824H Wheel Dozer, 825H Soil Compactor and 826H Landfill Compactor Electrohydraulic System Caterpillar

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Sudden movement or accidental starting of the machine can cause personal injury or death to persons on or near the machine. To prevent personal injury or death, perform the following: Park the machine on a smooth, level surface. Lower the work tool to the ground and engage the parking brake. Stop the engine and remove the key. Block the wheels and install the steering frame lock.

The pilot control valve is an electrohydraulic valve that controls the pilot oil pressure to each end of the main control valve spools. When the pilot control valve is activated, pilot oil pressure is supplied to one end of the spool in the main control valve. Next, the spool then moves away from the pressurized chamber in the main control valve.

This procedure measures the oil pressure in a chamber of the main control valve in order to assist in troubleshooting the implement hydraulic system.

1. Ensure that the transmission control is in the NEUTRAL position.

2. Engage the parking brake.

3. Connect the steering frame lock.

   Reference: For more information about preparing the machine for troubleshooting, refer to the Service Manual module Testing and Adjusting, "Machine Preparation for Troubleshooting".

4. Release the pressure in the hydraulic system.

   Reference: For more information about releasing the hydraulic pressure, refer to the Service Manual module Testing and Adjusting, "System Pressure - Release" for the machine that is being serviced.

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Illustration 1	g00871524

5. The main control valve is located on the top of the front frame. Remove the cover in order to access the main control valve.

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Illustration 2	g00871536
Main Control Valve (1) Backward blade tip. (2) Forward blade tip. (3) Blade tilt right. (4) Blade tilt left. (5) Blade raise. (6) Blade lower.

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Illustration 3	g00871537
Pilot Lines (7) Main control valve. (8) Hose. (9) fitting.

6. Ensure that the blade is on the ground. Install a 6V-3965 Fitting to 8T-8902 Tee. Loosen the nut and remove hose (8). Install the tee to the fitting (9) on the end cap of the main control valve. Install the hose to the tee. The tee will need to be placed on each end cap in order to check each pilot pressure. The pressure taps for the 824H Wheel Dozer are listed:
   • (1) Backward blade tip

   • (2) Forward blade tip

   • (3) Blade tilt right

   • (4) Blade tilt left

   • (5) Blade raise

   • (6) Blade lower

   Note: The 825H Soil Compactor is equipped with lift and tilt. The 826H Landfill Compactor is equipped with lift only.

Note: Assemble the 8T-3613 Fitting and the 6V-4143 Coupler to the 177-7860 Hose .

7. Attach the hose assembly with 8T-0857 Pressure Gauge to the pressure tap that is installed on the main control valve.

8. Start the engine and warm the hydraulic oil to a normal operating temperature of approximately 49 to 65°C (120 to 150°F).

9. Run the engine at low idle. Move the implement control lever to a maximum stroke position in order to pressurize the chamber that is being tested. Record the gauge reading. Repeat the procedure for each chamber of the main control valve.

10. The pressure readings for the lift function should be at least 1475 kPa (215 psi). This pressure is required in order to shift the spools in the main control valve to the FULL FLOW position.

11. The pressure readings for the lift function should be at least 2375 kPa (345 psi). This pressure is required in order to shift the spools in the main control valve to the FLOAT position.

12. The pressure readings for the tip function and the tilt function should be at least 1245 kPa (180 psi). This pressure is required in order to shift the spools in the main control valve to the FULL FLOW position.

    Note: A pressure reading on the low side of the tolerance for the specifications may still be too low. The pressure that is required to shift a spool varies slightly with the force of the springs that are installed in the valve. If the reading for the pilot chamber pressure is on the low side of the tolerance, it may be necessary to increase the pressure.

    If the pressure reading is not within specifications, there are several possible causes:

    • Proportional solenoids require calibration.

    • Pressure reduction valves for the pilot system need to be replaced.

    • The pilot pump is worn. The pump needs to be replaced.

    Reference: For additional information on testing the pilot system, refer to Testing and Adjusting, "Pilot System Pressure - Test".