Replace the rear differential pinion seal on a 2021-2024 Ford F-150 3.5L EcoBoost RWD to eliminate gear oil leaks at the driveshaft connection point.

Warnings

Tools required

Parts

• Rear differential pinion seal × 1 — Use OEM specification
• Pinion nut (if reuse not recommended) × 1 — Use OEM specification

Fluids

• Motorcraft 75W-140 Synthetic Gear Oil — 2 qt

Preparation

1. Park vehicle on level surface and engage parking brake
2. Chock front wheels securely
3. Raise rear of vehicle and support on jack stands at frame rails
4. Allow differential to cool completely if vehicle was recently driven
5. Clean area around pinion flange and driveshaft connection with degreaser

Procedure

1. 1

   Mark driveshaft alignment
   Use paint marker to mark the driveshaft yoke and pinion flange alignment. This ensures proper driveline balance upon reassembly and prevents vibration issues.
2. 2

   Remove driveshaft
   Remove the four driveshaft U-joint strap bolts connecting the rear driveshaft to the pinion flange. Support the driveshaft and carefully lower it away from the differential. Do not allow the driveshaft to hang unsupported as this can damage the U-joint. Secure the driveshaft to the frame or exhaust with wire to keep it out of the way.
   Torque spec

   Driveshaft U-Joint Straps20 Nm (15 lb-ft)
3. 3

   Measure pinion bearing preload
   Using an inch-pound torque wrench on the pinion nut, measure and record the rotational torque required to turn the pinion. Rotate the pinion several times to ensure consistent reading. Record this value - it will be needed during reassembly to verify proper bearing preload. Typical readings are 15-35 inch-pounds for used bearings.
4. 4

   Mark and remove pinion nut
   Mark the position of the pinion nut relative to the pinion shaft threads using paint marker. Count and record how many threads are visible above the nut. Using a pinion flange holding tool or impact wrench to prevent rotation, remove the pinion nut. This nut is extremely tight and requires significant force.
5. 5

   Remove pinion flange
   Using a suitable puller or by carefully tapping the flange from behind while supporting it, remove the pinion flange from the pinion shaft. The flange is press-fit and may require moderate force. Do not strike the pinion shaft directly. Inspect the flange seal surface for grooves or damage that could cause the new seal to leak.
6. 6

   Remove old pinion seal
   Using a seal puller or carefully positioned pry bar, remove the old pinion seal from the differential housing. Work around the seal circumference evenly to avoid cocking it. Be careful not to damage the seal bore in the housing or score the surface.
7. 7

   Clean and inspect
   Thoroughly clean the seal bore in the differential housing using brake cleaner and clean rags. Inspect the bore for any damage, scoring, or irregularities. Clean the pinion flange seal surface and inspect for wear grooves. If the flange has a worn groove in the seal contact area, it must be replaced to prevent repeat leaks.
8. 8

   Install new pinion seal
   Lightly coat the outer edge of the new seal with gear oil. Position the seal squarely in the bore with the seal lip facing inward toward the differential. Using a pinion seal installation tool or large socket that contacts only the outer edge of the seal, carefully drive the seal into the housing until it is flush with the housing face. Tap evenly around the circumference to prevent cocking.
9. 9

   Install pinion flange
   Lightly coat the pinion seal lip with clean gear oil. Apply a small amount of clean gear oil to the pinion shaft splines. Carefully slide the pinion flange onto the pinion shaft, aligning the splines. Be extremely careful not to damage the new seal lip with the flange during installation. Tap the flange fully onto the shaft if needed.
10. 10

    Install and torque pinion nut
    Install the pinion nut (new or reused based on manufacturer recommendation). Using the pinion flange holding tool, tighten the nut gradually while checking rotational torque frequently. Tighten until the nut reaches the alignment marks made during disassembly AND the rotational torque matches the value recorded in step 3 (within 5 inch-pounds). If rotational torque becomes too high before alignment marks are reached, the bearing preload is excessive - consult service information. Do not overtighten as this will damage bearings.
11. 11

    Reinstall driveshaft
    Raise the driveshaft into position and align the marks made during disassembly. Install the four U-joint strap bolts finger-tight first, then torque in a crisscross pattern to ensure even loading.
    Torque spec

    Driveshaft U-Joint Straps20 Nm (15 lb-ft)
12. 12

    Check and refill differential oil
    Check the rear differential fluid level through the fill plug. Add Motorcraft 75W-140 Synthetic Gear Oil as needed to bring the level to the bottom of the fill hole. The differential should hold approximately 2.0 quarts total. Install the fill plug when complete.

Reassembly

1. Verify all fasteners are properly torqued per specifications
2. Wipe down the differential housing and pinion area to remove any spilled gear oil
3. Check that the driveshaft alignment marks are properly matched

Verification

• Lower vehicle and test drive 10-15 miles, including highway speeds
• Verify no vibration or driveline noise during acceleration and deceleration
• Raise vehicle and inspect pinion seal area for any signs of leakage
• Check differential fluid level again after test drive and top off if needed
• Monitor for leaks over the next 100 miles of driving

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