Tools & PPE
- Safety: safety glasses, nitrile gloves, long sleeves, drip tray, absorbent pads, fire extinguisher nearby.
- Tools: floor jack, 2 jack stands or ramps, wheel chocks, creeper, torque wrench (in-lbs/ft-lbs), 10–19 mm sockets and ratchet, extension, screwdriver set, drain pan (6–12 qt), large funnel, clean rags, transmission fluid pump (hand pump) or squeeze bottle, flare-nut/wrench set (line wrenches), hose clamp pliers or pliers for spring clamps, pick set for O‑rings, hose cutter, bench vise (optional), garden hose (for flushing external cooler), hand-held pressure/vacuum pump with gauge (optional leak test), O-ring pick, new O-rings/line seals, clamp-type or crimp hose clamps, replacement transmission cooler (or radiator if integral cooler needs replacement), new transmission fluid (manufacturer spec), transmission filter & pan gasket (if you open pan).

Safety precautions (non-negotiable)
1. Park level, chock rear wheels, set parking brake. Work only on a cold engine/transmission to avoid burns.
2. Use jack stands — never rely on a jack alone.
3. Wear eye protection and gloves; have absorbents for spills and a small fire extinguisher.
4. Dispose of used ATF at an appropriate recycling facility.

Overview of procedure
- Diagnose leak location → drain/collect ATF → remove/replace/repair cooler or lines → flush cooler if reusing → install new components with new O‑rings/clamps → refill, bleed, check level & road test → recheck for leaks.

Step-by-step (typical external cooler or line repair on 4Runner)
1) Confirm leak and locate
- Clean area: wipe radiator, cooler lines and frame with rags. Start engine briefly to pressurize A/T lines, then shut off and inspect for active leak (use cardboard under suspected area if necessary).
- Common leak points: cooler hose ends, quick-disconnect fittings at radiator, line flare nuts, cooler core (external cooler), or radiator cooler tank (integral).

2) Prepare vehicle and drain some fluid
- Raise vehicle safely on jack stands. Place drain pan under transmission cooler lines/radiator.
- If line removal will spill, loosen fittings slowly with a flare-nut/line wrench to control flow. Alternatively, remove transmission pan (if doing filter service) to minimize spillage and catch fluid.
- Tip: Use a fluid pump or squeeze bottle to remove excess fluid from the pan before separating lines to limit mess.

3) Remove old/failed component
- For leaking rubber hose: remove spring clamps with hose clamp pliers, twist hose free; cut if seized, then remove.
- For flare-nut or hard-line fittings: use the correct size line wrench on the hex portion to avoid rounding. Hold the opposite fitting with another wrench if needed to prevent twisting lines.
- For quick-disconnect fittings at radiator: depress the clip (use specific disconnect tool if present) and pull apart. Keep a rag under to catch fluid.
- For radiator/trans cooler core replacement: drain coolant and ATF as needed, remove fan shroud, radiator hoses and mounting bolts to replace radiator or core per service manual. This step is labor-intensive; consider replacement radiator assembly if core is integral.

How to use specific tools
- Flare-nut (line) wrench: slide fully over fitting hex, pull steady torque to break fitting loose; never use a thin open-end wrench on transmission lines if a line wrench fits.
- Hose clamp pliers: compress spring-type clamps and slide back along hose to free. For screw clamps, use screwdriver or nut driver.
- Disconnect tool: slip tool into the collar inside the quick-disconnect and push to release the retaining ring, then pull the line out.
- Torque wrench: tighten fittings/bolts to specified torque. If you don’t have exact specs, snug line fittings until seated then add 1/8–1/4 turn; pan bolts typically 6–10 ft·lb on many Toyota transmissions—but check manual.

4) Replace seals and lines
- Always replace O‑rings/seals on quick-disconnects when separating lines; lubricate O‑rings with fresh ATF before installing to prevent pinching.
- Replace any corroded hard lines or hose that is soft, cracked, or swollen.
- If installing a replacement external cooler: bench-fit fittings using new O‑rings. For threaded fittings use appropriate thread seal only if specified (most transmission cooler fittings use metal-to-metal or o-rings, not pipe thread sealants).
- Use new clamps on rubber hoses (crimp or heavy-duty screw clamps), not old weak clamps.

