966G Series II Wheel Loader and 972G Series II 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.

Problem 1: The implement 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 main relief valve opens at low oil pressure.

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

4. The implement pump has too much wear.

Problem 2: The oil temperature is too high.

Probable causes:

1. The machine is being operated above the intended capacity or in ambient temperatures that are too high for the machine's design.

2. The viscosity of the oil is incorrect.

3. The setting of the main relief valve is too low. This causes the main relief valve to remain open.

4. The implement pump has too much wear.

5. There is a restriction in an oil passage.

6. There is a leak in one or more circuits.

7. The oil level is low.

8. The oil is contaminated.

9. The air flow through the hydraulic oil cooler is too low.

10. The oil flow through the hydraulic oil cooler is too low.

11. 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:

    • Return oil is flowing into the hydraulic tank above the level of oil that is in the tank.

    • There are leaks in the oil suction line between the implement pump and the hydraulic tank.

12. The recirculation seals are damaged or missing. This allows hot air to recirculate upstream of the hydraulic oil cooler.

13. The hydraulic oil temperature sensor is faulty.

Problem 3: The output of the implement pump is low.

Probable causes:

1. The oil level is low.

2. The viscosity of the oil is incorrect.

3. The implement pump has too much wear.

Problem 4: The oil pressure is low.

Probable causes:

1. The main relief valve opens at low oil pressure.

2. The implement pump has too much wear.

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

4. The main control valve and/or a stem may have an excessive amount of wear.

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

Probable causes:

1. The pilot valve and the valve spool have an excessive amount of wear.

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

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

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

5. The pilot control valve is not centering correctly.

Problem 6: The lift cylinder droops when the lift control lever is moved from the HOLD position to the RAISE position.

Droop is an initial movement of the hydraulic cylinders. The movement is in the opposite direction that is commanded by the operator. The cylinders then move in the correct direction.

Probable causes:

1. The oil is too cold.

2. The piston seals and/or the lift cylinders are worn.

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

Problem 7: The hydraulic force is too low when the lift control lever is moved to the RAISE position or to the LOWER position.

Probable causes:

1. The machine is being operated incorrectly.

2. The pilot system pressure is too low.

3. The rocker plate in the pilot valve is not adjusted correctly.

4. The main control valve is contaminated or damaged.

5. The makeup valve is sticking.

6. The pilot valve is contaminated or damaged.

7. The main relief valve setting is too low.

Problem 8: All implement functions are sluggish.

Probable causes:

1. The oil level is too low.

2. The implement pump is worn or damaged.

3. The main relief valve setting is too low.

4. The primary pressure reducing valve in the pilot valve (oil manifold) is faulty.

5. The oil in the hydraulic system is contaminated.

Problem 9: The implement functions are erratic or intermittent.

Probable causes:

1. The oil level is too low.

2. 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:

   • Return oil is flowing into the hydraulic tank above the level of oil that is in the tank.

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

3. The pump has too much wear.

4. The rocker plate on the pilot valve is not adjusted correctly.

5. The primary pressure reducing valve in the pilot valve (oil manifold) is worn or damaged.

6. The main control valve is contaminated or damaged.

7. 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 10: There is excessive lift cylinder drift.

Drift is a cylinder movement with the pilot valve in the HOLD position.

Probable causes:

1. The rocker plate on the pilot valve is not adjusted correctly.

2. The ride control diverter valve is worn or damaged. Also, the pressue balancing valve spool for ride control may be worn.

3. The piston seals and/or the lift cylinders are worn.

4. The components of the main control valve are worn or damaged.

5. 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 11: The hydraulic force is too low when the tilt control lever is moved to the TILT BACK position or to the DUMP position.

Probable causes:

1. The machine is being operated incorrectly.

2. The rocker plate on the pilot control valve is not adjusted correctly.

3. The primary pressure reducing valve in the pilot valve group is worn or damaged.

4. The settings of the line relief valves are too low.

5. The piston seals and/or the lift cylinders are worn.

6. The main control valve is contaminated or damaged.

7. The pilot valve is contaminated or damaged.

8. The main relief valve setting is too low.

Problem 12: The lift cylinders collapse during the lift operation.

Cylinder collapse is the sudden drop during normal operation.

