785G Off-Highway Truck Systems Caterpillar

Hydraulic System (General)

A visual inspection of the hydraulic system and of the components is the first step when you identify a problem.

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Do not check for leaks with your hands. Pin hole (very small) leaks can result in a high velocity oil stream that will be invisible close to the hose. This oil can penetrate the skin and cause personal injury. Use cardboard or paper to locate pin hole leaks.

1. Check all oil lines and the connections for damage and for leaks.

2. Check the pump and the connections for damage and for leaks.

3. Follow the pump lines to the tank and valves. Check the lines for damage and leaks.

4. Check the oil tank for damage and leaks.

5. Check the tank oil level. (See 1 for sight gauge locations.)
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   Illustration 1	g06192326
   Location of sight gauges on hydraulic oil tank (1) Hoist and Brake Cooling Sight gauge (2) Fill Cap (3) Sight gauge (with cylinders lowered) (4) Fluid sampling valve (5) Sight gauge (with cylinders raised)

6. While the engine is running, use the oil sampling port to get an oil sample. The oil sampling port is on the hydraulic tank. Check for air bubbles in the oil sample. (See fluid sampling valve (4) in Illustration 1.)

   Note: To collect the fluid sample, a screwdriver or wrench is required to open and close the sampling valve.

7. Remove the filter elements. Check for particles that were removed from the oil by the filter elements. A magnet will separate ferrous particles from nonferrous particles (piston rings, O-rings, or seals).

   Note: See System Operation, Location of Components (Power Train System) within this manual for the location of the filter elements.

Fast Fill System

Illustration 2	g06190879
The "Fast Fill" service panel is on the left side of the truck between the bumper and the front tire. (1) Engine Oil Fill Port (2) Steering Fluid Fill Port (3) Transmission Oil Fill Port (4) Hydraulic Oil "Fast Fill" Indicator (5) Engine Oil "Fast Fill" Indicator (6) Engine Coolant "Fast Fill" Indicator (7) Steering Fluid "Fast Fill" indicator (8) Transmission Oil "Fast Fill" Indicator (9) Hydraulic Fluid Fill Port (10) Engine Coolant Fill Port (11) Diesel Fluid Fill Port

A visual inspection of the components is the first step in diagnosing a problem. The "Fast Fill" service panel is on the left side of the machine between the front tire and the bumper.

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Do not check for leaks with your hands. Pin hole (very small) leaks can result in a high velocity oil stream that will be invisible close to the hose. This oil can penetrate the skin and cause personal injury. Use cardboard or paper to locate pin hole leaks.

1. Check the service panel for power.

2. Check the service panel for loose mounting hardware.

3. Check the service panel for the proper function of the indicator lamps before filling the fluids.

4. Check the fill ports at the service panel for damaged fittings and/or leaking fittings.

5. Check the line connections at the service panel for damage and/or leaks.

6. Check the lines from the service panel to the reservoirs for damaged lines and/or leaking lines.

7. Check the line connections at the reservoirs for damage and/or leaks.

8. Check the sensors in the reservoirs for damage and/or leaks.

9. Check the wiring for the sensors for the following conditions:

   • Loose connections

   • Broken wiring

   • Damaged insulation

   • Frayed wiring

   Note: See System Operation, Location of Components (Machine System) within this manual for sensor locations.

10. Check the wiring at the control box for the following conditions:

    • Loose connections

    • Broken wiring

    • Damaged insulation

    • Frayed wiring

Steering System

A visual inspection of the steering system and of the components is the first step when you identify a problem.

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Do not check for leaks with your hands. Pin hole (very small) leaks can result in a high velocity oil stream that will be invisible close to the hose. This oil can penetrate the skin and cause personal injury. Use cardboard or paper to locate pin hole leaks.

1. Check all oil lines and the connections for damage and for leaks.

2. Check the pump and the connections for damage and for leaks.

3. Follow the pump lines to the tank and valves. Check the lines for damage and leaks.

4. Check the oil tank for damage and leaks.

5. Check the tank oil level.
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   Illustration 3	g06186827
   The drive motor for the steering arms is located behind the front right wheel and to the right of the torque converter.

