setting. System pressure will not rise above Pressure
Compensator setting. Adjust Pressure Compensator to appropriate set-
ting. Refer to Adjustments.
Check external relief valve. External relief valve setting below Pressure Com-
pensator setting will prevent pressure compensa-
tion. Adjust external relief valve per manufacturer’s
recommendations. External relief valve must be
set above Pressure Compensator setting for
proper operation.
Inspect Pressure Compensator control spring. Broken, damaged, or missing spring will cause
erratic operation. Replace spring as required.
Inspect Pressure Compensator spool for wear. Wear of the Pressure Compensator spool will
cause internal leakage in the control. Replace the spool as required.
Inspect Pressure Compensator spool for proper
orientation. Improper orientation will result in poor operation. Correct orientation of spool.
Check Pressure Compensator control for con-
tamination. Contamination may interfere with movement of
the Pressure Compensator Spool. Clean Pressure Compensator control compo-
nents, take appropriate action to eliminate con-
tamination.
Table 5-12. High Inlet Vacuum
Item Description Action
Check fluid temperature. Low temperature increases viscosity. High fluid
viscosity causes high inlet vacuum. Allow system to warm up before operation.
Inspect inlet screen. Blocked or restricted inlet screen will cause high
inlet vacuum. Clean screen / remove blockage.
Check inlet piping. Too many fittings, bends, or long piping will cause
high inlet vacuum. Eliminate fittings to make path more direct.
Hydraulic fluid viscosity above acceptable limits. High fluid viscosity causes high inlet vacuum. Select fluid with appropriate viscosity for
expected operating temperature.