Replace the differential pinion seal to stop rear differential fluid leaks at the pinion shaft. Requires removal of driveshaft and pinion yoke.

Warnings

Tools required

Parts

• Differential pinion seal × 1 — Use OEM specification for AMC Model 20 rear axle
• Pinion nut × 1 — Use OEM specification, replace if deformed
• Rear differential fluid × 1 — API GL-5 80W-90 gear oil
• Rear axle gasket or RTV sealant × 1 — For differential cover

Fluids

• API GL-5 80W-90 gear oil — 3.5 qt

Preparation

1. Park vehicle on level surface and set parking brake firmly
2. Place wheel chocks in front of front wheels
3. Raise rear of vehicle and support securely on jack stands placed under frame or axle housing
4. Remove rear wheels for better access if needed
5. Clean area around differential cover and pinion yoke to prevent contamination

Procedure

1. 1

   Mark driveshaft alignment
   Using paint marker, make alignment marks on the rear driveshaft yoke and the pinion yoke flange. These marks ensure proper driveshaft phasing during reinstallation to prevent vibration. Mark both sides of the flange for clarity.
2. 2

   Remove driveshaft
   Remove the four U-joint strap bolts at the rear differential pinion yoke. Support the driveshaft and slide it rearward to separate the U-joint from the yoke. Lower the driveshaft and move it aside. Do not allow the U-joint caps to fall off; tape them if necessary.
3. 3

   Measure pinion rotating torque
   Using an inch-pound torque wrench on the pinion nut, measure and record the torque required to rotate the pinion through several revolutions. This is the pinion bearing preload and must be matched or slightly exceeded during reassembly. Typical preload is 15-25 inch-pounds for used bearings. If unable to measure accurately, note this for later reference.
4. 4

   Mark pinion nut position
   Make alignment marks on the pinion nut, pinion shaft, and yoke using paint marker. Count and record the number of threads visible beyond the nut. These references are critical for maintaining proper pinion depth and bearing preload during reassembly.
5. 5

   Remove pinion yoke
   Hold the pinion yoke stationary using a yoke holding tool or large pipe wrench on the yoke flange. Remove the pinion nut using appropriate socket. Remove the yoke by pulling straight out; it may require moderate force. If stuck, use a two-jaw puller. Do not strike the yoke with a hammer as this can damage pinion bearings.
6. 6

   Remove old pinion seal
   Using a seal puller or small pry bar, carefully extract the old pinion seal from the differential housing. Work around the seal perimeter to avoid damaging the seal bore. Inspect the seal surface on the pinion yoke for grooves or damage; if grooved, the yoke must be replaced or the new seal will leak.
7. 7

   Clean and inspect components
   Thoroughly clean the seal bore in the differential housing and the seal surface on the pinion yoke. Inspect the yoke seal surface for wear grooves, pitting, or damage. Check the seal bore for nicks or damage. Minor imperfections in the bore can be smoothed with fine emery cloth. Inspect the pinion shaft threads and clean if necessary.
8. 8

   Install new pinion seal
   Lightly coat the outer diameter of the new seal with non-hardening gasket sealant or gear oil. Coat the seal lip with fresh gear oil. Position the seal squarely in the bore with the lip facing inward toward the differential. Using a seal driver or large socket that contacts only the outer edge of the seal, drive the seal in until it is flush with or slightly below the housing face. Tap evenly around the perimeter to ensure straight installation.
9. 9

   Reinstall pinion yoke
   Coat the pinion shaft seal surface and splines with gear oil. Slide the pinion yoke onto the pinion shaft, aligning the reference marks made during disassembly. The yoke should slide on with moderate hand pressure. Install the pinion washer if equipped.
10. 10

    Install and torque pinion nut
    Apply a small amount of medium-strength thread locker to the pinion nut threads. Thread the pinion nut onto the shaft by hand. Hold the yoke stationary and tighten the nut gradually, checking bearing preload frequently. Tighten until the alignment marks are close to matching AND the rotating torque equals or slightly exceeds the original measurement (typically 5-10 inch-pounds higher than original for new seal drag). Do not overtighten. If preload becomes excessive before marks align, the collapsible spacer may be crushed and the job must be redone with new bearings and spacer.
11. 11

    Verify pinion preload
    With the pinion nut tightened to match reference marks, recheck the pinion rotating torque with an inch-pound torque wrench. The torque should be within 5-10 inch-pounds higher than the original reading. If significantly different, the nut may need minor adjustment. Make small adjustments (1/8 turn at a time) as needed. Once correct, the pinion nut should not be loosened or bearing preload will be lost.
12. 12

    Reinstall driveshaft
    Position the driveshaft and align the U-joint with the pinion yoke, matching the alignment marks made during disassembly. Install the U-joint straps and bolts. Tighten the strap bolts in a crossing pattern to ensure even clamping. Since no torque specification is provided, tighten securely to factory specification if available in service manual.
13. 13

    Check and refill differential fluid
    Place drain pan under differential cover. Remove the differential cover fill plug on the passenger side of the housing. Fluid level should be at the bottom of the fill hole. If low or if fluid was lost during seal replacement, add API GL-5 80W-90 gear oil until it reaches the fill hole. Install fill plug and tighten securely.

Reassembly

1. Reinstall rear wheels if removed and torque lug nuts to specification
2. Lower vehicle from jack stands carefully
3. Remove wheel chocks and test parking brake operation
4. Wipe down differential housing and pinion yoke area to check for leaks after test drive

Verification

• Start engine and verify no abnormal noises from differential
• Test drive vehicle 10-15 miles, including highway speeds and turns in both directions
• Listen for whining, howling, or clunking noises that may indicate incorrect pinion preload or bearing damage
• After test drive, inspect pinion seal area for any signs of fluid leakage
• Recheck differential fluid level at fill plug; top off if necessary
• Monitor for leaks over the next few days of normal driving

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