777F Off-Highway Truck Hydraulic System Caterpillar

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Personal injury or death can result from sudden machine movement. Sudden movement of the machine or the release of oil under pressure can cause injury to persons on or near the machine. To prevent possible injury before testing and adjusting the hydraulic system, perform the procedure in the Testing and Adjusting, "General Information" section.

Perform the visual checks before you troubleshoot a problem. Make sure that the transmission is in neutral and stop the engine. During the visual checks, use a magnet to separate ferrous particles (iron and steel) from nonferrous particles (aluminum, rubber and paper).

Check: Check the oil level in the torque converter, hoist, and brake hydraulic tank.

Illustration 1	g00690544
Torque converter, hoist, and brake hydraulic tank

Illustration 2	g01257244
(1) Torque converter, hoist, and brake oil coolers (2) Screens for torque converter, hoist, and brake oil coolers

Check the oil level in the torque converter, hoist, and brake hydraulic tank. Look for air or water in the sight gauge. Many problems in the torque converter are caused by low oil level or air in the oil. Add oil to the torque converter, hoist, and brake hydraulic tank, if necessary. See the Operation and Maintenance Manual for the correct type of oil.

Problem: There is an indication of water in the oil.

Probable Cause

• Failure of torque converter, hoist, and brake oil cooler (1)

Problem: There is an indication of air in the oil.

Probable Cause

• There is a leakage in the oil lines. See "Check: Check for oil leakage.".

Problem:The oil level is too low.

Probable Cause

• There is leakage in the oil lines. See "Check: Check for oil leakage.". Add oil before you perform any tests.

• Brake cooling oil can leak into the final drive. Check the oil level in each final drive.

Problem: The oil level is too high.

Probable Cause

• The engine crankshaft rear seal is allowing engine oil into the torque converter housing.

Check: Check for oil leakage.

Inspect all oil lines, all hoses and all connections for damage or for leaks. Look for oil on the ground under the machine.

Note: If oil can leak out of a fitting or out of a connection, air can enter the system. Air in the system can be as harmful as having too little oil.

Check: Check the brake system oil filter.

Illustration 3	g01256855
(3) Brake system oil filter

Remove brake system oil filter (3). Inspect brake system oil filter (3) for foreign material. Cut the filter open, if necessary.

Note: Brake system oil filter (3) has an oil bypass valve. The bypass valve allows the oil to bypass the filter elements whenever the pressure difference between the inlet oil and the outlet oil becomes too high. Any oil that does not flow through the filter element goes directly into the hydraulic circuit. Dirty oil can cause sticking valves and restrictions in valve orifices.

For the pressure setting of the bypass valve in brake system oil filter (3), refer to the Hydraulic System Specifications, "Oil filter (Parking, Service brakes)".

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 4	g01257930
Pump for hoist, brake cooling, and brake charging. (4) Hoist pump section (5) Brake cooling pump section (6) Brake charging pump section

Probable Cause

• Mechanical failure of hoist pump section (4).

• Mechanical failure of brake cooling pump section (5).

• Mechanical failure of brake charging pump section (6).

Check: Check the torque converter charging oil filter.

Illustration 5	g01256877
(7) Torque converter charging oil filter

Remove torque converter charging oil filter (7). Inspect torque converter charging oil filter (7) for foreign material. Cut the filter open, if necessary.

Note: Torque converter charging oil filter (7) has an oil bypass valve. The bypass valve allows the oil to bypass the oil filter element or the screen whenever the pressure difference between the inlet oil and the outlet oil becomes too high. Any oil that does not flow through the filter element or through the screen goes directly into the hydraulic circuit. Dirty oil can cause sticking valves and restrictions in valve orifices.

For the pressure setting of the bypass valve in the torque converter charging oil filter (7), refer to the Power train Specifications, "Oil filter (Torque converter charging)".

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 6	g01256872
Pump for Torque converter charging and scavenging, transmission charging and scavenging, and torque converter lockup clutch pilot. (8) Transmission charging and scavenging pump sections (9) Torque converter clutch pilot pump section (10) Torque converter charge and scavenge pump sections

Probable Cause

• Mechanical failure of the torque converter charge or scavenge pump section (10).

Check: Check the torque converter lockup clutch oil filter.

Illustration 7	g01257950
(11) Filter for torque converter lockup clutch valve

Remove torque converter lockup clutch valve oil filter (11). Inspect torque converter lockup clutch valve oil filter (11) for foreign material. Cut the filter open, if necessary.

Note: Torque converter lockup clutch valve oil filter (11) has an oil bypass valve. The bypass valve allows the oil to bypass the oil filter element or the screen whenever the pressure difference between the inlet oil and the outlet oil becomes too high. Any oil that does not flow through the filter element or through the screen goes directly into the hydraulic circuit. Dirty oil can cause sticking valves and restrictions in valve orifices.

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 8	g01256872
Pump for Torque converter charging and scavenging, transmission charging and scavenging, and torque converter lockup clutch pilot. (8) Transmission charging and scavenging pump sections (9) Torque converter lockup clutch pilot pump section (10) Torque converter charge and scavenge pump sections

Probable Cause

• Mechanical failure of the torque converter lockup clutch pilot pump section (9).

