Goal: get the Hino W04-series marine engine’s “Check Engine” / warning lamp diagnosed and fixed, starting from a beginner level. You’ll learn what each electrical/engine component is, why the lamp lights (theory), how the system works, what commonly fails, and a practical, safe troubleshooting sequence you can follow. No fluff.

Basic analogy to keep in mind
- ECU = brain; sensors = nerves giving the brain status; actuators = muscles the brain moves. The check-engine light is like a fever light: it tells you the brain detected something wrong with the body. Fixing it means finding and treating the cause, not just turning the fever light off.

Safety first
- Work only on a cool engine when possible. Use gloves and eye protection.
- Prevent accidental starts: key off, battery negative disconnected for electrical repairs. When running for tests, ensure propeller removed or engine secured; be aware of moving parts, exhaust, and hot surfaces.
- Marine environments corrode connectors—inspect for saltwater damage before probing.

Tools you’ll need (minimum)
- Multimeter (DC volts, resistance, continuity)
- Basic hand tools: screwdrivers, sockets, pliers
- Insulated jumper wires / back-probe probes
- Fuel pressure gauge (diesel-compatible) and/or mechanical oil-pressure gauge
- Infrared thermometer or temp probe (optional)
- Service/diagnostic tool that can read Hino codes (recommended) or the manufacturer’s service manual
- Clean rags, dielectric grease, contact cleaner
- Replacement fuses, crimp connectors

Key components and what each does (detailed)
- ECU / ECM (Engine Control Unit)
- Role: central controller. Reads sensors, runs protection logic, stores fault codes, controls actuators (injector timing/solenoid, idle speed, shutdown solenoid, turbo actuator if fitted).
- Inputs: crank/cam position, coolant temp, oil pressure (or oil pressure switch), fuel pressure/rail pressure (if common-rail), intake/boost pressure, air-temp, exhaust temp, throttle/gov position, water-in-fuel sensor, alternator charge signal, battery voltage.
- Outputs: warning light (lamp driving circuit), data to gauge panel, fuel shutoff solenoid, alarm relays, engine protection shutdowns.

- Warning lamp / panel cluster
- Role: visual indicator on helm/dashboard. The ECU drives it (ground or +12V) when a stored fault or active protection triggers.
- Can be a simple “lamp + resistor” circuit or a lamp driven by a relay. Also may be paired with audible alarms.

- Diagnostic connector / service port
- Role: access point to read stored fault codes and live data with a diagnostic tool. Could be proprietary to Hino or follow common protocols.

- Sensors (typical; which exist depends on the specific W04 variant)
- Coolant Temperature Sensor: tells ECU coolant temp; used for warm-up enrichment, high-temp alarm.
- Oil Pressure Switch / Sender: either gives a pressure signal to gauge or a switch that trips when pressure low. A low-oil-pressure switch typically triggers stop or alarm.
- Crank Position Sensor (CKP): tells engine position/rotation for timing and injection. If it fails, engine may not start or runs poorly and sets codes.
- Boost/Intake Pressure Sensor (MAP or turbo boost): on turbo models. Monitors turbo performance and can trigger fault if boost is abnormal.
- Fuel Pressure Sensor / Fuel Rail Pressure (if common-rail): critical for fuel control.
- Water-in-Fuel (WIF) sensor / fuel filter sensor: detects free water in fuel and triggers lamp if present.
- Intake Air Temperature (IAT), Exhaust Gas Temperature (EGT): protect engine from over-temp.
- Alternator/Voltage sense: alerts to charging issues.

- Actuators and protective devices
- Fuel shutoff solenoid/stop solenoid: closes fuel flow to stop the engine.
- Engine speed governor / actuator: maintains rpm; may be electric or mechanical.
- Turbo actuator (if fitted): controls boost; failure affects power and sets codes.
- Relays and fuses: supply power to ECU and sensors; blown fuse can cause lamp or loss of ECU function.

- Wiring harness and connectors
- Role: the nervous system connecting sensors/actuators to ECU. In marine use, these get corroded or shorted by salt and vibration.

Why the check-engine light is needed (theory)
- Safety and protection: The light warns of faults before they become catastrophic—low oil pressure, high coolant temp, fuel contamination, sensor failure, or loss of critical control signals.
- Diagnostic aid: It stores fault codes that point you to failing components or circuits.
- Preventive: By alerting early, it prevents costly failures (e.g., turbo, engine seizure, or fuel system damage).

