785G Off-Highway Truck Systems 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.

References

Reference: Operation and Maintenance Manual, Prepare the Machine for Maintenance

Reference: Operation and Maintenance Manual, System Pressure Release

Reference: For more information, refer to Relief Valve (Hoist Raise, Hoist Lower) - Test and Adjust on Caterpillar Channel 1.

Illustration 1	g06412532

Note: A CWS login is required to access Caterpillar Channel 1. Scan the QR code below with a QR enabled device or copy the link that follows.

Introduction

The relief valve must be set properly to prevent a pressure level that results in damage to the hoses, fittings, and other pressure sensitive locations of the hoist system. The relief valves are at the top of the hoist control valve. Relief valve pressure is adjusted by loosening the nut at the end of the valve and adjusting the retainer.

Note: Make sure that the filters and the screens are clean before you test any pressures. Make sure that the oil is at operating temperature before you perform this test.

Note: Before you perform any tests, visually inspect the entire hydraulic system for oil leaks and for damaged components.

Illustration 2	g06193787
(1) Low-pressure relief valve (2) Hoist dual stage relief valve (3) High-pressure relief valve (4) Hoist control valve

Hoist dual stage relief valve (2) is part of the hoist control valve (4). The relief pressure for the RAISE position is controlled by high-pressure relief valve (3). The relief pressure for the LOWER position is controlled by low-pressure relief valve (1).

Required Tools

Test Preparation - Relief Valve (High Pressure)

Illustration 3	g06193792
Location of pressure tap for high-pressure oil (see arrow)

1. Lower the dump body. Stop the engine.

2. Connect a hose assembly with an 8T-0860 Pressure Gauge ( 0 to 40000 kPa (0 to 5800 psi)) to pressure taps (5).

Test Procedure - Relief Valve (High Pressure)

1. Start the engine. Operate the dump body repeatedly until the temperature of the hydraulic oil is above 38°C (100°F).

2. Run the engine at high idle and move the hoist control to the RAISE position.

3. After the dump body is fully raised, look at the gauge. Hold the hoist control in the RAISE position. The high-pressure gauge reading is the relief valve pressure setting of high-pressure relief valve (3). The correct pressure setting is 17225 ± 700 kPa (2500 ± 102 psi).

Adjustment Procedure - Relief Valve (High Pressure)

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Illustration 4	g06193816
(1) Low-pressure relief valve (2) Hoist dual stage relief valve (3) High-pressure relief valve (5) Nut (6) Retainer

1. Loosen nut (5). Turn retainer (6) to adjust the setting of high-pressure relief valve (3).

2. When high-pressure relief valve (3) is correctly set, tighten nut (5).

Test Preparation - Relief Valve (Low Pressure)

Illustration 5	g06193818
Pressure tap for low-pressure oil

1. Lower the dump body. Stop the engine.

2. Connect a hose assembly with an 8T-0856 Pressure Gauge ( 0 to 6000 kPa (0 to 870 psi)) to the low-pressure tap.

Test Procedure - Relief Valve (Low Pressure)

1. Start the engine. Operate the dump body repeatedly until the temperature of the hydraulic oil is above 38°C (100°F).

2. Lower the dump body onto the truck frame.

   Note: The body down or up switch must be in the RAISE position before low-pressure relief valve (1) can be measured. If the body down or up switch is in the LOWER position, the Power Train Electronic Control Module will keep the hoist control valve in the SNUB position. Low-pressure relief valve (1) cannot be measured in this condition.

3. Run the engine at high idle and move the hoist control to the LOWER position.

4. Look at the gauge and hold the hoist control in the LOWER position. The low-pressure gauge reading is the relief valve pressure setting of low-pressure relief valve (1). The correct pressure setting is 3450 ± 350 kPa (500 ± 50 psi).

Adjustment Procedure - Relief Valve (Low Pressure)

1. Loosen nut for the low-pressure relief valve (1). Turn the retainer to adjust the setting of low-pressure relief valve.

2. When low-pressure relief valve (1) is correctly set, tighten the low-pressure nut.