784C, 785C and 785C HAA Off-Highway Truck Air System and Brakes 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.

Note: Before you perform any operation checks, open the drain valves in order to release any water in the air tanks. Water lowers the capacity of the air tanks. When the capacity of the air tanks is lowered, the air compressor can operate continually. Close the drain valve. Start the engine. Operate the machine until the machine is at the normal operating temperature.

Illustration 1	g00672126
Location of drain valve for primary air tank

Illustration 2	g00405338
Location of drain valve for secondary air tank

Operation checks can be used to find the source of air leakage in the air system and in the brakes. Operation checks can also be used to diagnose the reasons for bad performance in the air system and in the brakes.

Leakage Checks

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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.

Note: It is possible to hear the sound of air leakage. Air leakage can be observed by applying soapy water to connections, to valves and to hoses.

1. Start the engine and allow the air pressure to reach the cutout pressure. Stop the engine.

   Note: The governor cutout pressure is 830 ± 35 kPa (120 ± 5 psi). If the governor cutout pressure is not correct, see the Systems Operation/Testing And Adjusting, "Governor (Air Compressor)".

   Note: Include all hoses, lines and connections in your investigations.

2. Check from the air compressor to the primary air tank. The check should include the air compressor governor and the air dryers.

3. Check for air leakage at the drain valves of the air tanks. Check for air leakage at the relief valves for the air dryers and for the primary air tank.

4. Check for air leakage from the primary air tank to the service brake air control valve.

5. Check for air leakage in the service brake air control valve.

6. Check for air leakage from the service brake air control valve to the retarder air control valve.

7. Check for air leakage in the retarder air control valve.

8. Check all hoses, lines and connections between the following components: the service brake air control valve, retarder air control valve, the torque converter and front brake cooling diverter valve and the brake air relay valves.

9. Check the brake air relay valves and the torque converter and front brake cooling diverter valve for air leakage.

10. Check for air leakage from the brake air relay valves to the brake master cylinders.

11. Check the brake master cylinders for air leakage.

12. Check all hoses, lines and connections between the following components: secondary air tank, the secondary brake air control valve and the air control valve for the parking brake.

13. Check for air leakage in the air control valve for the secondary brake and in the air control valve for the parking brake.

14. Check from the air control valve for the parking brake to the parking and secondary brake valve.

15. Check the parking and secondary brake valve for air leakage.

16. Check all hoses, lines and connections between the following components: the primary air tank, the air start relay and the air starting motor gp.

17. Check from the secondary air tank to the air system pressure protection valve.

18. Check for air leakage in the air system pressure protection valve.

19. Check all hoses, lines and connections between the following components: the air system pressure protection valve, the circuit for the horn and the solenoid for the air starting motor gp.

Note: While you check for air leakage, check the exhaust passages for restrictions. (The restrictions may be from mud or from dirt.) A restriction in an exhaust passage will slow down the release of pressure air during normal operation. A restriction in an exhaust passage may even prevent the release of pressure air during normal operation.

Service Brake Check

Note: The service brake air control valve activates the front brakes and the rear brakes.

1. Start the engine and allow the pressure to reach the cutout pressure.

2. Move the parking brake control to the OFF position. Do not depress the secondary brake control.

3. Move the transmission control to the NEUTRAL position.

4. Depress the service brake control fully.

5. Move the transmission control to the FIRST GEAR position.

6. Increase engine speed to 1300 rpm. The machine should not move.

7. If the machine moves, inspect the brake system for wear or for damage. Correct the air pressure. See the Systems Operation/Testing And Adjusting, "Braking System Troubleshooting".

Parking Brake Check

Note: The air control valve for the parking brake activates all the wheel brakes. The wheel brakes are applied by spring pressure and the wheel brakes are released by oil pressure.

The parking brake tests consist of the following two tests.

Brake Drive Through Test

Note: This test should be conducted when the truck is empty.

1. Move the parking brake control to the ON position.

2. Move the transmission control to the FIRST GEAR position.

3. Increase engine speed to 1300 rpm. The machine should not move.

4. If the machine moves, inspect the brake system for wear or for damage. Correct the air pressure. See the Systems Operation/Testing And Adjusting, "Braking System Troubleshooting".

