JLG 1850SJ Service Manual - Manual (Page 263)

SECTION 5 - BASIC HYDRAULIC INFORMATION & HYDRAULIC SCHEMATICS
3121619 – JLG Lift – 5-93
SYSTEM PRESSURE NOT REACHING PC SETTING
HIGH INLET VACUUM
HIGH INLET VACUUM CAUSES CAVITATION WHICH CAN DAMAGE INTERNAL
PUMP COMPONENTS.
Item Description Action
Check PC control setting. System pressure will not rise above PC setting. Adjust PC to appropriate setting.
Check external relief valve. External relief valve setting below PC setting presents
pressure compensation. Adjust external relief valve according to manufacturer’s
recommendations. External relief valve must be set
above PC setting to operate properly.
Inspect PC control spring. Damaged or missing spring will cause erratic operation. Replace spring as required.
Inspect PC spool for wear. Wear of PC spool causes internal leakage in control. Replace spool as required.
Inspect PC spool for proper orientation. Improper orientation results in poor operation. Correct orientation of spool.
Check PC control for contamination. Contamination may interfere with movement of PC
spool. Clean PC control components. Take appropriate action to
eliminate contamination.
Item Description Action
Check fluid temperature. Low temperature increases viscosity. High fluid viscosity
causes high inlet vacuum. Allow system to warm up before operating.
Inspect inlet screen. Blocked or restricted inlet screen. Clean screen/remove blockage.
Check inlet piping. Too many fittings, bends, or long piping causes 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 oper-
ating temperature.