5) Flush cooler (if reusing)
- Remove cooler assembly or disconnect at radiator and push garden-hose water through cooler until clear (only for external coolers, not radiator cores). Blow out and make sure internal passages are clean. Transmission fluid is not compatible with water—ensure cooler is fully drained and dried before reinstall (use compressed air).
- Alternatively flush with clean ATF using a hand pump: pump fresh ATF through cooler until coming out clear.

6) Reinstall and torque
- Reassemble lines, replace mounting brackets and fasteners. Tighten flare nuts and fittings evenly, avoid cross-threading.
- Torque values: consult service manual. If unknown, tighten fluid lines until seated and then small additional turn—avoid over-torquing (can crack fittings).
- Reinstall any removed parts: fan shroud, splash guards, etc.

7) Refill transmission fluid
- Use manufacturer-specified ATF (consult owner’s manual/service manual; many Toyota 4Runners use Toyota WS for later models—verify your year). Typical fluid quantity varies; you will likely add 2–6 quarts depending on how much was drained. Have extra on hand.
- Start by adding the measured amount to bring level to cold fill mark if appropriate. Use a pump to add through the dipstick tube or fill port.

8) Bleed air and check level
- Warm vehicle to normal operating temperature (drive gently or let idle until transmission fluid temp reaches spec) — checking hot level procedure per manual:
- With engine idling and parking brake set, cycle through gears (P-R-N-D-3-2-1) pausing a few seconds in each to work fluid through the circuits, returning to Park.
- With engine idling on level ground, check dipstick hot range; add fluid slowly to bring to correct hot mark. Overfilling causes foaming and shifting problems; underfilling causes overheating.
- Look for leaks at all fittings while engine idling and during the gear cycle.

9) Road test and recheck
- Test drive under normal conditions (short drive). Recheck fluid level hot and inspect for leaks again. Re-torque any suspect fasteners after heat cycling.

Common pitfalls & how to avoid them
- Not replacing O‑rings/seals: always replace when disconnecting. Lubricate them with clean ATF.
- Using wrong ATF: causes harsh shifts and transmission damage—use manufacturer-specified fluid.
- Over-tightening fittings: may crack fittings or shear threads. Use correct wrenches and torque specs.
- Cross-threading or rounding fittings: use proper line wrenches; hold the fitting steady with a second wrench.
- Not flushing contaminated cooler: reusing a clogged/contaminated cooler will recontaminate new fluid; flush until clear.
- Not checking level hot: incorrect fluid level is the most common cause of post-repair problems.
- Not catching or disposing fluid properly: ATF on hot components is fire risk and environmental hazard; clean spills immediately.
- Reusing old clamps/hose: cheap clamp failure causes repeat leaks—use new quality clamps.

Replacement parts typically required
- O‑rings/seals for quick-disconnects
- Rubber cooler hoses or entire hard-line assemblies (if corroded)
- External aftermarket cooler or original-equipment cooler (if core failed)
- Radiator replacement if internal cooler core is leaking
- Transmission fluid (manufacturer spec), possibly filter and pan gasket if you drop the pan
- Hose clamps (new)

Final checks
- Ensure no leaks at idle and after short drive.
- Proper fluid level at operating temperature.
- Correct shift feel and no overheating.

If you need model-year specific torque values, fluid spec, or coolant/ATF capacities, consult the Toyota factory service manual for your 4Runner year.
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1) What the output shaft seal is and how it fails
- Purpose: a radial lip seal around the transfer case/transmission output shaft keeps gear oil/trans fluid inside while allowing the driven shaft to rotate. The lip rides on the shaft and uses line contact plus a thin oil film to seal.
- Common failures: lip wear or hardening, seal material shrinkage, torn lip from debris, improper installation, shaft scoring/corrosion, or excessive shaft runout/axial movement. Any loss of tight lip-to-shaft contact or a path past the lip lets gear oil leak.

2) Diagnosis (why you must confirm first)
- Clean area, run vehicle, observe leak source (front/back of transfer case vs transmission vs differential). Use cardboard/paper towels to pinpoint drip.
- Theory: many leaks appear similar; replacing the wrong seal wastes work and doesn’t stop the leak. Confirming fluid type (transfer case fluid vs ATF vs differential oil) tells you which component is failing.

