48RE Transmission Cooler 2003-20

If your 2003-2007 Ram 2500 or 3500 with the 5.9L Cummins is dripping ATF along the frame rail, has a cooler line rusted through near the front of the truck, or is running hot every time it pulls a trailer, this is the part most owners end up searching for. Despite the short title, this listing is a complete transmission cooler line kit for the 48RE, not a cooler core: two braided lines, the end fittings, and the sealing washers and O-rings needed to replace the factory steel tubes between the transmission and the cooler.

What the cooler lines do in a 48RE

The 48RE sends hot fluid out of the case through one line, forward along the frame to the heat exchanger ahead of the radiator, and back through the other line to lubricate the geartrain. On a Cummins tow rig that circuit carries a lot of heat, and the converter adds more any time it is unlocked. ATRA's GEARS Magazine puts the ideal maximum fluid temperature at about 200 F and notes that fluid and clutch life roughly halves for every 20 F above that, so a pinched or leaking line is not a cosmetic problem. The kit uses a PTFE-lined hose with a stainless and nylon outer braid, which will not rust through from road salt the way the original steel tubes do.

Symptoms of a bad 48RE cooler line

• ATF wetting a line, a fitting at the case, or the area around the heat exchanger connections
• Low fluid level that keeps coming back after a top-off
• High transmission temperature when towing, often with converter shudder or soft shifts once it is hot
• A line that is visibly kinked, crushed by a rock strike, or scaled with rust where it clips to the frame
• Late or slipping 3-4 shifts after a fluid loss, since the overdrive clutch is the first to suffer from low fluid and heat

How to confirm the lines are the problem

Flow-test before you buy anything. GEARS calls for at least about one gallon per minute of cooler flow with the engine warm at idle, checked with a flow meter or by running the return line into a bucket. A real external leak points straight at the lines or fittings. Low flow with no leak is a different problem: it can be a stuck bypass valve inside the factory heat exchanger, or worn pump and valve body circuits feeding the cooler. Our 48RE pump failure guide covers the pressure and flow checks, and the 48RE cooler line replacement guide walks the whole circuit step by step.

Fitment

This kit is listed for the 48RE in 2003-2007 Dodge Ram 2500 and 3500 trucks with the 5.9L Cummins diesel. The product name reads 2003-20 because the original listing title was cut off; the fitment is 2003-2007, which covers the full 48RE Cummins run. It is not listed for the 6.7L trucks or for 68RFE-equipped Rams. Routing and the fittings at the cooler end can differ between 2WD, 4WD and cab/chassis builds, so compare the kit's fittings against the ports on your transmission and cooler before you cut anything off. If you are not sure, send us the year and VIN and we will check it. Every part we carry for this transmission is on the 48RE parts page.

Install notes

• Put a drain pan under both ends. Fluid runs from the case side and the cooler side as soon as a fitting opens.
• Release the factory quick-connects with the correct release tool. Do not pry them; the case port has to stay undamaged.
• Do not overtighten the threaded fittings in the case. Sonnax notes the Chrysler RWD cooler fittings use tapered threads with no stop, and cranking on them can crack the case boss.
• Use the supplied crush washers and O-rings, route the braided lines clear of the exhaust and driveshaft, and secure them so they cannot rub. The hose is not meant to be bent tight or folded.
• Refill with ATF+4 (MS-9602) only and check the level hot, in Neutral, on level ground. Do not fill by a number; the 48RE holds 29-33 pints total and a line job drains an unknown amount.

If the heat exchanger itself is plugged or its bypass valve is stuck, new lines will not fix the heat. Replace it with the 48RE oil cooler heat exchanger for the 5.9 Cummins while the lines are off, and if the fluid came out dark or burnt, pair the job with a 48RE solenoid and filter service kit so the pan, filter and governor parts are fresh too.

Is this a transmission cooler or a cooler line kit?

It is a cooler line kit. You get two braided lines, the end fittings, and the crush washers and O-rings. It replaces the factory steel lines between the 48RE and the cooler; it does not replace the heat exchanger itself.

Will it fit my 2003-2007 Ram 2500 or 3500 Cummins?

It is listed for 2003-2007 Ram 2500/3500 trucks with the 5.9L Cummins and 48RE. Routing and cooler-end fittings can vary by drivetrain and cab/chassis, so compare the fittings to your ports first, or send us your year and VIN and we will confirm.

Why does the title say 2003-20?

The original listing title was truncated. The fitment for this kit is 2003-2007, the full 48RE Cummins production run. It is not listed for 2008-up trucks.

What fluid do I use after replacing the lines?

Chrysler ATF+4 meeting MS-9602. Do not refill by capacity; start the engine, cycle through every range, and check the level hot in Neutral on level ground, adding a little at a time.

My truck runs hot but I see no leak. Will new lines fix it?

Probably not. Flow-test the circuit first. Low flow with no leak usually points to a stuck bypass valve in the heat exchanger or worn pump and valve body oil circuits, not the external lines.

Should I replace both lines or just the leaking one?

On a high-mileage truck, do both. The second line has seen the same heat, salt and vibration, and the kit is built to replace the pair in one job.

• 48RE Pump Failure Symptoms and Replacement
• 48RE Cummins Transmission Problems: Failure Points, Upgrades, and What Actually Breaks
• 48RE Ram 2500 Cummins Common Problems and Fixes
• 48RE Ram 3500 Cummins Common Problems and Fixes