Safety first: work on a flat, supported vehicle; disconnect battery; use correct lifting/support gear; follow the factory service manual for torque, tolerances and safety procedures.

Overview — theory you must understand before repair (short):
- A manual gearbox is essentially an input shaft (driven by the clutch), a counter/lay shaft, and an output shaft with gears. Gears on the shafts mesh and either are fixed or rotate on splines; gear selection is done by sliding a hub/sleeve that locks a gear to the shaft or by engaging dogs.
- Synchronizers (blocking ring + friction cone + hub/sleeve) match speeds before teeth engage; shift forks and selector rails move the hubs; bearings support shafts and set gear mesh geometry (axial endplay and radial position).
- Failures come from wear (synchro friction surfaces, dog teeth, splines), bearing failure (noise, shaft misalignment), improper preload or backlash (chatter, noisy gear mesh), seals/leaks (loss of lubrication → accelerated wear), and damaged shift mechanisms (hard/shaky shifting).

Ordered repair procedure with underlying theory and how each action fixes faults:

1) Preparation and diagnosis (in-vehicle)
- What to do: Confirm symptoms (grinding in 2–3, whining in neutral, difficulty selecting certain gears, leaking oil, play in shifter). Check fluid level/type and leaks, clutch condition (slip, drag), linkage/cable play.
- Theory and fix: Symptoms narrow root cause. Low/contaminated oil or wrong oil affects synchronizer friction and bearing lubrication. Clutch drag/misadjustment mimics transmission issues; fixing clutch or fluid can resolve shifting problems without tearing down the gearbox.

2) Remove transmission from vehicle
- What to do: Disconnect battery, drain gearbox oil, remove driveshafts/propshaft, shift linkage, electrical connectors/speed sensors, starter and exhaust as required, support gearbox with jack, unbolt bellhousing from engine, lower gearbox.
- Theory and fix: Internal inspection/repair requires full removal so shafts and components can be disassembled; also prevents engine/transmission misalignment during reassembly.

3) Strip case and document layout
- What to do: Separate gearbox halves, remove shift rails, forks, hubs/sleeves, countershaft, input/output shafts and bearings. Mark parts and orientations; photograph assemblies if needed.
- Theory and fix: Careful disassembly preserves original shim/ordering data required to restore correct geometry. Many faults occur from incorrect reassembly (wrong shims, reversed forks) so documenting orientation fixes potential reassembly errors.

4) Inspect and measure every critical component
- What to inspect/measure:
- Bearings: spin, feel roughness, radial play, noise.
- Races: pitting/scoring.
- Gears: tooth profile, pitting, wear, chipped teeth, scoring.
- Synchro rings: wear at the friction cones (grooves, rounded edges), broken keys.
- Hubs/sleeves/dog teeth: mushroomed dogs, burrs, excessive wear on engagement faces.
- Splines: wear on clutch input splines and gear splines.
- Shift forks: worn pads or bent forks (causes misalignment/popping out).
- Endfloat and backlash: measure axial play of shafts and backlash between mating gears with dial indicators.
- Theory and fix: Measurements identify whether wear is within service limits. Replacing only the worn parts that exceed tolerances prevents recurring failures. For example, excessive backlash means gear/shaft bearing locations are wrong or bearing races are worn — replacing bearings and resetting shims restores correct gear mesh and removes noise and accelerated wear.

5) Replace worn/damaged components
- What to replace: bearings and races showing play or roughness; worn synchro rings and springs; damaged hubs/sleeves and dog gears; badly worn gears or shafts; seals and gaskets; shift forks if worn/bent; pilot bearing/bushing. Use OEM or high-quality replacements.
- Theory and fix:
- Bearings: new bearings restore concentric support and reduce radial runout; this restores correct gear mesh alignment and removes whining and accelerated tooth wear.
- Synchro rings: fresh friction surfaces and correct taper restore the synchronizer’s ability to slow a gear to hub speed and allow smooth engagement—fixes grinding/harsh shifts.
- Hubs/sleeves/dog teeth: replace to restore positive engagement and prevent popping out of gear.
- Seals: prevent oil loss; restoring lubrication reduces overheating and wear.
- Pilot bearing/bushing: corrects input shaft alignment in the crank and reduces vibration and bearing loads.

6) Press-fit and machine operations (if required)
- What to do: Install new races and bearings with correct press tools and heating if needed; replace or recondition shafts/gears only if within allowable repair specs; if gears are damaged beyond limits, replace entire gear set or shaft. If shimmed bearings are used, prepare correct shim stack.
- Theory and fix: Proper interference fits and bearing preloads ensure bearings don’t move under load, keeping gear position and backlash within spec. Improper fitted bearings cause movement, changing backlash and causing noise/gear damage.

