980G Wheel Loader Hydraulic System Caterpillar

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Personal injury can result from hydraulic oil pressure and hot oil. Hydraulic oil pressure can remain in the hydraulic system after the engine has been stopped. Serious injury can be caused if this pressure is not released before any service is done on the hydraulic system. Make sure all of the attachments have been lowered, oil is cool before removing any components or lines. Remove the oil filler cap only when the engine is stopped, and the filler cap is cool enough to touch with your bare hand.

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Personal injury or death can result from escaping fluid under pressure. Escaping fluid under pressure, even a very small pin-hole size leak, can penetrate body tissue and cause serious injury and possible death. If fluid is injected into your skin, it must be treated immediately by a doctor familiar with this type of injury. Always use a board or cardboard when checking for a leak.

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Sudden movement or accidental starting of the machine can cause personal injury or death to persons on or near the machine. To prevent personal injury or death, perform the following: Park the machine on a smooth, level surface. Lower the work tool to the ground and engage the parking brake. Stop the engine and remove the key. Block the wheels and install the steering frame lock.

The main hydraulic pump relief valve is common to the circuits. The auxiliary circuit and the tilt circuit have makeup valves and check valves in the head ends and in the rod ends in order to prevent cylinder drift during movement of the valve spool.

The relief valves in the auxiliary circuit and in the tilt circuit protect the components from pressures in the circuits when the control valves are in the HOLD position.

• The oil pressure for opening the relief valves for the main system and for the auxiliary circuits must be within the specifications. Pressures that are too low will cause a decrease in the lifting force and in the dig characteristics. Pressures that are too high will cause a decrease in the life of hoses and of other components.

• Circuit drift is caused by leakage past cylinder pistons, past O-ring seals in the control valves, past check valves that do not seat correctly or past makeup valves that do not seat correctly. Poor adjustments of the control valves or incorrect fit may also contribute to circuit drift.

• Cycle times that are longer than normal result from oil leakage, from pump wear, or from the pump speed.

If the basic tests indicate a leak in the circuit, install a flow meter and perform a test. This procedure will help to find the source of a leak.

Note: This machine is equipped with an electronic control module (ECM). The ECM has a built-in system diagnostics for detecting faults in the implement electronic control system. The fault codes are displayed on the Caterpillar Monitoring System when the system is in the service mode. Make sure that all faults are corrected before you troubleshoot the following problems.

Reference: For more information about diagnosing the system, refer to the Service Manual Troubleshooting, "Implement Electronic Control System" for the machine that is being serviced.

Problem 1: The pump makes noise and the cylinder rods do not move evenly. There are also air bubbles in the oil.

Probable causes:

1. The viscosity of the oil is incorrect.

2. The relief valve opens at low oil pressure.

3. There is a loose connection of the oil line on the inlet side of the pump.

4. The pump has too much wear.

Problem 2: The oil temperature is too high.

Probable causes:

1. The viscosity of the oil is incorrect.

2. The setting of the relief valve is too low. This causes the relief valve to open prematurely.

3. The implement pump has too much wear.

4. There is a restriction in an oil passage.

5. There is an oil leak in one of the circuits.

6. The load on the system is too high. This causes the relief valve to open.

7. The oil level is low.

8. There is a high pressure oil leak in one or more circuits.

9. The oil is contaminated.

10. There is air in the hydraulic oil.

    Note: A problem with air in the hydraulic oil must be corrected before the hydraulic system will operate at normal temperatures.

    There are two conditions that cause air to be in the hydraulic oil:

    1. Return oil flows into the hydraulic tank above the level of oil that is in the tank.

    2. There are oil leaks in the oil suction line between the pump and the tank.

11. The oil cooler bypass valve has failed.

12. The hydraulic oil cooler is plugged.

Problem 3: The pump output is low.

Probable causes:

1. The oil level is low.

2. The viscosity of the oil is incorrect.

3. The pump has too much wear.

Problem 4: The oil pressure is low.

Probable causes:

1. The relief valve opens at low oil pressure.

2. The pump has too much wear.

3. An O-ring seal in the system failed.

4. The control valve and the valve spool have a large amount of wear.

Problem 5: The attachment moves while the control lever is in the HOLD position.

Probable causes:

1. The control valve and the valve spool have a large amount of wear.

2. A piston seal in a cylinder has a large amount of wear.

3. There is a leak in a connection between the control valve and the cylinders.

4. A check valve is not closing because of dirt or because of a worn seat.

5. Control levers not centered.

Problem 6: The attachment droops when the control lever is moved from HOLD to LIFT.

Probable cause:

The load check valve is not closing because the spring is weak or broken or because there is dirt or debris in the check valve.

Problem 7: The ride control functions too slowly or the ride control functions too fast.

Probable cause:

The pressure equalization time for ride control needs to be adjusted.