How the system detects faults (how it works)
- Sensors send voltages or resistances to the ECU. The ECU expects values within known ranges for given conditions (RPM, temperature, load).
- If a sensor value is out of range, inconsistent with other sensors, or a circuit open/short occurs, the ECU logs a fault code and turns on the lamp.
- Some faults trigger an immediate protective action (reduced power, alarm, or engine shutdown); others just set the lamp for maintenance.
- Fault codes are stored in ECU memory and can usually be retrieved by a service tool or sometimes by a blink-code method.

Common failure modes (what can go wrong)
- Electrical and wiring faults
- Corroded connectors, broken wires, chafed harness, poor grounds — common in marine environments.
- Blown fuses or bad relays preventing sensor/ECU power.
- Sensor failures
- Coolant temp sensor stuck or shorted (wrong temperature reading -> wrong fueling/overheat alarm).
- Oil pressure switch failing (false low-oil alarms) or actual low oil pressure due to pump wear or low oil.
- Crank position sensor failure causing misfires or no-start.
- Water-in-fuel sensor triggered by real water contamination or sensor corrosion/short.
- Fuel system issues
- Clogged filters, contaminated fuel, air in lift lines, weak transfer pump, worn injection pump (loss of power; fault codes).
- Turbo/intercooler problems (turbo models)
- Boost leaks, wastegate actuator failure, turbo bearing wear — results in low power and certain boost/pressure codes.
- Overheating
- Blocked raw-water sea strainer, failed raw-water pump, thermostat stuck — causes high coolant-temp warnings.
- ECU or instrument cluster failures
- Rare but possible: corrupted ECU software, failed warning-lamp driver, or bad gauge cluster.

Step-by-step diagnostic procedure (beginner-friendly, logical order)
1) Observe and record symptoms
- Is lamp steady, flashing, or accompanied by alarm, loss of power, or shutdown?
- Note any visible smoke color, unusual noises, or performance loss.

2) Simple visual and electrical checks (quick wins)
- Battery voltage: with key on, measure 12–14V. Low battery/charging affects ECU and sensors.
- Fuses/relays: check engine/ECU related fuses; replace any blown ones.
- Lamp bulb: if lamp never lights on key-on, check bulb/indicator supply—lamp should illuminate briefly on key-on as a bulb test. If lamp never comes on at key-on, that could be a bulb/fuse/ECU output issue.
- Wiring/connectors: visually inspect ECU connectors, sensor connectors, and harness for corrosion, salt, or broken wires. Clean and re-seat.

3) Retrieve fault codes
- Best option: plug into ECU with the proper Hino diagnostic tool and read codes and live data.
- If you don’t have the tool: consult service manual for blink-code or self-diagnosis entry (some models let the ECU flash the lamp when a diagnostic connector is bridged). Don’t improvise by bypassing safety devices.
- Write down the codes and freeze-frame data. Codes point you to the suspect circuit.

4) Interpret codes and run targeted tests
- Electrical circuit tests:
- For a sensor that shows “open circuit” code: disconnect, measure sensor resistance and check for voltage supply and ground at the connector.
- For a short-to-ground or short-to-V: check continuity to chassis and to battery.
- Use back-probing to measure sensor voltages at operating conditions when safe.
- Sensor-specific checks:
- Coolant temp: cold resistance should be high; as it warms it drops (thermistor behavior). Alternatively check voltage at sensor as engine warms—value should rise smoothly.
- Oil pressure: if lamp indicates low pressure, connect a mechanical oil gauge to the engine’s oil-pressure port and compare to spec. If mechanical reading is good but switch indicates low, replace switch.
- CKP (crank sensor): with cranking, you should see an AC or pulsed DC signal at the sensor lead; no pulse = sensor or reluctor ring issue.
- Fuel pressure: attach fuel gauge; compare to spec. Low pressure -> check filters, lift pump, supply lines.
- WIF sensor: visually inspect drain; if water present, drain fuel, replace filter, clear water.
- Actuator tests:
- If ECU commands a shutdown solenoid, confirm it actuates with power applied (bench test). If it doesn’t actuate, check wiring and fuse.

