844K Wheel Dozer Caterpillar

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Sudden movement of the machine or release of oil under pressure can cause serious injury to persons on or near the machine. To prevent possible injury, perform the procedure that follows before testing and adjusting the hydraulic system.

Note: Prior to examining the hydraulic system review the hydraulic schematic and the System Operation/Test and Adjust manual for the 844K:

• Hydraulic Schematic, UENR4895, "844K Wheel Dozer (T4F & LRC) Hydraulic System Schematic"

• System Operation/Test & Adjust, UENR4599, "844K Wheel Dozer (T4F & LRC) Machine Systems Operation/Testing & Adjusting"

Problem List for the Hydraulic System

1. The temperature of the oil is too hot.

2. The pump makes an unusual noise and the cylinder rods do not move smoothly.

3. A large amount of air is in the oil.

4. The pump standby pressure is too low.

5. The pump standby pressure is too high.

6. There is a pause before the pressure is reached in all circuits.

7. Any one of the implements move, when the control lever is in the HOLD position.

8. The line relief valves are too noisy.

9. There is too much implement drift.

10. The implement slips at the fully raised position, when the implement is moved from a partially raised position to a fully raised position.

11. The cycle time for the lift cylinders or the tilt cylinders are too slow.

Problem 1

The temperature of the oil is too hot.

Possible Causes

1. The oil level of the hydraulic tank is too low.

2. The viscosity of the oil is wrong.

3. The pump is worn.

4. There is a restriction in flow due to a bent hose, a bent tube, or a restriction in an oil passage.

5. There is air in a hydraulic circuit.

6. There is a restriction or a reduction of air flow through the oil cooler.

7. The outside air temperature is too hot.

Problem 2

The pump makes unusual noise and the cylinder rods do not move smoothly.

Possible Causes

1. The oil level of the hydraulic tank is too low.

2. The viscosity of the oil is wrong.

3. There is a loose connection on the inlet side of the pump or the tube on the pump outlet is bent.

4. There is air in a hydraulic circuit.

5. Oil additives (friction modifying oils) were not added.

6. Hydraulic tank is not maintaining pressure.

Problem 3

A large amount of air is in the oil.

Possible Causes

1. There is a loose connection on the inlet side of the pump.

2. The hydraulic system was not purged of air after assembly, inspection, or testing.

3. There is leakage in the cylinder seals or around the cylinder seals.

4. Check for loose fittings or cracked fittings.

5. The oil level of the hydraulic tank is too low.

Problem 4

The pump standby pressure is too low.

Possible Causes

1. The margin pressure is low.

2. The pump is worn.

3. There is contamination in the pump controller.

Problem 5

Pump standby pressure is too high.

Possible Causes

1. The margin pressure is high.

2. There is contamination in the pump controller.

Problem 6

There is a pause before the pressure is reached in all circuits.

Possible Causes

1. The margin pressure is low.

2. There is air in the signal network.

3. There is leakage in the signal network.

4. There is contamination in the load sense signal line and /or network

Problem 7

When the control lever is in the HOLD position, the implements move.

Possible Causes

1. The main control valve spool or the bore may be worn excessively. This wear may cause leaks in the main control valve.

2. Contamination in the main control valve is preventing the spool from centering completely.

3. There is a leak around the seals of the piston in the cylinders that are affected.

4. There is a leak in the lines between the control valve and the cylinders.

5. The solenoid has contamination.

6. The makeup valves are open or the seals are damaged.

Problem 8

There is too much implement drift.

Possible Causes

1. There is a leak around the seals of the piston in the cylinder that is being affected.

2. There is a leak past the makeup valve in the circuit.

3. The spring for the main control valve is broken.

4. The solenoid is contaminated, causing the spool to stick.

5. The main control valve spool or bore may be worn excessively. This condition may cause leaks.

6. The manual lower valve is opened.

Problem 9

The implement droops, when the implement is moved from a partially raised position to a raised position.

Possible Causes

1. The signal passages in the main spool of the control valve that is affected is blocked.

2. The load check valve is not seating properly.

3. The margin pressure is set too low.

4. There is air in the signal network.

Problem 10

All implement cycle times are too slow.

Possible Causes

1. There is a blockage or a leakage in the signal network.

2. The standby pressure is set too low.

3. The pump does not upstroke fully. The actuator piston is stuck or the swashplate is blocked.

4. The pump is worn.

5. The pump high-pressure cutoff is set too low.

6. The pilot pressure is low.

7. The engine speed is too low.

8. The cylinders are leaking for the affected circuit.