Vehicle System Testing and Adjusting 33
3. The setting on the relief valve is too low.
4. Too many restrictions in the system.
5. Hydraulic oil level in the tank is too low.
6. High Pressure oil leak in one or more circuits.
7. Very dirty oil.
8. Air in the hydraulic oil.
NOTE: If the problem is because of air in the oil, it
must be corrected before the system will operate at
normal temperatures. There are two things that
cause air in the oil (aeration). These are:
a. Return oil to the tank goes in above the level of
the oil in the tank.
b. Air leaks in the oil suction line between the pump
and the tank.
Hydraulic System and Mast
During a diagnosis of the hydraulic systems,
remember that correct oil flow and pressure are
necessary for correct operation. The output of the
pump (oil flow) increases with an increase in engine
speed (rpm) and decreases when engine speed
(rpm) is decreased. Oil pressure is caused by
resistance to the flow of oil.
Visual checks and measurements are the first step
when troubleshooting a possible problem. Then do
the operation checks and finally, do instrument tests
with pressure gauges.
Use the Fittings Group, a stop watch, a magnet, a
thermometer, and an inch(mm) ruler for basic tests
to measure :
1. The pressure of the oil to open the relief valve.
Relief valve pressures that are too low will cause
a decrease in the lift and tilt characteristics of the
lift truck. Pressures that are too high will cause a
decrease in the life of hoses and components.
2. Drift rates in the cylinders. Cylinder drift is caused
by a leakage past cylinder pistons, O-ring seals in
the control valve, check valves that do not seat
correctly or poor adjustment or fit in the control
valve spools.
3. Cycle times in the lift and tilt circuits : Cycle times
that are too long are the result of leakage, pump
wear and/or pump speed (rpm).
Problem: The hydraulic system will not lift the
load.
Probable Hydraulic Cause:
1. There is an air leak, which lets air into the
hydraulic system on the inlet side of the hydraulic
pump.
2. The relief valve opens at low oil pressure.
3. The hydraulic pump has too much wear.
4. The priority valve does not work correctly.
5. The load is not correct.
6. Unload function doesn’t work correctly.
Probable Mechanical Cause:
1. The mast is not in alignment with the other lifting
components and does not move freely.
2. Not enough lubricant on the parts of the mast that
move.
3. The carriage or mast rollers and bearings are
worn and do not move (seized).
4. Mast shimmed too tight.
Problem: Lift cylinder extends too slowly.
Probable Hydraulic Cause:
1. Not enough oil supply to lift cylinder.
2. Defective lift cylinder seals.
3. Relief pressure set too low.
4. Unload function doesn’t work correctly.
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