5) Fix or replace the failed component
- If wiring/corrosion is the problem, clean connectors, apply dielectric grease, repair wiring and replace corroded parts.
- Replace faulty sensors with OEM or correct-spec parts.
- Clean or replace fuel filters, drain water separators.
- If ECU is indicated as faulty and diagnostics show communication errors across multiple unrelated sensors with good wiring, consider ECU replacement or professional repair.

6) Clear codes and retest
- After repair, clear codes with the diagnostic tool or by following the service manual procedure. Run engine, verify lamp extinguishes, monitor live data for stable readings.
- If lamp returns, re-check stored codes and proceed to the next likely cause.

Practical examples: symptom => likely causes
- Lamp on + loss of power, derate mode: turbo boost sensor, boost leak, fuel pressure low, or ECU-triggered derate (check codes).
- Lamp on with high coolant temp reading: check raw-water pump, sea strainer, thermostat, coolant level, coolant sensor.
- Intermittent lamp and erratic RPM: loose crank sensor connector, poor ground, or failing CKP.
- Lamp on after saltwater exposure: corroded connectors or water ingress into sensors.

Things beginners sometimes mis-handle
- Replacing a sensor without checking the connector/wiring first: often the wiring is the real problem.
- Clearing codes repeatedly without fixing root cause: lamp will return.
- Assuming ECU is dead first: it’s usually wiring or a sensor.
- Using non-marine parts: saltwater-rated connectors and dielectric grease matter.

Where to find correct values and exact test procedures
- Use the Hino W04-series service manual or wiring diagrams for exact connector pinouts, signal voltages, and sensor resistance/voltage specs. The manuals give ECU diagnostic pinouts and specific fault-code meanings for these models.
- Factory diagnostic tool will give the cleanest path to codes and real-time sensor values.

Maintenance & prevention (simple)
- Keep battery and alternator in good health.
- Replace fuel and oil filters regularly and drain water separators.
- Inspect and seal connectors; use marine-grade heat-shrink and dielectric grease.
- Keep raw-water strainers clean, and change coolant on schedule.
- Schedule sensor replacements if repeatedly failing in a salty environment.

When to call a professional
- If the ECU itself is suspected faulty and you don’t have access to the correct reprogramming or matching tool.
- If many unrelated electrical faults exist — may indicate ECU or complex wiring faults needing bench testing.
- If you cannot access the fault codes or need help interpreting them.

Quick checklist to follow now (do this first)
1. Turn key to ON: does lamp illuminate briefly? If not, check bulb/fuse.
2. Note lamp behavior (steady/flashing/with alarm).
3. Inspect connectors at ECU, sensors, and battery. Clean and re-seat.
4. Check battery voltage and fuses.
5. Read fault codes with a diagnostic tool or follow service manual method.
6. Test/replace the failed component indicated by code (start with cheap/reliable items: connectors, fuses, sensors).
7. Clear codes and retest.

Final practical tip
- Always trace a fault from the lamp back to the ECU code, then to wiring and sensor. The lamp is the symptom; the code is your map; the wiring and sensor/actuator is the usual offender. Fix the wiring and sensors first before swapping the ECU.

This gives you the full map and a step-by-step route to follow as a beginner mechanic. If you need a short checklist or a printable diagnostic flow for the most common codes on your exact W04 variant, get the factory fault code list and I can distill the top 8-10 codes into a focused troubleshooting card for you.
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Short, direct workshop-style guide to marine transmission repair for Hino W04D / W04C‑T / W04C‑TI installations (engine → reduction/transmission gearbox). This is a generic, beginner‑friendly but detailed manual-style explanation: components, theory, failure causes, step‑by‑step repair flow, measurement/setting procedures, tools/consumables, safety. Where exact torque, shim thickness, or clearance numbers are needed, get the OEM gearbox/engine manual — do not guess.

1) What we mean by “transmission” in this context
- Inboard marine: the transmission (a marine gearbox / reduction gear) sits between the engine flywheel/flex coupling and the propeller shaft. It provides forward/neutral/reverse, reduction ratio, and reverses rotation. It contains input and output shafts, gears, bearings, seals, shift mechanism and sometimes a wet clutch or torque converter depending on design.
- The Hino W04 series is the engine; the gearbox model varies (make: ZF, BorgWarner, Twin Disc, or proprietary). This guide covers common marine gearbox internals and repair tasks you’ll encounter.