3) Safety and prep (why each step matters)
- Park on level ground, chock wheels, lift and support with jack stands. Wear eye protection and gloves.
- Gather tools: ratchets, sockets, seal puller, drift/seal driver, soft hammer, pry bar, torque wrench, catch pan, clean rags, solvent, new seal, shaft lubricant (fluid), replacement bolts/gasket if required.
- Theory: safe support prevents collapse; clean work area avoids dirt entering the case; the right tools prevent damage to the bore and shaft which would re-create a leak.

4) Remove torque load and the driveshaft (ordered disconnect)
- Mark driveshaft/flange orientation relative to the output flange so balance is retained.
- Remove retaining bolts/nuts and separate the driveshaft from the output yoke. Support the driveshaft to avoid hanging from universal joints.
- Theory: leaving the driveshaft attached keeps torque on the flange and can make removal dangerous/difficult. Marking maintains dynamic balance and reduces vibration after reassembly.

5) Remove output flange/yoke to expose the seal
- Remove any retaining nut or circlip and pull flange/yoke (use a puller if needed). Be careful not to score output shaft splines.
- Theory: the seal is behind the flange; removing it gives direct access and prevents damage to the new seal when reinstalling the flange.

6) Remove the old seal correctly
- Pry or use a seal puller to extract old seal evenly. Avoid widening or gouging the bore.
- Theory: damaging the bore will prevent the new seal seating properly and recreate the leak. Inspect the removed seal for material condition — cuts, hardening, flattening tell you failure mode.

7) Inspect shaft and bore (critical theory step)
- Clean the shaft and bore with solvent. Visually and by feel inspect for grooves, nicks, corrosion, or high spots. Rotate to check for axial play/runout and check bearing play if accessible.
- If the shaft is lightly scored, polish with fine emery or a crocus cloth along shaft circumference (not around), or install a sleeve if recommended. Deep grooves require shaft replacement or professional repair.
- Theory: the seal lip needs a smooth, concentric surface. Surface defects or excessive runout prevent uniform lip contact, concentrate pressure on the lip edge, and cause leaks and rapid seal wear.

8) Prepare and install the new seal
- Confirm correct seal orientation (open side with the lip faces the fluid side). Lightly lubricate the seal lip with the correct fluid.
- Use a seal driver or appropriately sized socket to press the seal squarely into the bore to the specified depth. Tap evenly around circumference; do not cock the seal.
- Theory: correct orientation and seating ensure the lip rides on the shaft with uniform pressure and that the seal’s inner spring (if present) maintains radial load. Lubrication prevents dry start and immediate wear.

9) Reassemble flange/yoke and driveshaft
- Reinstall output flange/yoke, new O-ring if present, and any retaining hardware. Torque fasteners to factory specs and use threadlocker or new bolts/nuts per manual.
- Reinstall driveshaft aligned to marks and torque U-joint or flange bolts to spec.
- Theory: correct torque and alignment preserve concentricity and prevent vibration that would stress the seal. New fasteners/gaskets prevent seepage along bolt holes.

10) Refill fluids and set levels
- Refill transfer case/transmission to the correct fluid type and level per the service manual. Replace any fill plug gaskets.
- Theory: correct fluid maintains lubrication and hydraulic pressure. Underfill can let air enter and overfill can churn foam that increases internal pressure and leak risk.

11) Test and verify
- Run the vehicle at idle and visually inspect for leaks with the driveshaft turning. Road test under normal operating conditions, then recheck for leaks and re-torque bolts if necessary.
- Theory: dynamic testing confirms the seal holds under rotation, temperature changes, and normal differential/transfer case pressure cycles.

12) Why the repair fixes the fault (concise theory)
- Replacing the seal restores a proper elastomeric lip that establishes controlled, uniform contact with a smooth, concentric shaft surface. This restores the intended fluid boundary while allowing rotation. Proper inspection and repair of the shaft/bore and correct installation eliminate the underlying causes (damaged lip surface, rough shaft, misalignment, or improper seating) so the leak no longer has a path.

Notes and cautions
- Always use the exact replacement seal and fluid specified for your model year. Torque specs and fluid types vary — consult the factory service manual.
- If the output shaft has significant scoring, or if bearings show excessive play, replacing only the seal is a temporary fix; the underlying mechanical damage must be addressed to prevent recurrent leaks.

This is the ordered procedure plus the engineering reasons each action prevents or corrects the leak.
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