JLG 740A, Service And Maintenance Manual

Page: 311 / 408

SECTION 5 - HYDRAULICS
3121160 – JLG Lift – 5-25
Table 5-9. Low Pump Output Flow
Item Description Action
Check fluid level in reservoir. Insufficient hydraulic fluid will limit output flow and
cause internal damage to pump. Fill the reservoir to proper level.
Hydraulic fluid viscosity above acceptable limits. Fluid viscosity above acceptable limits or low fluid
temperature will not allow the pump to fill or con-
trol to operate properly. Allow system to warm up before operation or use
fluid with the appropriate viscosity grade for
expected operating temperatures.
Check external system relief valve setting. External relief valve set below Pressure Compen-
sator setting will cause low output flow. Adjust external relief valve per manufacturer’s
recommendation. External relief valve setting
must be above Pressure Compensator setting for
proper operation.
Check Pressure Compensator and Load Sense
control setting. Low Pressure Compensator setting will prevent
the pump from achieving full stroke. Low Load
Sense setting will limit output flow. Adjust Pressure Compensator and Load Sense
setting. Refer to Adjustments.
Check pump inlet pressure / vacuum. High inlet vacuum will cause low output flow. Correct inlet pressure conditions.
Check input speed. Low input speeds decrease flow. Adjust input speed.
Check pump rotation. Incorrect rotational configuration will cause low
flow. Use pump with appropriate rotational configura-
tion.
Table 5-10. 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 elimi-
nate source of air ingression.
Check control spools. Sticking control spools will cause erratic opera-
tion. 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
Adjustments.
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 Com-
pensator setting. Insufficient pressure differential between Pres-
sure Compensator Pressure Compensator set-
ting and external relief valve. Adjust external relief valve or Pressure Compen-
sator control settings to appropriate level. Relief
valve setting must be above Pressure Compen-
sator setting for proper operation.
Check external relief valve. Chattering external relief valve may cause unsta-
ble feedback to pump control. Adjust or replace relief valve.