2) Key components — detailed descriptions and function
- Gearbox housing (case)
- Cast housing that holds gears, shafts, bearings and oil. Acts as oil reservoir and structural frame.
- Input flange / coupling / bell housing
- Connects gearbox to engine flywheel housing via flexible coupling or spline. Transmits engine torque into gearbox.
- Input shaft
- Receives torque from engine coupling. May carry pinion or cluster gear.
- Counter/cluster gear (if reduction gear)
- A set of gears that provide reduction between input and output. Think of bicycle chainrings: large and small sprockets change speed/torque.
- Output shaft / tailshaft
- Drives the propeller shaft (usually via a splined coupling or flange). May have thrust bearings to absorb prop thrust.
- Gears (helical/straight)
- Teeth that transfer torque. Helical gears are quiet and take axial load; straight cut are simpler but noisier.
- Shift forks and shift rail
- Mechanical arms that move sliding gears/dog clutches to engage forward, neutral, or reverse.
- Dog clutch / synchromesh (marine gearboxes often use dog clutches)
- Dog clutch slides to lock a gear to a shaft for engagement. No synchromesh in many marine gears — must shift at correct rpm.
- Bearings (roller, tapered, ball)
- Carry radial and axial loads; support shafts and maintain gear mesh geometry.
- Thrust bearings
- Carry axial forces from the propeller pushing/pulling on the shaft.
- Seals (shaft seals, O-rings, cover gaskets)
- Prevent gearbox oil from leaking and seawater from entering.
- Oil pump / cooling passages (if fitted)
- Maintain lubrication and oil flow/cooling for gearbox.
- Oil filler/dipstick/drain plugs
- For oil change and level check.
- Reverse idler gear (in reverse gearboxes)
- Engages to reverse output rotation (often a separate gear engaging to reverse direction).
- Clutch plates (if wet clutch)
- Friction plates that connect/disconnect power; require correct clearance and condition.
- Shift linkage / external control
- Remote levers/cables/pneumatics/servo that move internal shift rails.
- Mounts and alignment shims
- Position gearbox relative to engine; control alignment to prop shaft.

Analogies:
- Gears: like bicycle gears — big gear = more torque less speed; small gear = more speed less torque.
- Bearings: like wheel bearings on a skateboard — worn bearings make noise and wobble.
- Gear mesh/backlash: like the gap between two interlocked combs — too tight = binding and heat; too loose = clunking and wear.

3) Why repair is needed — theory of failure
- Wear and fatigue: Teeth wear (pitting, spalling) from long use, misalignment, poor lubrication, contaminated oil, or overload. Repeated load cycles cause micro-cracks that propagate.
- Bearings fail from contamination, loss of lubrication, misalignment, or axial/radial overload.
- Seals fail from aging, cuts, hardening, or shaft wear → oil loss or seawater ingress → accelerated internal corrosion and bearing/gear destruction.
- Misalignment and improper coupling cause excessive loads, vibration, and rapid wear.
- Gear backlash or endfloat out of spec → noisy operation or gear jump.
- Shift mechanism wear leads to incomplete engagement → dog tooth damage.
- Overheating from oil breakdown or inadequate cooling → loss of lubricant properties → rapid wear.
- Corrosion from seawater contamination (marine environment) causing pitting, rust and fast failures.
What can go wrong in plain terms:
- Noisy whining = worn bearings or tight gear mesh.
- Clunking on shift = too much backlash or damaged dog teeth.
- Oil leaks = seal/gasket failure.
- Loss of drive = broken tooth, failed coupling, or sheared shaft key.
- Overheating/oil smoking = low oil or wrong oil.

4) Tools, equipment, consumables
- Tools: engine hoist or gearbox jack, heavy-duty hand tools, torque wrench(s), impact wrench (careful), pullers, bearing/seal drivers, drift punches, snap ring pliers, straight edge, feeler gauges, micrometer, vernier calipers, dial indicator with magnetic base, plastigauge, depth micrometer, micrometer for shafts, gear tooth calipers, press for bearing removal/installation, soft mallet, heat gun, bench vise, cleaning brushes.
- Measuring: micrometers (0–25 mm, 25–50 mm), dial indicator (0.01 mm resolution), plastigauge for bearing clearances, bore gauges if available.
- Consumables: OEM gearbox oil, new gaskets and seals, replacement bearings/gears as required, anti-seize, Loctite/threadlocker per spec, replacement studs/bolts if corroded, shaft key, assembly grease.
- Safety: lifting straps, gloves, safety glasses, PPE, drip pans for oil, absorbents.

