Transmission Cooler Lines

This procedure covers the removal and replacement of transmission cooler lines on a 2007 Ram 3500 with 5.9L Cummins diesel and automatic transmission, including line routing and fluid refill.

Warnings

Tools required

Parts

• Transmission cooler line set (transmission to radiator) × 1 — Use OEM specification or exact replacement
• Transmission cooler line fittings/seals × 1 — Often included with cooler lines

Fluids

• ATF+4 — 4 qt

Preparation

1. Park vehicle on level surface and apply parking brake
2. Place wheel chocks behind rear wheels
3. Allow transmission to cool if vehicle was recently operated
4. Raise front of vehicle and support securely on jack stands at frame rails
5. Remove engine skid plate if equipped (4 bolts)
6. Locate transmission cooler lines running from transmission to radiator-mounted cooler

Procedure

1. 1

   Drain transmission fluid from cooler lines
   Position drain pan beneath transmission cooler line connections at the radiator. The cooler lines connect to the lower portion of the radiator on the passenger side. Expect 2-4 quarts of fluid to drain from the lines when disconnected. This procedure covers the removal and replacement of transmission cooler lines on a 2007 Ram 3500 with 5.9L Cummins diesel and 48RE automatic transmission, including line routing and fluid refill.
2. 2

   Disconnect cooler lines at radiator
   Using a flare nut wrench to prevent rounding, loosen the cooler line fittings at the radiator. The fittings are typically 5/8" and use a tube nut design. Turn counterclockwise to remove. Support the line while loosening to prevent twisting the radiator fitting. Once loose, allow fluid to drain completely into pan.
3. 3

   Disconnect cooler lines at transmission
   Locate the transmission cooler line connections on the driver side of the transmission case, near the front. Clean the area around fittings with brake cleaner and wire brush. Using a flare nut wrench, loosen both cooler line fittings at the transmission. These are also typically 5/8" tube nuts. Allow remaining fluid to drain from lines.
4. 4

   Remove cooler line mounting clips
   Trace the cooler lines along the driver side frame rail from transmission to radiator. Remove any plastic clips, metal brackets, or clamps securing the lines to the frame rail, crossmember, or body. On 4WD models, lines typically route above the front differential and along the frame. Note routing position and clip locations for reinstallation.
5. 5

   Remove old cooler lines
   Carefully pull the old cooler lines out from their routing path. Inspect the frame rail area for any rubbing or contact points that may have damaged the old lines. Note which line connects to which port on both the transmission and radiator - typically the lower radiator port connects to the upper transmission port for proper flow direction.
6. 6

   Inspect and clean fittings
   Inspect the radiator cooler fittings and transmission case fittings for damage, corrosion, or thread wear. Clean all fittings thoroughly with brake cleaner and a wire brush. Check that threads are clean and undamaged. If radiator fittings are damaged, radiator replacement may be required.
7. 7

   Route new cooler lines
   Route the new cooler lines along the same path as the original lines, following the driver side frame rail. Ensure lines do not contact the exhaust, driveshaft, steering components, or suspension. Maintain adequate clearance from heat sources and moving parts. On 4WD models, verify clearance above the front differential through full steering travel.
8. 8

   Connect cooler lines to transmission
   Install the cooler lines to the transmission case fittings hand-tight first to ensure proper thread engagement. The return line typically connects to the upper port. Once hand-tight, use a line wrench to snug the fittings. Tighten firmly but do not over-torque - approximately 18-20 ft-lbs is typical for these fittings though exact specification is not provided.
9. 9

   Connect cooler lines to radiator
   Connect the cooler lines to the radiator fittings, ensuring correct port assignment matches original configuration. Hand-tighten fittings first to verify proper thread engagement. Use a line wrench to snug the fittings - tighten firmly but be careful not to over-torque aluminum radiator fittings. Approximate torque should be 18-20 ft-lbs.
10. 10

    Install mounting clips and brackets
    Reinstall all mounting clips, brackets, and clamps in their original locations to secure the cooler lines to the frame rail and body. Ensure lines are properly supported and cannot vibrate or contact other components. Verify adequate clearance through full suspension travel and steering range.
11. 11

    Add transmission fluid
    With vehicle still raised and supported, locate the transmission dipstick/fill tube. Add ATF+4 through the dipstick tube using a transmission fluid funnel to replace the fluid lost from the cooler lines, then verify level by dipstick reading rather than a fixed quantity. This replaces fluid lost from the cooler lines and circuit.
12. 12

    Check for leaks and circulation
    Start the engine and allow it to idle. With your foot on the brake, shift through all gear ranges (P-R-N-D-2-1) pausing 2-3 seconds in each position, then return to Park. Inspect all cooler line connections at both the transmission and radiator for leaks. Check the fluid level on the dipstick with engine running and add ATF+4 as needed to bring to the proper level on the HOT range.

Reassembly

1. Reinstall engine skid plate if equipped, ensuring proper bolt torque
2. Lower vehicle from jack stands
3. Recheck transmission fluid level after vehicle reaches operating temperature
4. Top off transmission fluid to HOT range as needed - transmission should be at operating temperature with engine running in Park

Verification

• Test drive vehicle through multiple heat cycles and check for leaks at all cooler line connections
• Verify transmission shifts properly through all gears with no slipping or delayed engagement
• Recheck fluid level after test drive with transmission at operating temperature - fluid should be in HOT range on dipstick
• Inspect ground beneath vehicle for any signs of ATF leakage after overnight parking
• Verify no contact between cooler lines and frame, exhaust, or suspension components during driving

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