The oil level is low.

Add oil in order to acquire the proper oil level. Refer to Transmission Fluid Check .

The engine idle speed is too low.

Refer to Idle Speed Adjustment in Diesel Fuel Injection.

The oil cooler lines are restricted or leaking.

1. Check the lines for kinks or leaks.
2. Reroute or replace the lines as necessary. Refer to Transmission Oil Cooler Line Replacement .

The transmission is excessively leaking oil internally.

Overhaul the transmission.