5) Safety and preparation (non-negotiable)
- Work on a stable dock/shop bench; boat properly supported on stands; disconnect batteries; isolate fuel; ensure ventilation.
- Drain oil before removal; catch and dispose properly.
- Use a hoist for heavy gearbox removal; do not lift gearbox by shift levers.
- Mark and tag every cable, hose and bolt for reassembly.
- Wear PPE: gloves, eye protection, steel toes if heavy parts.
- Have engine and gearbox manuals to reference specs.

6) Diagnostic workflow (before tear-down)
- Symptom list: noise (type, frequency, when present), leak location, loss of drive or slippage, shifting difficulty, overheating.
- Basic checks:
- Oil level and condition (contamination, emulsified = water).
- External leaks and corrosion.
- Check coupling: is the flexible coupling intact? Any sheared bolts or cracked rubber?
- Prop shaft and propeller: free to rotate? Fouling?
- Check alignment marks on engine/gearbox mounts.
- Run at low idle in neutral and listen; note when noise occurs (idle, forward under load, reverse).
- If suspect internal damage: prepare for removal and rebuild. If only seals and oil, you may sometimes repair without full strip.

7) Removal of gearbox (general steps)
1. Document everything: photos, mark alignment on coupling and flanges, tag wires and hoses.
2. Drain gearbox oil via drain plug; remove propeller and propshaft coupling (or flange).
3. Disconnect shift linkage, control cables, electrical connections (neutral switch).
4. Support gearbox with jack/hoist; remove mounting bolts between gearbox and engine.
5. Separate gearbox from engine; ensure coupling connection is free; be ready to lift heavy weight.
6. Remove gearbox from boat and place on workbench on cradle.

8) Disassembly (systematic, label everything)
- Clean external grime before opening to reduce contamination entry.
- Remove covers, top plate, and intermediate covers; keep bolts in order labeled.
- Remove shift rails and forks; mark their positions.
- Drain remaining oil and remove oil pump (if fitted).
- Remove output flange/tailshaft assembly so you can access bearings.
- Remove bearings using press or puller; document orientation of races, shims and spacers.
- Remove gears and shafts: note any keyed parts, woodruff keys, and orientation marks.
- Inspect internal parts for damage; clean parts using solvent (degreaser) and compressed air (blow away from you).

9) Inspection — what to look for, measurement
- Gears:
- Inspect teeth for pitting, cracking, wear patterns, broken teeth. Look for polished “wipe” marks and scoring.
- Check contact pattern — paint or prussian blue to identify gear mesh pattern; pattern should be centered on tooth face.
- Measure tooth thickness and pitch where applicable.
- Bearings:
- Spin and feel for roughness; inspect rollers/races for pitting or brinelling.
- Replace bearings showing any roughness or dimension changes.
- Shafts:
- Measure journal diameters with micrometer; check for scoring or out-of-round.
- Check for straightness: mount on V-blocks and use dial indicator to rotate.
- Seals:
- Replace all seals/gaskets if removed. Shaft grooves and seal seats must be smooth.
- Backlash and gear mesh:
- Measure backlash with dial indicator on gear tooth: move output gear back and forth and note play.
- End float:
- Check axial play of shafts; compare to manual.
- Housing:
- Inspect bores and bearing seats for ovality or wear.
- Oil contamination:
- Water/emulsion indicates seawater ingress — inspect seals, check for corrosion on components.

10) Rebuild principles (what must be correct)
- Cleanliness: everything must be spotless during reassembly. Dirt = instant wear.
- Bearings and shafts must be pressed/installed to specified seating depths and orientations.
- Gear mesh (contact pattern) and backlash must be set within OEM tolerances — achieved by shims or adjusting spacers.
- End float and preload for tapered-roller bearings must be set correctly (not too loose, not too tight).
- Shift fork alignment and dog engagement clearance must be correct to avoid tooth damage.
- Use correct oil grade and fill level.

