Replace transmission cooler lines connecting the transmission to the external cooler/radiator to address leaks or damage on 2021-2024 Ford F-150 3.5L EcoBoost RWD models.

Warnings

Tools required

Parts

• Transmission cooler line set (inlet and outlet) × 1 — Use OEM Ford specification
• Transmission line O-rings/seals × 1 — Typically included with line set
• Transmission drain plug crush washer × 1 — Use OEM specification if equipped

Fluids

• Motorcraft MERCON LV ATF — 3 qt

Preparation

1. Park vehicle on level surface and apply parking brake firmly
2. Place wheel chocks behind rear wheels
3. Allow engine and transmission to cool if recently operated
4. Raise front of vehicle with floor jack and support securely on jack stands at frame rails
5. Remove engine splash shield/skid plate if equipped (retain all fasteners)
6. Identify transmission cooler line routing from transmission to radiator/cooler assembly
7. Position drain pans under transmission cooler line connections at transmission and radiator

Procedure

1. 1

   Drain transmission fluid (optional but recommended)
   If equipped with drain plug, remove transmission drain plug using appropriate socket. Allow fluid to drain completely into drain pan. If no drain plug is present, proceed to next step understanding more fluid will be lost during line removal. Install drain plug with new crush washer if removed.
   Torque spec

   Drain Plug39 Nm (29 lb-ft)
2. 2

   Disconnect cooler lines at radiator/cooler
   Locate transmission cooler line connections at the radiator or external transmission cooler (typically on driver side of radiator). Clean connections thoroughly with degreaser and shop towels. Using appropriate quick-disconnect tool, depress locking tabs and carefully pull lines straight out from radiator fittings. Be prepared for fluid spillage. Cap or plug radiator fittings to prevent contamination.
   ⚠Do not twist or bend lines excessively during removal as this can damage radiator fittings.
3. 3

   Disconnect cooler lines at transmission
   Trace lines to transmission case connections (typically on right side of transmission). Clean connection points thoroughly. Using quick-disconnect tool or appropriate flare nut wrench, disconnect cooler lines from transmission fittings. The 10R80 transmission uses quick-disconnect fittings at the case. Depress locking tabs and pull lines straight away from transmission. Allow remaining fluid to drain into pan.
   ⚠Ensure drain pan is positioned directly under fittings as significant fluid may drain when disconnected.
4. 4

   Remove cooler line mounting brackets
   Follow cooler lines along their routing and identify all mounting brackets and clips securing lines to frame, crossmember, or body. Remove bolts or release clips securing lines. Note exact routing and position of all brackets for reinstallation. The lines typically route along the passenger side frame rail.
5. 5

   Remove old cooler lines
   Carefully maneuver old cooler lines down and out from under vehicle, taking care not to damage surrounding components. Inspect old lines for failure points (cracks, corrosion, damage from road debris). Clean all connection points at transmission and radiator using shop towels and degreaser.
6. 6

   Inspect fittings and install new seals
   Inspect transmission and radiator cooler fittings for damage, corrosion, or contamination. Clean thoroughly with lint-free shop towels. Install new O-rings or seals on new cooler lines if not pre-installed. Lightly lubricate new O-rings with clean MERCON LV transmission fluid to ease installation and prevent damage.
   ℹ️Do not reuse old O-rings or seals. Always install new seals provided with replacement lines.
7. 7

   Route new cooler lines
   Route new cooler lines from transmission to radiator following exact path of original lines. Ensure lines are not kinked, twisted, or in contact with exhaust components, driveshaft, or any sharp edges. Verify adequate clearance for suspension travel and steering components.
8. 8

   Connect cooler lines at transmission
   Insert cooler line fittings into transmission case connections. Push firmly until you hear or feel a distinct click indicating the quick-disconnect has fully seated. Gently pull on each line to verify it is locked in place and will not pull free. Ensure alignment is straight and no cross-threading has occurred.
9. 9

   Connect cooler lines at radiator/cooler
   Insert cooler line fittings into radiator or external cooler connections. Push firmly until quick-disconnect locks engage with an audible click. Verify both lines are fully seated by pulling gently on each line. Confirm correct line routing (inlet to inlet, outlet to outlet).
   ⚠Reversed cooler line connections can cause transmission overheating and damage. Verify routing matches original configuration.
10. 10

    Reinstall mounting brackets and clips
    Secure cooler lines using all original mounting brackets and clips in their original positions. Ensure lines are properly supported along entire length with no sagging or excessive tension. Verify no contact with moving components or heat sources.
11. 11

    Fill transmission with fluid
    Locate transmission fill plug on driver side of transmission case. Remove fill plug. Using appropriate funnel and fluid pump, add approximately 3 quarts of Motorcraft MERCON LV ATF through fill hole to replace lost fluid. Do not overfill at this stage; final level will be verified with engine running at operating temperature.
12. 12

    Initial leak check and test
    Reinstall splash shield/skid plate if removed. Lower vehicle to ground. Start engine and allow to idle in Park. While engine idles, move shifter through all gear positions, pausing 3 seconds in each position, then return to Park. Check all cooler line connections at transmission and radiator for leaks. Observe fluid level and add as necessary through fill plug.
    ⚠️Keep hands and tools clear of rotating components and hot exhaust while engine is running.
13. 13

    Check fluid level at operating temperature
    With engine running and transmission in Park, drive vehicle briefly to bring transmission fluid to operating temperature (160-180°F, approximately 10-15 minutes of driving). Park on level surface with engine running. Check fluid level at fill plug; correct level should be at bottom of fill plug hole with slight weeping. Add fluid if necessary. Install fill plug when level is correct.
    Torque spec

    Fill Plug39 Nm (29 lb-ft)

Reassembly

1. Ensure all mounting brackets and clips are securely fastened with no loose cooler lines
2. Verify engine splash shield or skid plate is properly reinstalled with all fasteners tight
3. Confirm no tools or parts were left in engine compartment or under vehicle

Verification

• With engine at operating temperature and running in Park, verify no leaks at any cooler line connections
• Verify transmission fluid level is correct at operating temperature (slight weeping at fill plug)
• Test drive vehicle through all gears confirming smooth shifts with no slipping or delayed engagement
• Recheck all connections for leaks after test drive while engine is running
• Monitor transmission fluid level and condition over next few days of operation

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