Short theory first (so each step’s purpose is clear)
- Purpose of the differential: accept torque from the driveshaft (pinion), translate it 90° to the axle shafts via the ring gear and carrier, and allow the two wheels to rotate at different speeds by the spider/side gears. The ring & pinion must have correct mesh (contact pattern), the carrier and pinion bearings must have correct preload, and the enclosure must keep lubricant in and contaminants out. Noise, overheating, leaks, or premature wear come from improper oil, contaminated oil, worn bearings/gears, incorrect backlash/preload, or leaking seals.

Ordered service procedure with theory and how each repair fixes faults
1) Prepare and safety
- Action: Park on level ground, chock wheels, jack and support axle with stands, wear eye/hand protection.
- Theory/fix: Safe access prevents accidents. Having the axle supported at ride height helps avoid distortion when setting backlash/pattern.

2) Identify the job scope
- Action: Decide if this is a fluid-only service or a full inspection/rebuild (fluid change vs. checking gear tooth pattern, bearings, shims, seals, etc.).
- Theory/fix: Fluid-only fixes contaminants and lubrication issues; deeper inspection fixes mechanical wear, preload/mesh faults that cause noise and accelerated wear.

3) Drain and initial fluid inspection
- Action: Remove fill plug (top) then drain or remove cover to drain. Catch fluid, inspect for metal particles, smell/burnt odor, and color.
- Theory/fix: Draining removes degraded oil and contaminants. Copper/steel flakes indicate bearing or gear wear; a burnt smell indicates overheating. Fluid inspection tells you whether damage is superficial (contaminated oil) or structural (metal chunks → rebuild).

4) Clean magnetic drain surfaces and inspect debris
- Action: Clean magnets (or magnet areas) and cover with solvent/clean rag; collect and examine metal shavings (size/type).
- Theory/fix: Magnets trap ferrous particles; cleaning removes embedded debris that abrades. Large flakes or chunks indicate advanced wear of bearings/gears and require deeper repair.

5) Remove cover (if doing full inspection)
- Action: Remove differential cover, pry gently to separate; catch remaining fluid.
- Theory/fix: Allows full visual inspection of gears, carrier, bearing caps, and seals.

6) Visual gear inspection
- Action: Inspect ring gear, pinion (if visible), carrier, spider gears and axle splines for wear patterns, pitting, scoring, broken teeth, discoloration.
- Theory/fix: Gear tooth surface must show a consistent contact pattern where loads are carried. Pitting/scoring indicates lubrication failure or contamination; broken teeth require replacement of components. Identifying the wear mode answers root cause (low oil, wrong oil, misalignment, insufficient preload).

7) Check backlash and gear mesh pattern (if investigating noise)
- Action: Rotate ring while holding pinion and use a dial indicator on ring tooth to measure backlash; mark gear teeth and use gear marking compound (Prussian blue) to show contact pattern; evaluate pattern relative to tooth face.
- Theory/fix: Backlash controls clearance between ring and pinion. Proper backlash and mesh pattern keep load distributed across the tooth. If pattern is too close to toe/heel or root/face, adjust carrier shims/pinion depth. Correcting mesh spreads load, reduces localized wear and whining.

8) Check bearing play and pinion preload
- Action: Check lateral play of carrier and axial preload of pinion (feel for endplay, spin pinion to detect rough bearings). Full pinion preload adjustment requires removal of pinion nut and shims/crush sleeve methods.
- Theory/fix: Bearings must be preloaded to eliminate excessive movement while not being over-tight. Too loose → gear misalignment and impact wear; too tight → overheated bearings and premature failure. Adjusting preload removes play and stabilizes gear mesh.

9) Inspect seals and axle splines
- Action: Inspect pinion seal, axle shaft seals (rear wheel bearings seal area) for leaks/drying/cracks.
- Theory/fix: Seals keep lubricant in and contaminants out. Replace leaking seals to prevent oil loss and contamination that cause bearing and gear failure.

10) Replace worn components if required
- Action: Replace bearings, races, pinion, ring, carrier, shims, or axle shafts as determined. Use new carrier bearings when replacing ring & pinion; always replace crush sleeve or use recommended shim method if pinion removed.
- Theory/fix: Worn bearings and gears cannot be corrected by lubrication alone. Replacing restores correct geometry and preload capability. Using new components eliminates runout and wear surfaces that produce noise and metal debris.

11) Reassemble with correct gasket/sealant and torque
- Action: Clean the mating surfaces, install new gasket or gasket maker per spec, torque cover bolts to manufacturer spec, install/torque fill plug.
- Theory/fix: Proper sealing retains lubricant and prevents ingress. Correct bolt torque prevents distortion of the cover that can cause leaks or uneven loading.

12) Refill with correct gear oil and additives
- Action: Fill to bottom of fill hole with manufacturer-specified gear oil (commonly GL-5 75W-90 synthetic on many TJ diffs; use LSD additive if axle has limited-slip). Fill until it begins to flow from fill hole.
- Theory/fix: Proper viscosity and additive chemistry provide oil film strength, EP protection, and friction characteristics. Incorrect oil causes poor film, overheating, and wear; the correct oil restores lubrication and extends life.

13) Road test and recheck
- Action: Drive and load the vehicle, listen for noise, feel for vibration, then recheck for leaks and re-torque fasteners after a short break-in.
- Theory/fix: Operational verification ensures the repair addressed the original symptoms and that seals hold and preload/mesh are stable once components seat under load.

Common fault-to-repair mappings (how the repair fixes the fault)
- Symptom: Gear whine at certain speeds → Causes: incorrect pinion depth or worn bearings. Repair: set correct pinion depth/backlash and replace worn bearings. How it fixes: correct mesh moves contact pattern to the proper area and removes excessive play so teeth engage smoothly instead of howling.
- Symptom: Grinding/clunking under load → Causes: broken/ badly worn teeth, insufficient backlash, or collapsed bearings. Repair: replace damaged gears/bearings and set backlash/preload. How it fixes: eliminated damaged contact surfaces and restored proper gear spacing so load transfers evenly.
- Symptom: Leaks → Causes: degraded or torn seals or leaking cover gasket. Repair: replace seals and gasket, clean surfaces, use proper RTV/gasket, torque bolts. How it fixes: restores pressure boundary to keep lubricant in and contaminants out.
- Symptom: Frequent oil change needed or contamination → Causes: internal metal wear or water intrusion. Repair: flush, replace oil, inspect and fix component wear or seals. How it fixes: removes contaminants and fixes source of contamination so new oil can protect.
- Symptom: Overheating/ burning smell → Causes: insufficient oil level/ wrong oil or binding bearings. Repair: correct fluid level and type, replace bearings if binding. How it fixes: restores lubrication and reduces frictional heating.

Useful practical notes (concise)
- Use a drain-first method only if you don’t need a full inspection. Removing cover is required for checking gear pattern/backlash.
- Always replace crush sleeves or use correct shims when pinion is disturbed; pinion preload is critical.
- Use proper gear marking compound to check pattern; small adjustments alter pattern significantly.
- If you see significant metal chunks, plan for a rebuild or component replacement, not just oil change.
- Use the Jeep/TJ service manual for torque specs and exact fluid type and capacity.

That’s the ordered procedure with the why and how each action cures the fault.
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