Probable causes:

1. The primary pressure reducing valve in the pilot valve (oil manifold) is worn or damaged.

2. The ride control diverter valve may be worn or damaged. Also, the pressure balancing valve spool for ride control may be worn.

3. The nitrogen precharge pressure in the ride control accumulator is too low.

4. The piston seals and/or the lift cylinders are worn.

5. The main control valve is contaminated or damaged.

6. The makeup valve is sticking.

7. 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 13: There is low power to the implements.

Probable causes:

1. The machine is operating above the intended capacity.

2. The engine power is too low.

3. The implement pump has too much wear.

4. The rocker plate on the pilot valve is not adjusted correctly.

5. The pressure setting of the primary pressure reducing valve in the pilot valve group is too low.

6. The main relief valve setting is too low.

7. The settings of the line relief valves are too low.

8. The piston seals and/or the cylinders are worn.

9. The load check valve is damaged or contaminated.

Problem 14: There is too much tilt cylinder drift.

Drift is a cylinder movement with the joystick control in the HOLD position.

Probable causes:

1. The rocker plate on the pilot valve is not adjusted correctly.

2. The setting of the line relief valve is too low.

3. The piston seals and/or the tilt cylinder is worn.

4. The components of the main control valve are worn or damaged.

5. The load check valve is contaminated or damaged.

Problem 15: The tilt cylinder collapses during the tilt operation.

Cylinder collapse is the sudden drop during normal operation.

Probable causes:

1. The rocker plate on the pilot valve is not adjusted correctly.

2. The setting of the line relief valve for the tilt cylinder is too low.

3. The piston seals and/or the tilt cylinder is worn.

4. There is dirt or debris in the main control valve.

5. The main control valve spool and mating surfaces may be worn or damaged.

6. The makeup valve is sticking.

Problem 16: The attachment droops during the tilt function.

Droop is an initial movement of the hydraulic cylinders. The movement is in the opposite direction that is commanded by the operator. The cylinders then move in the correct direction.

Probable causes:

1. The oil is too cold.

2. The piston seals and/or the lift cylinder is worn.

3. 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 17: There is too much drift in the auxiliary cylinders.

Drift is a cylinder movement with the pilot valve in the HOLD position.

Probable causes:

1. The rocker plate on the pilot valve is not adjusted correctly.

2. The setting of the line relief valve is too low.

3. The piston seals and/or the auxiliary cylinders are worn.

4. 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.

5. The main control valve is contaminated or damaged.

Problem 18: The auxiliary cylinders collapse during the auxiliary function.

Cylinder collapse is the sudden drop during normal operation.

Probable causes:

1. The rocker plate on the pilot valve is not adjusted correctly.

2. The setting of the line relief valve is too low.

3. The piston seals and/or the auxiliary cylinders are worn.

4. There is dirt or debris in the main control valve.

5. The main control valve spool and mating surfaces may be worn or damaged.

Problem 19: The attachment droops during the auxiliary function.

Droop is an initial movement of the hydraulic cylinders. The movement is in the opposite direction that is commanded by the operator. The cylinders then move in the correct direction.

Probable causes:

1. The oil is too cold.

2. The piston seals and/or the auxiliary cylinders are worn.

3. 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 20: The hydraulic force is too low during auxiliary functions.

Probable causes:

1. The machine is being operated above the intended capacity.

2. The rocker plate on the pilot valve is not adjusted correctly.

3. The primary pressure reducing valve in the pilot valve group is faulty.

4. The setting of the line relief valve is too low.

5. The piston seals and/or the auxiliary cylinders are worn.

6. The main control valve spool and the mating surfaces are worn or damaged.

7. The spool in the pilot valve and the mating surfaces are worn or damaged.

Problem 21: One implement function is sluggish or nonfunctional.

Probable causes:

1. The rocker plate on the pilot valve is not adjusted correctly.

2. The setting of the line relief valve is too low.

3. The spool in the ride control diverter valve and the mating surfaces are worn or damaged.

4. The main control valve spool and the mating surfaces are worn or damaged.

Problem 22: The implements are not functioning.

Probable causes:

1. The ball valve (hydraulic lockout) is in the LOCKED position.

2. The pilot pump is worn or damaged.

3. The implement pump has too much wear.

4. The primary pressure reducing valve in the pilot valve group is faulty.

5. The main relief valve setting is too low.