7) Set gear mesh geometry: shims, endplay and backlash adjustments
- What to do: With main and countershaft assembled, use dial indicator to set backlash to factory spec by adding/removing shims or adjusting spacer thickness; set main shaft axial endplay per spec. Check pinion depth where applicable.
- Theory and fix: Backlash and pinion depth control tooth contact pattern and load distribution. Correct backlash prevents gear tooth impact and noise, and reduces wear. Correct endplay prevents the shaft from moving under load and losing gear engagement.

8) Reassemble synchronizers and shift mechanism correctly
- What to do: Install synchro cones/blocker rings in correct orientation, hubs and sleeves to correct alignment, shift forks on their rails with correct clearance; ensure selector detents function and spring/balls in place. Replace worn detent springs/balls.
- Theory and fix: Synchronizer assembly orientation matters: blocker rings must seat and pivot; if installed wrong, synchronization fails and you get grinding. Properly adjusted forks prevent overshift and ensure full engagement of hubs—fixes popping out and hard shifting.

9) Lubrication and seals
- What to do: Replace all gaskets/seals and fill with the correct grade and amount of gear oil specified by the manufacturer (API GL-4/GL-5 as specified). Fit new output shaft seals, speedo seals, and case gaskets.
- Theory and fix: Correct lubrication prevents metal-to-metal contact at friction surfaces and bearings. Some synchronizers depend on oil viscosity for correct operation — using the wrong oil can cause slow or harsh shifts. New seals prevent leakage and pressure/effect losses.

10) Refit clutch components and transmission
- What to do: Replace clutch pilot bearing/bushing, inspect/replace clutch disc, pressure plate, and throw-out bearing if worn. Refit gearbox to engine, torque bellhousing and mount bolts to spec, reconnect linkages, sensors, and driveshafts. Fill gearbox and if hydraulic clutch used, bleed clutch.
- Theory and fix: A properly functioning clutch and pilot bearing centers the input shaft and prevents lateral movement that would alter gear engagement. Correct torque and alignment prevent misalignment-induced wear.

11) Functional testing and break-in
- What to do: Check for leaks at idle, test shifting through all gears in a controlled environment, test drive under various loads. Re-check gear oil level after initial warm-up and cooling cycles. Listen/feel for remaining noises, check for gear pop-out or slipping.
- Theory and fix: Operational testing verifies synchronizers match and bearings run quietly under real load. Any remaining problem typically points to missed measurement or incorrect assembly that must be rechecked.

12) If problems persist: targeted re-check steps
- What to do: Re-measure backlash, endplay, bearing preload; re-inspect synchro engagement surfaces; check alignment of clutch/pilot bearing and bellhousing; ensure correct oil viscosity.
- Theory and fix: Persistent issues most often come from incorrect gear mesh (backlash/pinion depth), wrong bearing selection/installation, or missed worn components (dog faces/hubs). Re-checking these removes the root cause.

Practical measurement & tools checklist (minimum):
- Factory service manual (specs, torque, shims) — indispensable.
- Dial indicator, magnetic base (backlash/endplay measurement).
- Micrometers/calipers.
- Bearing puller/press and bearing drivers.
- Torque wrench, impact/ratchets, sockets.
- Snap ring pliers, drift punches, seal drivers.
- Clean solvent, assembly lube, new gaskets/seals, correct gear oil.

Common symptom → likely internal cause → repair that fixes it (quick reference):
- Grinding during shift → worn synchro cones or blocker rings → replace synchros and check hubs/sleeves.
- Gear noise (whine in neutral/high speed) → worn bearings or incorrect backlash/pinion depth → replace bearings; reset backlash/shims.
- Popping out of gear under load → worn dog teeth, worn hub engagement, weak detents → replace hubs/dog gears, check fork alignment, replace detents.
- Hard to engage 1st/2nd (especially when cold) → worn synchromesh, wrong oil viscosity → replace synchros; use correct oil.
- Oil leak → worn seals/gaskets → replace seals and fill to proper level (prevents lubrication-related wear).

End note (brief): Follow factory tolerances for backlash, bearing preload and endplay exactly — those are what make the theoretical fixes work in practice. Use OE-quality replacement synchros and bearings; cheap parts often fail quickly and reintroduce the same faults.
rteeqp73

Sort of hidden feature in many Nissan vehicles. Useful for diagnosis. #thecarfixer #automotive

2012 NISSAN Altima - Warning and Indicator Lights Warning and Indicator Lights The warning and indicator lights on your vehicle will help you stay aware of many systems and their ...

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