777F Off-Highway Truck Hydraulic System Caterpillar

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.

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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.

Illustration 1	g01258000
(1) High-pressure relief valve (2) Low-pressure relief valve (3) Hoist dual stage relief valve (4) Hoist control valve

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

Relief Valve Test (High Pressure)

Illustration 2	g01258002
(5) Pressure tap for high-pressure oil

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).

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

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

5. After the dump body is fully raised, look at the gauge. Hold the hoist control in the RAISE position for at least 5 seconds. The high-pressure gauge reading is the relief valve pressure setting of high-pressure relief valve (1). The correct pressure setting is 18950 + 1000 - 500 kPa (2750 + 145 - 72.5 psi).
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   Illustration 3	g00531382
   (1) High-pressure relief valve (2) Low-pressure relief valve (3) Hoist dual stage relief valve (6) Nut (7) Retainer

6. Loosen nut (6). Turn retainer (7) in order to adjust the setting of high-pressure relief valve (1).

7. When high-pressure relief valve (1) is correctly set, tighten nut (6).

Relief Valve Test (Low Pressure)

Illustration 4	g01258252
(8) 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 pressure tap (8).

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

4. 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 (2) can be measured. If the body down or up switch is in the LOWER position, then the Power Train Electronic Control Module will keep the hoist control valve in the SNUB position. Low-pressure relief valve (2) cannot be measured in this condition.

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   Illustration 5	g01313893

5. Remove the magnet (A) from the body. When the magnet is removed and the body is in the down position, the indicator light on the dash turns on.

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

7. 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 (2). The correct pressure setting is 3450 ± 350 kPa (500 ± 50 psi).
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   Illustration 6	g00531539
   (1) High-pressure relief valve (2) Low-pressure relief valve (3) Hoist dual stage relief valve (9) Retainer (10) Nut

8. Loosen nut (10). Turn retainer (9) in order to adjust the setting of low-pressure relief valve (2).

9. When low-pressure relief valve (2) is correctly set, tighten nut (10).