48RE Transmission Oil Cooler Heat Exchanger 5.9L Cummins

This is the coolant-to-oil heat exchanger that bolts to the driver side of the 5.9L Cummins block and cools the transmission fluid coming off the 48RE torque converter. It is not an air-to-oil cooler in front of the radiator - engine coolant runs through one side of the core and automatic transmission fluid runs through the other, so the transmission comes up to temperature fast in winter and stays out of trouble when the truck is loaded. The unit ships with the mounting base, and it replaces Mopar part numbers 68004317AA, 05086946AA, 05086946AB, 05191791AA and 68253200AA.

Fitment is 2003 through 2009 Dodge Ram 2500 and Ram 3500 with the 5.9L Cummins diesel. On those trucks the cooler sits low on the block behind the oil filter, with two transmission lines into the top of the housing and coolant passages through the base.

The failure these units are known for is internal cross-leaking. Once the core corrodes through, coolant pushes into the transmission circuit and the fluid on the dipstick comes up pink, milky or foamy - or ATF goes the other direction and shows up as a slick film in the radiator and overflow bottle. Coolant in a 48RE destroys the clutch frictions and the converter lining in short order, so the cooler gets replaced at the same time as the transmission, not after. If you are rebuilding a 48RE that came apart with pink fluid in the pan, this part is not optional.

To change it: drain the coolant below block level, disconnect and cap both transmission cooler lines, unbolt the assembly from the block, and clean the mounting face before the new base goes on. Use new O-rings and gaskets at the block, refill the cooling system, then fill and level the transmission with the engine running - the new cooler and lines hold fluid that the pan fill alone will not account for.