Brake Slope Test

Note: This test should be conducted when the truck is loaded to the normal rated capacity.

1. Load the truck to the normal rated capacity.

2. Park the machine on a slope of no more than 15 percent.

3. Depress the service brake control.

4. Shift the transmission control to NEUTRAL.

5. Move the parking brake control to the ON position.

6. If the machine moves, inspect the brake system for wear or for damage. Correct the air pressure. See the Systems Operation/Testing And Adjusting, "Braking System Troubleshooting".

Secondary Brake Check

Note: The secondary brake air control valve activates all the wheel brakes. The wheel brakes are applied by spring pressure and the wheel brakes are released by oil pressure.

1. Start the engine. Allow the pressure to reach the cutout pressure.

2. Do not depress the secondary brake control.

3. Move the transmission control to the NEUTRAL position.

4. Depress the secondary brake control fully.

5. Move the transmission control to the FIRST GEAR position.

6. Increase engine speed to 1300 rpm. The machine should not move.

7. If the machine moves, inspect the brake system for wear or for damage. Correct the air pressure. See the Systems Operation/Testing And Adjusting, "Braking System Troubleshooting".

Retarder Brake Check

Note: The retarder air control valve activates the front brakes and the rear brakes.

1. Stop the machine in a level area.

2. Pull the retarder control completely to the DOWN position.

3. Move the transmission control to the NEUTRAL position.

4. Move the parking brake control to the OFF position.

5. Move the transmission control to the FIRST GEAR position.

6. Increase engine speed to 1300 rpm. The machine should not move.

7. If the machine moves, inspect the brake system for wear or for damage. Correct the air pressure. See Systems Operation/Testing And Adjusting, "Braking System Troubleshooting".

Check The Warning System (Air System)

1. Turn on the battery disconnect switch.

2. Start the engine.

3. Allow the pressure to reach approximately 690 kPa (100 psi).

4. At speeds less than 8 km/h (5 mph), the air pressure indicator for the brake and the action light for low air pressure will operate until the pressure is approximately 415 ± 35 kPa (60 ± 5 psi).

5. At speeds that are greater than 8 km/h (5 mph), the action alarm, the air pressure indicator for the brake, and the action light for low air pressure will operate until the pressure is approximately 415 ± 35 kPa (60 ± 5 psi).

6. If the action alarm and/or action light do not operate, stop the engine.

7. Check the electrical connections for the action alarm, for the action light and for the pressure switches. Check the bulb for the action light.

8. If the connections are good, connect a digital multimeter across the terminals of one of the pressure switches.

   Note: The air pressure must be below the specifications of the pressure switches.

9. If there is no continuity between the terminals, install a new pressure switch.

10. If the action alarm does not operate, connect a digital multimeter across the terminals of the action alarm.

11. If there is no continuity between the terminals, install a new alarm.

12. If the action alarm and/or action light operate above 415 kPa (60 psi), stop the engine.

13. Install a new pressure switch.

14. Start the engine.

15. Allow the pressure to reach approximately 690 kPa (100 psi).

16. Slowly open the drain valve.

17. The action alarm and the action light must operate at 415 ± 35 kPa (60 ± 5 psi).

18. Whenever the parking brake is activated the parking brake indicator light should be on.

19. Start the engine.

20. Allow the pressure to reach approximately 690 kPa (100 psi).

21. With the parking brake engaged, shift the transmission control to REVERSE or to any FORWARD gear.

22. The action alarm and the action light will operate until the parking brake is released or the transmission control is shifted back to NEUTRAL.

23. If the action alarm and/or action light do not operate, check the electrical connections for the action alarm, for the action light and for the pressure switch. Check the bulb for the action light.

24. The horn does not operate until air pressure reaches 550 kPa (80 psi).

Brake Cooling System Check

Refer to Systems Operation/Testing And Adjusting, "Braking System Troubleshooting" for an hydraulic system that is overheated.