The shift cable is not in adjustment.

Adjust the shift cable. Refer to Shift Cable Adjustment .

The fourth clutch is inoperative and is dragging.

Rebuild the fourth clutch. Refer to Allison service manuals.

The shift cable is faulty or broken.

Replace the shift cable. Refer to Shift Cable Replacement .

The main pressure is low.

Test the pressure. Refer to Allison service manuals.

The shift selector is not engaged at the control valve.

Install or replace the parts that are involved. The parts are found inside the oil pan. Refer to Allison service manuals.

The oil level is low.

Fill to the proper oil level. Refer to Transmission Fluid Check .