789B 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 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 (aluminum, rubber and bronze).

Check : Check the oil level in the hydraulic tank.

Illustration 1	g00327367
(1) Torque converter and front brake cooling oil cooler

Illustration 2	g00313884
(2) Oil cooler for the right rear brake (3) Screen for the right rear brake (4) Oil cooler for the left rear brake (5) Screen for the left rear brake

Check the oil level in the 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 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 and front brake cooling oil cooler (1)

• Failure of the right rear brake oil cooler (2)

• Failure of the left rear brake oil cooler (4)

Problem : There is an indication of air in the oil

Probable Cause

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

Problem : The oil level is too low

Probable Cause

• There is a leakage in the oil lines. See "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 cover.

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 hydraulic filter.

Illustration 3	g00327368
(6) Parking brake release hydraulic filter.

Remove parking brake release hydraulic filter (6) . Inspect parking brake release hydraulic filter (6) for foreign material. Cut the filter open, if necessary.

Note: Parking brake release hydraulic filter (6) 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.

The pressure setting of the bypass valve in parking brake release hydraulic filter (6) is 248 ± 1 kPa (36 ± 3 psi) .

Problem : Rubber particles are found

Probable Cause

• Seal failure

• Hose failure

Illustration 4	g00313886
Pump for the torque converter, the parking brake release, and brake cooling. (7) Cover for the torque converter scavenge screen (8) Torque converter scavenge pump section (9) Torque converter charging pump section (10) Parking brake release pump section (11) Section for rear brake cooling

Problem : Shiny steel particles are found

Probable Cause

• Mechanical failure of parking brake release pump section (10) .

Check : Check the torque converter hydraulic filter.

Illustration 5	g00327371
(12) Torque converter hydraulic filter.

Remove torque converter hydraulic filter (12) . Inspect torque converter hydraulic filter (12) for foreign material. Cut the filter open, if necessary.

Note: Torque converter hydraulic filter (12) 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.

The pressure setting of the bypass valve in torque converter hydraulic filter (12) is 140 kPa (20 psi) .

Problem : Rubber particles are found

Probable Cause

• Seal failure

• Hose failure

Problem : Shiny steel particles are found

Probable Cause

• Mechanical failure of the torque converter charging pump section (9)

Check : Check 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 torque converter scavenge screen (7) . Inspect torque converter scavenge screen (7) for foreign material or restrictions.

Problem : Rubber particles are found

Probable Cause

• Seal failure

• Hose failure

Illustration 6	g00327377
(13) Oil pump drive (14) Gear pump for rear brake cooling ( front section ) (15) Gear pump for the hoist (rear section)

Problem : Shiny steel particles are found

Probable Cause

• Mechanical failure of the oil pump drive (13) .

• Mechanical failure of the parking brake release oil pump section (10) . See "Check the parking brake release hydraulic filter" .

• Mechanical failure of the torque converter charging pump section (9) See "Check the torque converter hydraulic filter" .

Problem : Aluminum particles are found

Probable Cause

• Mechanical failure of the torque converter

Problem : Bronze particles are found

Probable Cause

• Worn lockup clutch in the torque converter

Check : Check the torque converter and front brake cooling screen. Check the oil cooler screens for the rear brakes.

Illustration 7	g00313884
(2) Oil cooler for the right rear brake (3) Screen for the right rear brake (3) Oil cooler for the left rear brake (5) Screen for the left rear brake

Illustration 8	g00327464
(16) Screen for the torque converter and front brake cooling (23) Filter bypass switch

Remove screens (3) , (5) , and (16) for the front and rear brake oil coolers. Inspect screens (3) , (5) , and (16) .

Problem : Rubber particles are found

Probable Cause

• Seal failure

• Hose failure

Problem : Shiny steel particles are found in the torque converter and front brake cooling screen

Probable Cause

• Mechanical failure of torque converter charging pump section (9) See "Check the torque converter hydraulic filter" .

Problem : Large quantities of aluminum particles are found in the torque converter and front brake cooling screen

Probable Cause

• Mechanical failure of the torque converter

Problem : Bronze particles are found in the torque converter and front brake cooling screen

Probable Cause

• Worn lockup clutch in the torque converter

Problem : Shiny steel particles are found in the screen for the right rear brake or in the screen for the left rear brake

Probable Cause

• Mechanical failure of the rear brake cooling section of the torque converter pump (11) .

Problem : Aluminum particles are found in the screen for the right rear brake or in the screen for the left rear brake

Note: A small amount of aluminum particles is normal.

Probable Cause

• Mechanical failure of the hoist and rear brake cooling gear pump section (14) or (15) . See "Check the hoist and rear brake cooling screens" .

Check : Check the hoist and rear brake cooling screens.

Illustration 9	g00347135
(17) Oil screen for the rear brake cooling (18) Oil screen for hoist

Remove oil screens (17) and (18) . Inspect screens (17) and (18) for foreign material or for restrictions.

Problem : Rubber particles are found

Probable Cause

• Seal failure

• Hose failure

Problem : A large quantity of aluminum particles are found in the hoist screen or in the rear brake cooling screen

Note: A small amount of aluminum particles is normal.

Probable Cause

• Mechanical failure of the hoist and rear brake cooling gear pump section (14) or (15) .

Check : Check the parts inside the hydraulic tank.

Illustration 10	g00313892
(19) Return screen for oil from the hoist (20) Suction screen for torque converter and parking brake release oil (21) Suction screen for rear brake cooling oil (22) Suction screen for hoist and rear brake cooling oil

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 suction screens (20) , (21) , and (22) and clean the suction screens. Remove screen (19) and inspect the screen for foreign materials.

Problem : Rubber particles are found

Probable Cause

• Seal failure

• Hose failure

Problem : Shiny steel particles are found

Probable Cause

• Mechanical failure of the torque converter scavenge pump section (8)

• Mechanical failure of parking brake release pump section (10) . See "Check the parking brake release hydraulic filter" .

• Mechanical failure of the torque converter charging pump section (9) . See "Check the torque converter hydraulic filter" .

• Mechanical failure of oil pump drive (13) . See "Check the torque converter scavenge screen" .

• Mechanical failure of rear brake cooling pump section (11) . See "Check the torque converter and front brake cooling screen. Check the oil cooler screens for the rear brakes". Inspect screen (5) .

Problem : Aluminum particles are found

Probable Cause

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

Problem : A large quantity of aluminum particles are found

Note: A small amount of aluminum particles is normal.

Probable Cause

• Mechanical failure of the hoist and rear brake cooling gear pump section (14) or (15) .

Problem : Pieces of red, nonferrous material are found. These pieces are part of the brake disc

Probable Cause

• Failure of the wheel brakes

Problem : Bronze particles are found

Probable Cause

• There is a worn lockup clutch in the torque converter. See "Check the torque converter scavenge screen" .

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.