797F and 797F XQ Off-Highway Truck Air System Caterpillar

Illustration 1	g02021935
Location of the drain valve for the air tanks

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When opening the drain valve, wear protective gloves, protective face shield, protective clothing and protective shoes. Pressurized air could cause debris to be blown and result in personal injury.

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The air lines to and from the air dryer must be at atmospheric pressure. If the air lines are not at atmospheric pressure, personal injury could result. Release the air pressure from the air system completely before performing maintenance.

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To prevent personal injury, stop the engine and drain the air reservoir before performing maintenance on the air dryer.

Check for Moisture in the System

1. Check for moisture in the air system by opening the drain cocks or valves on the air tanks. If moisture is present, the desiccant may require replacement. The following conditions may also cause water to accumulate and these conditions should be considered.

   1. An outside source of air has been used in order to charge the air system. This air did not pass through the drying bed.

   2. The amount of air that is being used is exceptionally high, and may be due to the demands for accessory air or some unusual requirements for air. This demand does not allow time for the loading and unloading of the compressor in a normal fashion. Check for high leakage in the air system.

   3. The air dryer has been installed in a system that has been previously without an air dryer. This type of system will be saturated with moisture and the system may take several weeks of operation in order to dry.

   4. In areas with more than a 30 degree range of temperature occurring in one day, small amounts of water can accumulate in the air system due to condensation. Under these conditions, the presence of a small amount of moisture is normal.

   Note: A small amount of oil in the system may be normal and the oil should not be a reason to consider replacing the desiccant. A desiccant that is strained with oil will work adequately.

2. Check mounting bolts for tightness.

3. Visually check for physical damage to the air dryer such as broken lines, missing parts, and bad electrical connections.

If either of the previous conditions are adding moisture to the air system, the desiccant may be okay.

Operation and Leakage Tests

1. Test the check valve that is in the outlet port by building the air system to the governor cutout and observing the air gauge that is installed in the reservoir. A rapid loss of pressure could indicate a failed check valve in the outlet port. Confirm by purging the air system, by removing the check valve assembly from the end cover and by applying air pressure to the unit. Apply a soap solution to the exhaust side of the check valve. Leakage should not exceed a 1 inch bubble in one second.

2. Check for excessive leakage around the purge valve. While the compressor is loaded, apply a soap solution to the exhaust port of the housing of the purge valve and observe the leakage. The leakage should not exceed a 1 inch bubble in one second. If the leakage is excessive service the purge valve.

3. Close all drain cocks on the reservoir. Build up the system pressure to the governor cutout and note the escape of air by the purge valve. Use the drain valve for the air tanks shown in Illustration 1 in order to reduce the air system pressure to the governor cut-in. After the pressure is reduced, the air system should build up to full pressure. Then in about 30 seconds, the air system should have a purge cycle.

4. Check the operation of the safety valve by pulling the exposed stem while the compressor is loaded. There must be an exhaust of air while the stem is held. When the stem is released, the valve should reseat.

5. Check all the lines and the fittings that lead in and out of the air dryer for leakage and integrity.

Changing the Desiccant

The procedure for removing the desiccant from the air dryer is described in the Disassembly and Assembly, "Air Dryer - Disassemble". The procedure for installing the new desiccant is in the Disassembly and Assembly, "Air Dryer - Assemble" section.