The oil has not been changed at proper intervals.

1. Drain the oil.
2. Replace the oil filter. Refer to External Transmission Oil Filter Replacement .
3. Replace the external oil filter. Refer to Transmission Fluid Pan and Filter Replacement .
4. Fill the transmission with new oil.

The heat of the oil is excessive.

Check the cooling system.

The clutch is inoperative.

Overhaul the transmission.

Refer to applicable Allison Transmission Service Manual.

The filter or filter elements are damaged.

Replace the external filter. Refer to External Transmission Oil Filter Replacement .

The engine rear oil seal is faulty.

Replace the engine rear oil seal. Refer to Crankshaft Rear Oil Seal Replacement in Engine Mechanical.