Transmission Oil Cooler

This procedure covers removal and replacement of the transmission oil cooler, which is integrated into the radiator on 2007 Ram 3500 4WD vehicles with the 5.9L Cummins diesel and automatic transmission.

Warnings

Tools required

Parts

• Transmission oil cooler (radiator assembly with integrated cooler) × 1 — Use OEM specification
• Upper radiator hose × 1 — Reuse if condition is good, replace if deteriorated
• Lower radiator hose × 1 — Reuse if condition is good, replace if deteriorated
• Transmission cooler line O-rings × 2 — Use OEM specification
• Radiator mounting brackets/insulators × 1 — Inspect and replace if damaged

Fluids

• ATF+4 — 20 qt
• HOAT coolant 50/50 — 28 qt

Preparation

1. Park vehicle on level surface and apply parking brake
2. Allow engine and cooling system to cool completely (minimum 2 hours)
3. Disconnect negative battery cable
4. Place large drain pan under radiator drain cock
5. Remove engine cover/doghouse from top of engine if equipped
6. Raise and support vehicle on jack stands if additional clearance is needed for lower access

Procedure

1. 1

   Drain cooling system
   Open radiator drain cock at bottom of radiator (passenger side) and drain coolant into appropriate container. Once drained, remove radiator cap to ensure complete drainage. If drain cock is seized, disconnect lower radiator hose instead.
2. 2

   Remove upper radiator hose
   Use hose clamp pliers or flathead screwdriver to loosen hose clamps at radiator and thermostat housing. Twist hose to break seal, then pull off both ends. Inspect hose condition and replace if cracked or deteriorated.
3. 3

   Remove lower radiator hose
   Loosen hose clamps at radiator outlet and water pump inlet. Twist and remove lower radiator hose. Expect residual coolant to drain.
4. 4

   Disconnect transmission cooler lines
   Position large drain pan under transmission cooler line connections on radiator (driver side). Using appropriate line wrench, carefully loosen transmission cooler line fittings at radiator. Verify cooler line fitting size and type against OEM documentation before selecting wrench, as configuration varies. Once loosened, allow transmission fluid to drain completely. Plug lines with rubber caps or transmission line plugs to minimize fluid loss and prevent contamination. Use proper line wrench to avoid rounding off cooler line fittings. Standard wrenches can damage the fittings.
   ⚠Use proper line wrench to avoid rounding off cooler line fittings. Standard wrenches can damage the soft brass fittings.
5. 5

   Disconnect cooling fan shroud
   Remove bolts securing upper fan shroud to radiator (typically four 10mm bolts along top edge). Lift fan shroud up and position it to rest on fan blades. This allows radiator removal without removing mechanical fan assembly.
6. 6

   Disconnect auxiliary transmission cooler lines (if equipped)
   If vehicle is equipped with auxiliary external transmission cooler in front of radiator, disconnect cooler lines at auxiliary cooler. Use line wrenches and have drain pan ready. Some models route these lines differently; trace from radiator to identify configuration.
7. 7

   Remove radiator mounting hardware
   Remove upper radiator mounting brackets (typically two brackets with 13mm bolts). Remove any hold-down straps or clips. Note position of rubber mounting insulators at bottom of radiator for reinstallation.
8. 8

   Remove radiator assembly
   Carefully lift radiator straight up and out of engine bay. Radiator sits on lower rubber insulators and does not bolt at bottom. Tilt top toward engine slightly to clear fan shroud and A/C condenser. Be careful not to damage A/C condenser fins. Drain any remaining coolant from radiator into drain pan.
   ⚠Radiator is heavy when full of residual fluid. Get assistance if needed to avoid dropping and damaging radiator or A/C condenser.
9. 9

   Inspect and prepare new radiator
   Inspect lower mounting insulators and replace if cracked or deteriorated. Install new transmission cooler line O-rings onto fittings of new radiator, lubricating lightly with ATF+4. Verify new radiator has all necessary mounting bracket holes and transmission cooler fittings in correct positions.
10. 10

    Install new radiator assembly
    Carefully lower new radiator into position, seating lower mounting tabs into rubber insulators. Align radiator so fan shroud can be repositioned properly and cooler line fittings are accessible. Ensure A/C condenser is not damaged during installation.
11. 11

    Install radiator mounting hardware
    Install upper radiator mounting brackets and tighten bolts securely. Reinstall any hold-down straps or clips. Verify radiator is seated properly in lower insulators and sits level.
12. 12

    Reconnect transmission cooler lines
    Remove plugs from transmission cooler lines. Thread cooler line fittings into radiator by hand first to ensure proper thread engagement. Tighten fittings with line wrench until snug, then an additional 1/4 turn. Do not overtighten as fittings can strip easily. Reconnect auxiliary cooler lines if equipped.
13. 13

    Reconnect radiator hoses and fan shroud
    Install lower radiator hose with new clamps, positioning clamps away from hose ends to avoid cutting into hose. Install upper radiator hose similarly. Position fan shroud back against radiator and install mounting bolts.
14. 14

    Refill cooling system
    Close radiator drain cock. Fill cooling system with 50/50 HOAT coolant mixture through radiator cap opening. Fill until coolant is visible at bottom of filler neck. Install radiator cap. Fill coolant overflow reservoir to FULL COLD mark. Verify total cooling system capacity against OEM service data before filling.
15. 15

    Refill and check transmission fluid
    With engine off, add ATF+4 to transmission through dipstick tube. Add approximately 4-6 quarts initially (amount lost from cooler and lines). Start engine and check transmission fluid level in PARK on level ground at the fluid temperature specified in the OEM procedure. Add fluid as needed to reach proper level on dipstick. Check for leaks at cooler line connections. Verify transmission total fill capacity against OEM data for the fitted transmission (48RE). The 48RE dry-fill is roughly 17-19 quarts depending on cooler. Check level multiple times during initial operation.
    ℹ️Transmission may require up to 20 quarts total capacity for complete fill. Check level multiple times during initial operation.

Reassembly

1. Reinstall engine cover/doghouse if removed
2. Reconnect negative battery cable
3. Lower vehicle if raised
4. Double-check all hose clamps and fittings are secure

Verification

• Start engine and allow to reach operating temperature while monitoring coolant level
• Check for coolant leaks at radiator hoses and connections
• With engine at operating temperature, shift transmission through all gears
• Check transmission fluid level with engine running in PARK or NEUTRAL on level surface
• Verify no transmission fluid leaks at cooler line connections
• Check cooling system overflow reservoir level after engine cools
• Test drive vehicle and monitor transmission temperature gauge if equipped
• Recheck transmission fluid level after test drive and top off as needed
• Inspect under vehicle for any fluid leaks after first 50 miles of operation

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