Hydraulic Valve Troubleshooting

Listed below are areas that you can diagnose in hydraulic valves. When working on a specific machine, refer to a machine’s technical manual for more information.

a. Pressure-Control Valves. The following lists information when troubleshooting relief, pressure-reducing, pressure sequence, and unloading valves:

(1) Relief Valves. Consider the following when troubleshooting relief valves because they have low or erratic pressure:

• Adjustment is incorrect.
• Dirt, chip, or burrs are holding the valve partially open.
• Poppets or seats are worn or damaged.
• Valve piston in the main body is sticking.
• Spring is weak.
• Spring ends are damaged.
• Valve in the body or on the seat is cocking.
• Orifice or balance hold is blocked.

Consider the following when troubleshooting relief valves because they have no pressure:

• Orifice or balance hole is plugged.
• Poppet does not seat.
• Valve has a loose fit.
• Valve in the body or the cover binds.
• Spring is broken.
• Dirt, chip, or burrs are holding the valve partially open.
• Poppet or seat is worn or damaged.
• Valve in the body or on the seat is cocking.

Consider the following when troubleshooting relief valves because they have excessive noise or chatter:

• Oil viscosity is too high.
• Poppet or seat is faulty or worn.
• Line pressure has excessive return.
• Pressure setting is too close to that of another valve in the circuit.
• An improper spring is used behind the valve.

Consider the following when troubleshooting relief valves because you cannot adjust them properly without getting excessive system pressure:

• Spring is broken.
• Spring is fatigued.
• Valve has an improper spring.
• Drain line is restricted.

Consider the following when troubleshooting relief valves because they might be overheating the system:

• Operation is continuous at the relief setting.
• Oil viscosity is too high.
• Valve seat is leaking.

(2) Pressure-Reducing Valves. Consider the following when troubleshooting pressure reducing valves because they have erratic pressure:

• Dirt is in the oil.
• Poppet or seat is worn.
• Orifice or balance hole is restricted.
• Valve spool binds in the body.
• Drain line is not open freely to a reservoir.
• Spring ends are not square.
• Valve has an improper spring.
• Spring is fatigued.
• Valve needs an adjustment.
• Spool bore is worn.

(3) Pressure-Sequence Valves. Consider the following when troubleshooting pressure sequence valves because the valve is not functioning properly:

• Installation was improper.
• Adjustment was improper.
• Spring is broken.
• Foreign matter is on a plunger seat or in the orifices.
• Gasket is leaky or blown.
• Drain line is plugged.
• Valve covers are not tightened properly or are installed wrong.
• Valve plunger is worn or scored.
• Valve-stem seat is worn or scored.
• Orifices are too large, which causes a jerky operation.
• Binding occurs because moving parts are coated with oil impurities (due to overheating or using improper oil).

Consider the following when troubleshooting pressure-sequence valves because there is a premature movement to the secondary operation:

• Valve setting is too low.
• An excessive load is on a primary cylinder.
• A high inertia load is on a primary cylinder.

Consider the following when troubleshooting pressure-sequence valves because there is no movement or the secondary operation is slow:

• Valve setting is too high.
• Relief-valve setting is too close to that of a sequence valve.
• Valve spool binds in the body.

(4) Unloading Valves. Consider the following when troubleshooting these valves because a valve fails to completely unload a pump:

• Valve setting is too high.
• Pump does not build up to the unloading valve pressure.
• Valve spool binds in the body.

b. Directional-Control Valves. Directional-control valves include spool, rotary, and check valves. Consider the following when troubleshooting these valves because there is faulty or incomplete shifting:

• Control linkage is worn or is binding.
• Pilot pressure is insufficient.
• Solenoid is burned out or faulty.
• Centering spring is defective.
• Spool adjustment is improper.

Consider the following when troubleshooting directional-control valves because the actuating cylinder creeps or drifts:

• Valve spool is not centering properly.
• Valve spool is not shifted completely.
• Valve-spool body is worn.
• Leakage occurs past the piston in a cylinder.
• Valve seats are leaking.

Consider the following when troubleshooting directional-control valves because a cylinder load drops with the spool in the centered position:

• Lines from the valve housing are loose.
• O-rings on lockout springs or plugs are leaking.
• Lockout spring is broken.
• Relief valves are leaking.

Consider the following when troubleshooting directional-control valves because a cylinder load drops slightly when it is raised:

• Check-valve spring or seat is defective.
• Spool valve’s position is adjusted improperly.

Consider the following when troubleshooting directional-control valves because the oil heats (closed-center systems):

• Valve seat leaks (pressure or return circuit).
• Valves are not adjusted properly.

c. Volume-Control Valves. Volume-control valves include flow-control and flow-divider valves. Consider the following when troubleshooting these valves because there are variations in flow:

• Valve spool binds in the body.
• Cylinder or motor leaks.
• Oil viscosity is too high.
• Pressure drop is insufficient across a valve.
• Oil is dirty.

Consider the following when troubleshooting volume-control valves because of erratic pressure:

• Valve’s poppet or seat is worn.
• Oil is dirty.

Consider the following when troubleshooting volume-control valves because of improper flow:

• Valve was not adjusted properly.
• Valve-piston travel is restricted.
• Passages or orifice is restricted.
• Valve piston is cocked.
• Relief valves leak.
• Oil is too hot.

Consider the following when troubleshooting volume-control valves because the oil heats:

• Pump speed is improper.
• Hydraulic functions are holding in relief.
• Connections are incorrect.

Hydraulic Directional Control Valves

Directional-control valves also control flow direction. However, they vary considerably in physical characteristics and operation. The valves may be a

• Poppet type, in which a piston or ball moves on and off a seat.
• Rotary-spool type, in which a spool rotates about its axis.
• Sliding-spool type, in which a spool slides axially in a bore. In this type, a spool is often classified according to the flow conditions created when it is in the normal or neutral position. A closed-center spool blocks all valve ports from each other when in the normal position. In an open-center spool, all valve ports are open to each other when the spool is in the normal position.

Directional-control valves may also be classified according to the method used to actuate the valve element. A poppet-type valve is usually hydraulically operated. A rotary-spool type may be manually (lever or plunger action), mechanically (cam or trip action), or electrically (solenoid action) operated. A sliding-spool type may be manually, mechanically, electrically, or hydraulically operated, or it may be operated in combination.

Directional-control valves may also be classified according to the number of positions of the valve elements or the total number of flow paths provided in the extreme position. For example, a three-position, four-way valve has two extreme positions and a center or neutral position. In each of the two extreme positions, there are two flow paths, making a total of four flow paths.

Spool valves (see Figure 5-11) are popular on modern hydraulic systems because they—
• Can be precision-ground for fine-oil metering.
• Can be made to handle flows in many directions by adding extra lands and oil ports.
• Stack easily into one compact control package, which is important on mobile systems.

Spool valves, however, require good maintenance. Dirty oil will damage the mating surfaces of the valve lands, causing them to lose their accuracy. Dirt will cause these valves to stick or work erratically. Also, spool valves must be accurately machined and fitted to their bores.