Check: Check the torque converter scavenge screen.

Illustration 9	g01256879
(12) Cover for the torque converter scavenge screen

Make sure that the oil has been drained from the hydraulic oil tank and from the torque converter for the next check.

Remove cover (12) for the torque converter scavenge screen. Inspect the torque converter scavenge screen for foreign material or restrictions.

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 10	g01256872
Pump for Torque converter charging and scavenging, transmission charging and scavenging, and torque converter lockup clutch pilot. (8) Transmission charging and scavenging pump sections (9) Torque converter lockup clutch pilot pump section (10) Torque converter charge and scavenge pump sections

Probable Cause

• Mechanical failure of the torque converter lockup clutch pilot pump section (9). "Check: Check the torque converter lockup clutch oil filter."

• Mechanical failure of the torque converter charge or scavenge pump section (10). See "Check: Check the torque converter charging oil filter.".

Problem: Aluminum particles are found.

Probable Cause

• Mechanical failure of the torque converter

Problem: Black fiber is found.

Probable Cause

• Worn lockup clutch in the torque converter

Check: Check the screen for the torque converter, hoist, and brake oil coolers.

Illustration 11	g01257244
(1) Torque converter, hoist, and brake coolers (2) Screens for the torque converter, hoist, and brake oil coolers

Remove screens (2) for the torque converter, hoist, and brake oil coolers. Inspect screens (2).

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 12	g01257930
Pump for hoist, brake cooling, and brake charging. (4) Hoist pump section (5) Brake cooling pump section (6) Brake charging pump section

Illustration 13	g01256872
Pump for Torque converter charging and scavenging, transmission charging and scavenging, and torque converter lockup clutch pilot. (8) Transmission charging and scavenging pump sections (9) Torque converter lockup clutch pilot pump section (10) Torque converter charge and scavenge pump sections

Probable Cause

• There is a mechanical failure of the hoist pump section (4).

• There is a mechanical failure of the brake cooling pump section (5). See "Check: Check the brake system oil filter.".

• There is a mechanical failure of brake charging pump section (6). See "Check: Check the brake system oil filter.".

• Mechanical failure of the torque converter lockup clutch pilot pump section (9). See "Check: Check the torque converter lockup clutch oil filter.".

• There is a mechanical failure of the torque converter charge or scavenge pump sections (10). See "Check: Check the torque converter charging oil filter.".

Problem: Large quantities of aluminum particles are found.

Probable Cause

• Mechanical failure of the torque converter

Problem: Black fiber is found.

Probable Cause

• Worn lockup clutch in the torque converter

Check: Check the inside of the hydraulic tank (torque converter, hoist, and brake section).

Illustration 14	g00635050
(13) Suction screen for the torque converter charging pump section and for the torque converter clutch pilot pump section (14) Suction screen for hoist pump and brake cooling pump

Drain the oil from the hydraulic tank. Remove the outer cover from the hydraulic tank. Inspect the bottom of the hydraulic tank for debris. Remove the hoses from the back of the hydraulic tank in order to remove suction screens (13) and (14). Inspect the suction screens. Clean the suction screens.

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 15	g01257930
Pump for hoist, brake cooling, and brake charging. (4) Hoist pump section (5) Brake cooling pump section (6) Brake charging pump section

Illustration 16	g01256872
Pump for Torque converter charging and scavenging, transmission charging and scavenging, and torque converter lockup clutch pilot. (8) Transmission charging and scavenging pump sections (9) Torque converter lockup clutch pilot pump section (10) Torque converter charge and scavenge pump sections

Probable Cause

• There is a mechanical failure of the hoist pump section (4).

• There is a mechanical failure of the brake cooling pump section (5). See "Check: Check the brake system oil filter.".

• There is a mechanical failure of brake charging pump section (6). See "Check: Check the brake system oil filter.".

• Mechanical failure of the torque converter lockup clutch pilot pump section (9). "Check: Check the torque converter lockup clutch oil filter."

• Mechanical failure of the torque converter charge or scavenge pump section (10). See "Check: Check the torque converter charging oil filter.".

Problem: Aluminum particles are found.

Probable Cause

• Mechanical failure of the torque converter See "Check: Check the torque converter scavenge screen.".

Problem: Black fiber is found.

Probable Cause

• Failure of the wheel brakes

• There is a worn lockup clutch in the torque converter. See "Check: Check the torque converter scavenge screen.". Also see "Check: Check the screen for the torque converter, hoist, and brake oil coolers.".

Problem: Shiny steel particles and/or chrome particles are found

Probable Cause

• There is a mechanical failure of a hydraulic cylinder and/or of a cylinder rod. Inspect the cylinder rods for scoring.

Note: If any of the above particles are found during any visual checks, all components of the hydraulic system and of the brake cooling system must be cleaned. Do not use any damaged parts. Damaged parts must be replaced with new parts.