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 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.

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Personal injury can result from hydraulic oil pressure and hot oil. Hydraulic oil pressure can remain in the hydraulic system after the engine has been stopped. Serious injury can be caused if this pressure is not released before any service is done on the hydraulic system. Make sure all of the attachments have been lowered, and the oil is cool before removing any components or lines. Remove the oil filler cap only when the engine is stopped, and the filler cap is cool enough to touch with your bare hand.

Specifications

Introduction

Periodically the hoist cylinders should be tested for signs of excessive drift relative to the temperature of the hydraulic fluid contained in the cylinders.

Test Preparation

1. Start the engine. Operate the engine at high idle.

2. Move the hoist control to the RAISE position. Hold the hoist control in the RAISE position for 5 seconds after the dump body is fully raised. Then, lower the dump body all the way to the truck frame.

3. Repeat Step 2 until the temperature of the hydraulic oil is above 38°C (100°F).

After the hydraulic oil has reached the specified temperature, proceed with the following steps.

1. Lower the dump body to the truck frame.

2. Operate the engine at high idle.

3. Raise the dump body to the fully raised position. Use a stopwatch to measure the required time.

The required time for full extension of the hoist cylinders is approximately 11 seconds.

Test Procedure

Note: Perform this test with an empty dump body and hydraulic oil above 38°C (100°F).

1. Move the hoist control to the RAISE position.

2. When the first stage of the hoist cylinders is extended by 305 mm (12 inch), move the hoist control to the HOLD position.

3. The dump body will push the cylinder head of the first stage by 6.35 mm (0.250 inch) onto the second stage of the hoist cylinders. Use a stopwatch and record the required time.

The acceptable time for cylinder drift is relative to the temperature of the hydraulic oil in the hoist cylinders. See Table 2 for the acceptable time.

If the cylinder drift does not fall within the given parameters shown in Table2, excessive cylinder drift can be caused by the following conditions:

• Leaks in the lines between the hydraulic oil tank and the hoist cylinders.

• Worn control valve spool.

• Worn seals in the hoist cylinders.