Testing and Adjusting 784C and 785C Off-Highway Truck/Tractors Power Train. Transmission Troubleshooting

Testing and Adjusting 784C and 785C Off-Highway Truck/Tractors Power Train

Media Number --07

Publication Date -01/03/2004

Date Updated -08/03/2004

Transmission Troubleshooting

SMCS - 3030

Before troubleshooting a problem, perform the visual checks. Perform the checks with the engine OFF and with the parking brakes ON. Put the transmission control in NEUTRAL and the transmission rotary selector spool in NEUTRAL-1. During the checks, use a magnet to separate ferrous particles from nonferrous particles.

Check: Check the oil level in the transmission section of the hydraulic tank.

Check the oil level in the transmission section of the hydraulic tank. Look for air or for water at the sight gauge. Many problems in the transmission are caused by low oil levels or by air in the oil. if necessary, add oil to the transmission section of the hydraulic tank. In order to determine the correct oil to use, refer to your Operation and Maintenance Manual.

Problem: The oil level is too high.

Probable Cause

• Oil is leaking from the pump drive into the transmission gear pump.
• The oil from the differential and bevel gear leaks into the transmission case. One potential reason for the leaks is the failure of the pinion seal in the differential. The leaks can also occur if the center plug of the differential pinion is not installed or if the center plug does not fit properly.

Problem: There is an indication of water in the oil.

Probable Cause

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Illustration 1	g00666175
(1) Transmission oil cooler

• Failure of transmission oil cooler (1)

Problem: There is an indication of air in the oil.

Probable Cause

• Perform "Check for oil leakage".

Problem: The oil level is too low.

Probable Cause

• Perform "Check for oil leakage". Add oil before any tests are performed.

Check: Check for oil leakage.

Inspect all the oil lines, the hoses, and the connections for damage or for leaks. Look for oil on the ground under the machine.

Note: If oil can leak out of a fitting or a connection, air can enter the system. Air in the system can be as harmful as not enough oil.

Check: Check the transmission charging hydraulic filter.

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Illustration 2	g00666185
(2) Transmission charging hydraulic filter

Remove transmission charging hydraulic filter (2). Inspect the filter for foreign material.

Note: Transmission charging hydraulic filter (2) has an oil filter bypass valve. An oil filter bypass valve allows oil to bypass the oil filter element whenever the pressure difference between the inlet oil and the outlet oil at the oil filter gets too high. Any oil that does not go through the filter element flows directly into the hydraulic circuit. Dirty oil causes restrictions in valve orifices and in sticking valves.

The difference of the pressure setting at the bypass valve in transmission charging hydraulic filter (2) is approximately 250 kPa (36 psi).

Problem: Rubber particles are found.

Probable Cause

• Seal failure
• Hose failure

Problem: Shiny steel particles are found.

Probable Cause

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Illustration 3	g00367714
Transmission gear pump (3) Transmission scavenge pump section (4) Transmission lubrication section of the pump (5) Transmission charging pump section

• Mechanical failure of transmission charging pump section (5)

Check: Check the transmission magnetic screens.

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Illustration 4	g00290397
Lower right side of the transmission case (8) Magnetic screens

Remove magnetic screens (8). Inspect magnetic screens (8) for foreign material and/or for restrictions.

Problem: Rubber particles are found.

Probable Cause

• Seal failure
• Hose failure

Problem: Black coarse material is found.

Probable Cause

• Worn clutches in the transmission
• Failure of a thrust disc

Problem: Thin strands that are elastic are found.

Probable Cause

• Worn clutches in the transmission

Problem: Iron chips or steel chips are found.