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   Illustration 4	g06192397
   Location of oil sampling valve on the oil filter for the hydraulic fan and steering (see arrow)

6. While the engine is running, use the oil sampling valves to get an oil sample. An oil sampling valve is on the forward side of the hydraulic storage tank. Check for air bubbles in the oil sample.

7. Remove the filter elements. Check for particles that were removed from the oil by the filter elements. A magnet will separate ferrous particles from nonferrous particles (piston rings, O-rings, or seals).

Power Train System

Torque Converter

Illustration 5	g06192491
Location of the power train torque converter (1) Torque Converter Hydraulic Screen (2) Torque Converter Charging Hydraulic Filters (2) (3) Torque Converter Sight Gauge (4) Torque Converter Magnetic Screen (5) Torque Converter Hydraulic Fill Cap

Perform a visual inspection at the beginning of troubleshooting a problem. Perform the inspection while the engine is turned off. Put the transmission control in the NEUTRAL position. Engage the parking brake.

• Check the oil level.

Inspect the oil level in the torque converter sump.

Note: Many problems in the power train are caused by low oil levels or by air in the oil. If the engine has not been started for several minutes, this oil level check will ensure that oil is in the transmission and that the engine can be started. If the machine has not been moved overnight or an extended period and the engine has not been started, the oil level will be high. An accurate oil level check can be performed after the oil is hot.

• Check for leaks.

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Do not check for leaks with your hands. Pin hole (very small) leaks can result in a high velocity oil stream that will be invisible close to the hose. This oil can penetrate the skin and cause personal injury. Use cardboard or paper to locate pin hole leaks.

Inspect all oil lines, hoses, and 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 a connection, air can leak into the system. Air in the system can be as bad as a low amount of oil.

• Check the electrical system.

Inspect the electrical harnesses for damaged wires or for broken wires. Disconnect each connector and look for pins and sockets that have been bent, broken, or removed. Look for any foreign material inside the connectors. The connectors must be tightened with normal force. The connectors must be disconnected with the same amount of force.

• Check the batteries.

Check the batteries. Turn the battery disconnect switch ON.

• Check the filters and the screens.

Inspect the torque converter charging hydraulic filter, the torque converter hydraulic filter, the torque converter magnetic screen, and the suction screen.

Note: The torque converter charging hydraulic filter, the torque converter hydraulic filter, and the torque converter magnetic screen have a bypass valve. A bypass valve allows oil to bypass the oil filter elements whenever the difference in pressure between the inlet oil and the outlet oil in the oil filter is too high. Any oil that does not go through the filter elements goes directly in the hydraulic circuit. Dirty oil causes restrictions in the valve orifices, sticking valves, and so forth.

All the bypass valves are next to bypass switches. The Messenger will notify the operator of a restriction.

If any contamination is found in the filter elements or the screens, all the components of the transmission hydraulic system must be cleaned. Do not use any damaged parts. Any damaged parts must be removed and new parts must be installed.

Power Train and Rear Axle

Perform a visual inspection at the beginning of troubleshooting a problem. Perform the inspection while the engine is turned off. Put the transmission control in the NEUTRAL position. Engage the parking brake.

• Check the oil level at the Torque Converter sump.

Note: Many problems in the power train are caused by low oil levels or by air in the oil. If the engine has not been started for several minutes, this oil level check will ensure that oil is in the transmission and that the engine can be started. If the machine has not been moved overnight or an extended period and the engine has not been started, the oil level will be high. An accurate oil level check can be performed after the oil is hot.

Inspect all oil lines, hoses, and 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 a connection, air can leak into the system. Air in the system can be as bad as a low amount of oil.

• Check the electrical system.

Inspect the electrical harnesses for damaged wires or for broken wires. Disconnect each connector and look for pins and sockets that have been bent, broken, or removed. Look for any foreign material inside the connectors. The connectors must be tightened with normal force. The connectors must be disconnected with the same amount of force.

• Check the batteries.

Check the batteries. Turn the battery disconnect switch ON.

Illustration 6	g06192474
Location of fluid filters and screens for the transmission and rear axle (1) Rear Axle Housing Filter (2) Transmission Filter (3) Transmission Magnetic Screens (2)

Check the filters and the screens.