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.

This check determines if the following conditions exist:

• The transmission solenoids are leaking.

• The transmission clutches engage properly.

Procedure for Manual Shifts of the Transmission

1. Be certain that a cover is installed on the transmission case before any tests are performed.

2. Be certain that the transmission shift lever is in the NEUTRAL position.

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

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

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Illustration 2	g00287667

4. Remove the plug for the rotary selector spool from the left side of the transmission case while the engine is stopped.

5. Use a 9S-1721 Extension (1/4 inch square drive) and a 9U-6635 Reversible Ratchet to turn the transmission rotary selector spool. When the ratchet is fully turned in a clockwise direction, the rotary selector spool is in the NEUTRAL-1 position. The order of the detent positions is NEUTRAL-1, NEUTRAL-2, REVERSE, and FIRST speed through SIXTH speed.

   Note: There is approximately 30 degrees of rotation between each detent of the rotary selector spool.

Procedure To Check For Leaking Solenoids

1. Disconnect all the electrical harness connectors from upshift solenoid (1), downshift solenoid (2), and lockup clutch solenoid (3). Start the engine and run the engine at low idle.

2. Engage the parking brakes and the service brakes.

3. Manually shift the transmission to the FIRST speed. After the manual shift is performed, move away from the machine.

4. Operate the engine at low idle with the transmission in the FIRST speed for five minutes.

5. Return the rotary selector spool to the NEUTRAL-1 position.

6. Properly connect the harnesses to solenoids (1), (2), and (3) .

Problem: The transmission shifts into REVERSE.

Probable Cause

• Downshift solenoid (2) is leaking.

Problem: The transmission shifts into SECOND.

Probable Cause

• Upshift solenoid (1) is leaking.

Procedure To Check For Proper Clutch Engagement

1. Disconnect all the electrical harness connectors from upshift solenoid (1), downshift solenoid (2), and lockup clutch solenoid (3). Start the engine and run the engine at low idle.

2. Engage the parking brakes and the service brakes.

3. Manually shift the transmission to the desired speed. After the manual shift is performed, move away from the machine.

   Note: An event will be present in the Vital Information Management System (VIMS) when the upshift solenoid (1), downshift solenoid (2), or lockup clutch solenoid (3) is disconnected. This event must be acknowledged before the torque converter oil temperature is displayed.

   Note: On machines that are equipped with the VIMS, put the information about the torque converter oil temperature on the display of the VIMS. Regularly check the torque converter oil temperature. Do not allow the torque converter oil temperature to increase too much during this shift sequence. The system can be damaged by high oil temperature. Watch for any temperature warnings from the VIMS. If the oil temperature becomes too hot, put the transmission in NEUTRAL. The engine should then be run at approximately 1500 rpm until the temperature returns to the normal operating range.

Note: In order to maintain 1300 rpm for this test, disconnect the throttle position switch. Then use the throttle backup switch.

4. Gradually increase the engine rpm until the engine speed is approximately 1300 rpm or until the machine begins to creep forward.

5. If the engine speed is 1300 rpm and the machine does not begin to creep forward, look at the rear end of the drive shaft. When the transmission clutch is operating correctly, only a small amount of rotation will be observed. The amount of rotation will be less than one complete turn. In NEUTRAL, the drive shaft will rotate.

6. Decrease the engine rpm to low idle. Shift to the next speed position. Repeat Step 3, Step 4 and Step 5 for all of the forward speeds, for reverse speed and for NEUTRAL.

7. Return the rotary selector spool to the NEUTRAL-1 position.

8. Connect the throttle position switch. Properly connect the harnesses to solenoids (1), (2), and (3) .

Problem: The machine begins to creep forward before the engine rpm achieves 1300 rpm.

Probable Cause

• The probable cause is the brake system. Refer to the Air System and Brakes Systems Operation/Testing and Adjusting, "Braking System Troubleshooting".

Problem: The drive shaft does not turn in NEUTRAL.

Probable Cause

• The torque converter is damaged. Look for metal chips in the torque converter hydraulic filter. Refer to Testing and Adjusting, "Torque Converter and Transmission Hydraulic System Troubleshooting". Perform checks.

• There is an improperly engaged transmission clutch. Refer to Testing and Adjusting, "Transmission Hydraulic Control Clutch Pressure - Test". This can cause damage to the transmission.

Problem: When a shift is made to REVERSE or to any forward speed, the engine stops running.

Note: Be sure that the lockup clutch solenoid is disconnected.

Probable Cause

• The lockup clutch is stuck in the engaged position. Look for metal chips in the torque converter hydraulic filter. Refer to Testing and Adjusting, "Torque Converter and Transmission System Troubleshooting". Perform checks.

Problem: The drive shaft turns in more speeds than NEUTRAL, but the wheels do not turn. (A clutch or the clutches do not engage in the transmission.)

Probable Cause

• There is a slipping clutch. Look for debris from the clutches. Refer to Testing and Adjusting, "Torque Converter and Transmission System Troubleshooting". Perform checks.

Problem: The transmission will not stay in the selected speed position. Manual shifts are difficult in any direction (upshift or downshift).

Probable Cause

• The probable cause is solenoid leakage. A shift to a higher speed from the selected speed is an indication of a bad upshift solenoid. A shift to a lower speed from the selected speed is an indication of a bad downshift solenoid.