11) Typical reassembly procedure (step-by-step, general)
1. Replace all bearings, seals, and gaskets rather than reuse. Press in new bearings with proper tools and heat if required (heat bearing outer race to expand slightly).
2. Install shafts and gears dry-fit to check mesh; rotate to ensure free rotation.
3. Set bearing preload / end float:
- For tapered roller bearings, tighten pre-load nut or install correct shim thickness. Use a torque-angle or torque method per manual, and check rotational torque.
4. Set gear backlash:
- Use dial indicator on gear tooth and adjust shims/spacers to achieve specified backlash. Typical procedure: lock one gear, move the other back and forth and measure runout/backlash.
- Use gear compound/pattern test (Prussian blue) to check contact patch and move shim to center pattern on face.
5. Reinstall shift forks/rails ensuring fit into dog clutch slots; check free movement and correct engagement.
6. Fit seals and cover plates with fresh gaskets and correct sealant where required. Torque bolts to spec.
7. Refit output flange and torque fasteners (replace studs/bolts if corroded).
8. Reinstall oil pump and ensure strainer is clean.
9. Fill with OEM-specified gearbox oil to correct level.
10. Reinstall coupling to engine mating surface using alignment marks; use new coupling bolts or studs if required. Torque to spec.
11. Reattach mounts but leave final alignment adjustments for in-situ alignment on the boat.

12) Setting alignment and final installation
- Align engine and gearbox couplings inline: use feeler gauges or dial indicators to check concentricity and angular misalignment; correct with shims under mounts.
- Check prop shaft alignment with gearbox output flange; ensure propeller shaft coupling aligns.
- Secure all control linkages and test neutral position with engine off — gear should be in neutral and engagement should be smooth manually.
- Torque all engine/gearbox mounting bolts to spec.

13) Break-in and testing
- With boat on stands or test tank: run engine at low rpm in neutral and check for leaks, odd noises, and oil pressure.
- Shift through gears at low rpm to ensure smooth engagement. If dog-clutch tooth or synch issues, stop and adjust.
- Gradually load test: bring to cruising rpm and check temperature and oil level after run; re-torque bolts if necessary.
- On-water sea trial: progressive loading, listen for noises; check for overheating, leaks, and vibration.

14) Diagnostics of common symptoms and likely internal cause (quick reference)
- High-pitched whining at all speeds: worn/damaged bearings in input/counter shafts.
- Low rumbling/growl: gear tooth wear or improper tooth contact (mesh).
- Clunk on engagement: excessive backlash, worn dog teeth, or loose shift forks.
- Slipping under load: worn clutch plates (if clutch type), damaged teeth not fully engaging, or hydraulic actuator issue.
- Water in oil / milky oil: failed seals, seawater ingress → immediate strip and clean/replace corroded parts.
- Overheating: low oil, contaminated oil, blocked cooler or pump failure.
- Vibration: misalignment, bent shaft, propeller issues, or unbalanced gears.

15) What you must not do (common beginner mistakes)
- Reuse seals, bearings, or gaskets thinking “they look OK.”
- Reassemble without cleaning and drying parts.
- Guess torque or clearance values — use manual.
- Ignore coupling/alignment — most premature failures start there.
- Run gearbox with wrong oil or incorrect level.
- Force parts together without noting orientation and using correct presses/drifts.

16) Measurement checks you should perform before final assembly
- Gear backlash via dial indicator
- Bearing preload or torque to rotate
- Shaft runout and straightness
- Gear contact pattern using marking compound
- End float (axial play) for shafts
- Clearance across dog clutch engagement faces

17) Parts likely needed on a rebuild
- Full bearing set
- Main oil seals and O-rings
- Gasket kit for covers
- Replacement gears or gear sets if pitted or cracked
- Shift fork/regulating parts if bent or worn
- Coupling bolts, studs, and keys
- Oil and filter, oil cooler lines if applicable

18) Disposal, environmental and documentation
- Dispose of used oil and contaminated parts per local regulations.
- Keep a log of parts replaced, serial numbers, and measured clearances for future reference.

19) Final tips
- If you find cracked teeth, excessive pitting or deep groove scoring — replace gears; do not try welding or patching unless an experienced marine gearbox machinist does it.
- If you’re not confident setting backlash and bearing preload, get help or send gearbox to qualified rebuilder. Wrong setting → rapid catastrophic failure.
- Keep the engine and gearbox OEM manuals close — they contain critical torque and clearance specs.

If you need a concise checklist for the actual teardown and reassembly steps (bolt-by-bolt procedure) I can produce one — but you must have the specific gearbox model and OEM manual on hand for torque and shim data.
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