793C Off-Highway Truck Power Train 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.

Special Characteristics of the 793C ICM Transmission Hydraulic Control

The operation of the transmission hydraulic control on the 793C is unique in comparison to any other off-highway truck because of the operation of the priority reducing valve. On the 793C, the priority reducing valve regulates one of two pressures. When the machine is in torque converter drive, the priority reducing valve regulates the pilot pressure. When the machine is in direct drive, the priority reducing valve regulates the pressure of the transmission charging pump section. The system is balanced in order to provide the correct pilot pressure in torque converter drive when the pump pressure is correctly set in direct drive. The priority reducing valve should not be adjusted in order to control pilot pressure in torque converter drive.

Note: This test describes the proper procedure in order to obtain the pilot pressure. The pilot pressure should not be adjusted. The pilot pressure is a result of setting the pressure of the transmission charging pump section when the machine is in direct drive. Refer to the Power Train Testing and Adjusting, "Transmission Hydraulic Control (Pressure Pack) - Test and Adjust" for the proper procedure to adjust the pressure of the transmission charging pump section in direct drive.

Note: Make sure that the filters and the screens are clean before you test any pressures. Make sure that the oil is at operating temperature before you perform this test.

Procedure

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Illustration 1	g00681909
(1) Upshift solenoid (2) Downshift solenoid (3) Lockup clutch solenoid

1. Disconnect the wiring harness from upshift solenoid (1), downshift solenoid (2), and lockup clutch solenoid (3) .

2. Remove the solenoid guard and the large cover from the transmission case.

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Illustration 2	g00288789
Transmission hydraulic control (M) Pressure tap for the pilot pressure of the transmission hydraulic control

3. Remove the plug for pressure tap (M) .

4. Install a 3B-6552 Elbow (3.175 mm (0.1250 inch) NPT) at the location for pressure tap (M). Connect a 3B-7282 Pipe Nipple (3.175 mm (0.1250 inch) NPT) (152 mm (6 inch) long) with a 3B-6487 Coupling to the elbow. Add a 8T-3613 Valved Nipple .

5. Use a hose and a coupler to connect a 8T-0855 Pressure Gauge (4000 kPa (580 psi)) to the nipple at pressure tap (M) .

6. Put the large cover on the top of the transmission case. Use a piece of plastic or a piece of paper with the cover to prevent dirt from entering the transmission case. Use a piece of plastic or a piece of paper with the cover to prevent the spraying of oil.

   Alternate Step 6. Install the FT1775 Transmission Test Cover in place of the small cover. Use four bolts that are evenly spaced in order to hold the cover in place.

7. Start the engine. Run the engine at the low idle rpm with the transmission in NEUTRAL-1. Keep the parking brakes engaged.

8. The pressure on the gauge should be 2415 to 2760 kPa (350 to 400 psi) in torque converter drive.

   Note: The pilot pressure should not be adjusted. The pilot pressure is a result of setting the pressure of the transmission charging pump section when the machine is in direct drive. Refer to the Power Train Testing and Adjusting, "Transmission Hydraulic Control (Pressure Pack) - Test and Adjust" for the proper procedure to adjust the pressure of the transmission charging pump section in direct drive.

9. When the test is complete, stop the engine.

10. Properly connect the harnesses for upshift solenoid (1), downshift solenoid (2), and lockup clutch solenoid (3) .