793D Off-Highway Truck Power Train: Torque Converter and Transmission Hydraulic System Troubleshooting PART1

Perform the visual checks before you troubleshoot a problem. Make sure that the transmission is in neutral and stop the engine. During the visual checks, use a magnet to separate ferrous particles (iron and steel) from nonferrous particles (aluminum, rubber and paper).

Note: If any of the particles that are listed in the checks are found during a visual check, all the components of the torque converter and the transmission systems must be cleaned. Do not use any damaged parts. Damaged parts must be removed and new parts must be installed.

Check: Check the oil level in the torque converter sump

Check the oil level in the torque converter sump. Look for air or for water at the sight gauge. Many problems in the torque converter and in the transmission are caused by low oil levels or by air in the oil. Add oil to the torque converter sump, if necessary. See the Operation and Maintenance Manual for the correct type of oil.

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

Right side of the engine(1) Oil cooler for the torque converter and the transmission

Probable Cause

Failure of oil cooler (1) for the torque converter and the transmission

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

Probable Cause

There is leakage in the system. See "Check for oil leakage".
The oil level in the torque converter sump is too high or too low.

Problem: The oil level is too low.

Probable Cause

There is leakage from the system. See "Check for oil leakage". Add oil before any tests are performed.

Problem: The oil level is too high.

Probable Cause

The engine crankshaft rear seal is allowing engine oil into the torque converter cover.
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 pinion leaks.
The pump shaft seal for one or more of the pumps that is fastened to the pump drive has failed. Oil from the pump is leaking through the pump drive into the transmission sump.

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

(2) Transmission charging hydraulic filter

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

Note: The transmission charging hydraulic filter has a bypass valve. A 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 differential pressure setting of the bypass valve in transmission charging hydraulic filter (2) is approximately 345 kPa (50 psi).

Problem: Rubber particles are found.

Probable Cause

Seal failure
Hose failure

Problem: Shiny steel particles are found.

Transmission and torque converter gear pump
(3) Transmission lubrication section
(4) Transmission charging pump section
(5) Torque converter charging pump section(6) Transmission scavenge pump section

Probable Cause

Mechanical failure of transmission charging pump section (4)

Check: Check the torque converter hydraulic filter

(7) Torque converter hydraulic filter

Remove torque converter hydraulic filter (7). Inspect the filter for foreign material.

Note: Torque converter hydraulic filter (7) has a bypass valve. A 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 differential pressure setting of the bypass valve in torque converter hydraulic filter (7) is approximately 127 kPa (18 psi).