JLG 680S, Service And Maintenance Manual

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SECTION 5 - BASIC HYDRAULICS INFORMATION & SCHEMATICS
3121234 5-51
Table 5-12. Pressure or Flow Instability
Item Description Action
Check for air in system. Air in system will cause erratic operation. Activate Pressure Compensator, allowing system to bleed
air. Check inlet line for leaks and eliminate source of air
ingression.
Check control spools. Sticking control spools will cause erratic operation. Inspect spools for free movement in bore. Clean or replace
as needed.
Check Load Sense setting. Low Load Sense setting may cause
instability. Adjust Load Sense setting to proper level. See Adjust-
ments.
Check Load Sense signal line. Blocked Load Sense signal line will interfere with proper
Load Sense operation. Remove blockage.
Check external relief valve and Pressure Compensator set-
ting. Insufficient pressure differential between Pressure Com-
pensator Pressure
Compensator setting and external relief valve. Adjust external relief valve or Pressure Compensator con-
trol settings to appropriate level. Relief valve setting must
be above Pressure Compensator setting for proper opera-
tion.
Check external relief valve. Chattering external relief valve may cause unstable feed-
back to pump control. Adjust or replace relief valve.
Table 5-13. 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 setting.
Check external relief valve. External relief valve setting below Pressure Compensator
setting will prevent pressure compensation. Adjust external relief valve per manufacturer’s recommen-
dations. 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 orienta-
tion. Improper orientation will result in poor operation. Correct orientation of spool.
Check Pressure Compensator control for contamination. Contamination may interfere with movement of the Pres-
sure Compensator Spool. Clean Pressure Compensator control components, take
appropriate action to eliminate contamination.