992G Wheel Loader Electrohydraulic System Caterpillar

Illustration 1	g00288150
Flow Chart

Note: The flow chart should be used as a reference to the detailed procedure that follows.

The variable displacement piston pump has a pressure cutoff valve that is mounted on the pump. The pressure cutoff valve limits the maximum pressure from the pump. The swashplate strokes to a position of zero flow when the pressure rises to the pressure setting of the valve. There is also a backup relief valve in the main control valve (right).

The pressure setting of the cutoff valve is 28960 kPa (4200 psi). The pressure setting of the backup relief valve is 32750 kPa (4750 psi).

The pressure cutoff valve will not stroke the pump to zero flow before the backup relief valve opens. This is true when the pressure setting of the backup relief valve is less than 32400 kPa (4700 psi). Oil will then flow across the backup relief valve. An undesirable increase in hydraulic oil temperature and a decrease in the engine power that is available will be the result.

This procedure isolates the variable displacement piston pump from the fixed displacement piston pump. The variable displacement piston pump must be isolated from the fixed displacement piston pump in order to obtain an accurate reading. The variable displacement pump supplies oil only to the main control valve (right). The implement electronic control module (ECM) will activate only the main control valve (right) during this check. All of the flow from the fixed displacement piston pump flows through the deactivated main implement control valve (left) to the hydraulic tank.

Both a pump pressure check and a double stall test must be within specifications in order to verify that the pump cutoff valve and the backup relief valve are properly adjusted.

Test

1. Connect the steering frame lock link.

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Illustration 2	g00287052
Manifold for Implement Hydraulics (1) Pressure tap for variable displacement piston pump.

2. Attach a 5P-4816 Gauge (50000 kPa (7500 psi)) to pressure tap (1) on the manifold.

3. Engage the parking brake.

4. Shift the transmission to neutral.

5. Start the engine. Warm the hydraulic oil to normal operating temperature of approximately 49 to 65°C (120 to 150°F).

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Illustration 3	g00287034
Right Console (2) Keypad.

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Illustration 4	g00287036
Front Dash (3) Diagnostic message screen.

6. Press the "OK" key on keypad (2) repeatedly. The hour meter will be displayed on diagnostic message screen (3) .

7. On the keypad, enter "SERV" and then press the "OK" key. Press the ">" key until the diagnostic message screen displays the following message:
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   Table 2

   SELECT CALIBRATION: IMPL RLF - VAR PUMP

8. Press the "OK" key. The following message will be displayed on the diagnostic message screen:
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   Table 3

   "IMPL RLF - VAR PUMP" "PRESS" "OK" "TO START"

9. Press the "OK" key. The following messages will be displayed on the diagnostic message screen:
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   Table 4

   "RLF PRES CHECK MODE" "IS TURNED ON"

   "RIGHT VALVE ONLY" "WILL BE COMMANDED"

   "PROCEED W/ STANDARD" "SERVICE CHECKS"

   The following message will alternate with the previous message in five second intervals:

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   Table 5

   "PRESS" "<" "OR" ">" "TO SEE SPEC LIST"

   The displayed specifications on the diagnostic message screen are the expected results for the test. The specifications in this manual may not agree with the specifications that are displayed on the diagnostic message screen. The service personnel must use the specifications that are displayed on the diagnostic message screen for testing. These specifications could be changed by future updates in the software.

   The diagnostic message screen lists the specifications that follow:

   • pressure cutoff of the variable pump

   • relief pressures (dump line)

   • lift time with variable pump

   • single stall (torque converter)

   • double stall (torque converter and variable pump)

   The single stall is the torque converter stall. The double stall occurs when the hydraulic load of the variable displacement piston pump at main relief pressure is added to the torque converter stall.

10. Increase the engine speed to high idle.

11. Fully move the tilt control lever to the TILT BACK position. The bucket will contact the stops. Hold the tilt control lever in this position. There will be a pressure spike of 2070 to 3450 kPa (300 to 500 psi) above the cutoff pressure. The pressure will then drop back to the actual pressure. Record the final gauge reading.

    The cutoff pressure for the variable displacement piston pump should be 28960 kPa (4200 psi).

12. Decrease engine speed to low idle and lower the bucket to the ground.

The pressure reading from Step 11 should be the cutoff pressure of the variable displacement piston pump. However, if the pressure setting of the backup relief valve is less than 32400 kPa (4700 psi), the pressure reading in Step 11 is not the cutoff pressure of the variable displacement piston pump.

When the pressure setting of the backup relief valve is less than 32400 kPa (4700 psi), the operation of the two relief valves may affect each other. The piston pump may be partially destroked. The main relief valve may be dumping partial flow. In this situation, the pressure reading in Step 11 will not accurately indicate either setting because neither relief valve is relieving all of the flow. One or both of the relief valves will need to be adjusted before an accurate reading can be obtained.

If the pressure reading from Step 11 is not 28960 kPa (4200 psi), adjust the cutoff pressure of the variable displacement piston pump. The following procedure explains the adjustment of the cutoff pressure. Once the cutoff pressure has been set to 28960 kPa (4200 psi) VERIFY that the backup relief valve has been set correctly by proceeding to the Testing and Adjusting, "Torque Converter Stall and Variable Displacement Pump Double Stall - Check".

Note: When the pressure setting of the backup relief valve is less than 32400 kPa (4700 psi), the pressure reading does not change as the adjustment screw is being turned. Even though the cutoff pressure is adjusted higher, the pressure reading does not change because the backup relief valve is controlling the pressure. If this occurs, the backup relief valve will need to be adjusted.

Reference: For the correct procedure in order to Adjust the pressure setting of the backup relief valve, refer to Testing and Adjusting, SENR1367, "992G Wheel Loader Hydraulic System", "Variable Displacement Pump Backup Relief Valve Pressure - Test/Adjust.".

Adjust

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Illustration 5	g00287053
Variable Displacement Piston Pump (bottom view) (4) Pump control valve. (5) Screw for pressure cutoff valve adjustment. (6) Locknut.

1. Remove the cap from adjustment screw (5). There are two adjustment screws on the pump. The screw for the pressure cutoff valve adjustment is the lower of the two adjustment screws. Loosen locknut (6) .

Note: One turn of the adjustment screw will change the pressure setting approximately 20395 kPa (2960 psi).

2. Turn adjustment screw (5) clockwise in order to increase the cutoff pressure. Turn adjustment screw (5) counterclockwise in order to decrease the cutoff pressure.

3. Tighten the locknut.

   Note: Before the new setting is verified, be sure that the hydraulic oil temperature is still within the normal operating range 49 to 65°C (120 to 150°F). Allow the pump to operate for 3 minutes to 5 minutes after the adjustment screw has been moved. This will ensure that the components of the pump control are properly seated.

   Perform the "Test" procedure again in order to verify the new setting.

4. When the pressure cutoff is correct, install the cap on the adjustment screw.