776D, 777D and 777D HAA Off-Highway Truck/Tractors Air System and Brakes Pulled Unit Hydraulics, Torque Converter, and Brake System Troubleshooting Caterpillar

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Personal injury or death can result from sudden machine movement. Sudden movement of the machine or release of air pressure or oil pressure can cause injury to persons on or near the machine. To help prevent possible injury before testing and adjusting any air system or 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 (bronze, rubber, and paper).

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

Illustration 1	g00647477
(1) Pulled unit, torque converter, and brake hydraulic tank

Illustration 2	g00647479
(2) Brake oil cooler

Illustration 3	g00690434
(3) Pulled unit, torque converter, and brake oil cooler

Check the oil in the pulled unit, torque converter, and brake hydraulic tank (1). Look for air or water in the sight gauge. Many problems are caused by low oil level or air in the oil. Add oil to pulled unit, torque converter, and brake hydraulic tank (1), if necessary. Refer to Operation and Maintenance Manual, "Lubricant Viscosities and Refill Capacities" for the correct type of oil.

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

Probable Cause

• A failure of brake oil cooler (2)

• A failure of pulled unit, torque converter, and brake oil cooler (3)

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

Probable Cause

• There is a leak in the lines. Refer to "Check for oil leakage".

Problem: The oil level is too low

Probable Cause

• There is a leak in the oil lines. Refer to "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.

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 parking brake release oil filter.

Illustration 4	g00647480
(4) Parking brake release oil filter

Remove the element of the parking brake release oil filter (4). Inspect the element for foreign material. Cut the filter open, if necessary.

Note: Parking brake release oil filter (4) 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.

The pressure setting of the bypass valve in the parking brake release oil filter (4) is 248 ± 21 kPa (36 ± 3 psi).

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 5	g00696136
(5) Torque converter scavenge pump section (6) Torque converter charging pump section (7) Parking brake release pump section

Probable Cause

• There is a mechanical failure of parking brake release pump section (7) .

Check: Check the torque converter charging oil filter

Illustration 6	g00696137

Remove torque converter charging oil filter (8). Inspect torque converter charging oil filter (8) for foreign material. Cut the filter open, if necessary.

Note: Torque converter charging oil filter (8) has an oil bypass valve. The bypass valve allows the oil to bypass the 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 the screen goes directly into the hydraulic circuit. Dirty oil can cause sticking valves and restrictions in valve orifices.

The pressure setting of the bypass valve in torque converter charging oil filter (8) is 140 kPa (20 psi).

Problem: Rubber Particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 7	g00696136
(5) Torque converter scavenge pump section (6) Torque converter charging pump section (7) Parking brake release pump section

Probable Cause

• A mechanical failure of the torque converter charging pump section (6)

Check: Check the torque converter scavenge screen.

Illustration 8	g00696138
(9) 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 torque converter scavenge screen (9). Inspect torque converter scavenge screen (9) for foreign material or restrictions.

Problem: Rubber particles are found.

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 9	g00696136
(5) Torque converter scavenge pump section (6) Torque converter charging pump section (7) Parking brake release pump section

Illustration 10	g00696140
(10) Pulled unit pump (11) Pump drive

Probable Cause

• There is a mechanical failure of pump drive (11) .

• A mechanical failure of parking brake release pump section (7). Refer to "Check the parking brake release oil filter".

• There is a mechanical failure of the torque converter charging pump section (6). Refer to "Check the torque converter charging oil filter".

Problem: Aluminum particles are found.

Probable Cause

• There is a mechanical failure of the torque converter.

Problem: Black fiber is found.

Probable Cause

• There is a worn lockup clutch in the torque converter.

Check: Check the screens for the brake oil coolers.

Illustration 11	g00696142
(12) Brake oil cooler screen

Illustration 12	g00696143
(13) Screen for the pulled unit, torque converter, and brake oil cooler

Remove screens (12) and (13) for the brake oil coolers. Inspect screens (12) and (13) .

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found in the screen for the pulled unit, torque converter, and brake oil cooler.

Illustration 13	g00696136
(5) Torque converter scavenge pump section (6) Torque converter charging pump section (7) Parking brake release pump section

Illustration 14	g00696140
(10) Pulled unit pump (11) Pump drive

Probable Cause

• A mechanical failure of the torque converter charging pump section (6). Refer to "Check the torque converter charging oil filter".

Problem: Large quantities of aluminum particles are found in the screen for the pulled unit, torque converter and brake oil cooler.

Probable Cause

• Mechanical failure of the torque converter

Problem: Black fiber is found in the screen for the pulled unit, torque converter, and brake oil cooler.

Probable Cause

• A worn lockup clutch in the torque converter

Problem: Shiny steel particles are found in the brake oil cooler screen.

Illustration 15	g00696140
(10) Pulled unit pump (11) Pump drive

Probable Cause

• There is a mechanical failure of the pulled unit pump (10) .

Problem: Aluminum particles are found in the brake oil cooler screen.

Illustration 16	g00696140
(10) Pulled unit pump (11) Pump drive

Note: A small amount of aluminum particles are normal.

Probable Cause

• There is a mechanical failure of pulled unit pump (10). Refer to "Check the screens for the brake oil coolers".

Check: Check the inside of the hydraulic tank.

Illustration 17	g00916461
(12) Pulled unit screen (13) Torque converter and brake suction screens

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 (12) and (13) and clean the suction screens.

Problem: Rubber particles are found.

Probable Cause

• Seal failure

• Hose failure

Problem: Shiny steel particles are found.

Illustration 18	g00696136
(5) Torque converter scavenge pump section (6) Torque converter charging pump section (7) Parking brake release pump section

Illustration 19	g00696140
(10) Pulled unit pump (11) Pump drive

Probable Cause

• There is a mechanical failure of torque converter scavenge pump section (5) .

• There is a mechanical failure of parking brake release pump section (7). Refer to "Check the parking brake release oil filter".

• There is a mechanical failure of the torque converter charging pump section (6). Refer to "Check the torque converter charging oil filter".

• There is a mechanical failure of the pump drive (11). Refer to "Check the torque converter scavenge screen".

Problem: Aluminum particles are found.

Probable Cause

• There is a mechanical failure of the torque converter. Refer to "Check the torque converter scavenge screen".

Problem: A large quantity of aluminum particles are found.

Illustration 20	g00696140
(10) Pulled unit pump (11) Pump drive

Note: A small amount of aluminum particles are normal.

Probable Cause

• There is a mechanical failure of pulled unit pump (10) .

Problem: Black fiber is found.

Probable Cause

• Failure of the wheel brakes

• There is a worn lockup clutch in the torque converter. Refer to "Check the torque converter scavenge screen" and "Check the screens for the brake oil coolers".

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

Probable Cause

• There is a mechanical failure of a hydraulic cylinder and/or 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 pulled unit, of the torque converter, and of the brake cooling system must be cleaned. Do not use any damaged parts. Damaged parts must be replaced with new parts.