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Transmission Subcategories
Only subcategories containing verified fitment products are shown.
Differential parts divide torque without forcing equal wheel speed
On a bend, the outside wheel travels farther than the inside wheel. The differential's gears permit that speed difference while the final drive reduces rotational speed and multiplies torque.
The same housing also locates the pinion and carrier accurately enough for a controlled tooth contact pattern under substantial load.
Main differential arrangements
| Arrangement | Torque behaviour | Additional parts | Service implication |
|---|---|---|---|
| Open differential | Allows free speed difference; usable axle torque is limited by the lower-grip side. | Side gears, pinions/spiders, cross shaft and thrusts. | Simple, but internal wear can create backlash and clunk. |
| Clutch limited-slip | Friction packs resist excessive speed difference. | Clutch plates, preload springs and special side gears. | Oil/friction specification affects chatter and locking action. |
| Helical limited-slip | Helical gears bias torque while both sides retain some traction. | Matched helical pinions and pockets. | Gear condition, end float and correct lubricant matter. |
| Viscous coupling | Silicone fluid transfers increasing torque with speed difference. | Sealed plate coupling. | Usually replaced as an assembly; heat can degrade response. |
| Selectable locker | Mechanically couples both outputs when commanded. | Dog clutch, fork and pneumatic/electric actuator. | Confirm disengagement and wiring/air controls. |
| Electronic active differential | Controlled clutches vary torque distribution. | Pump or motor, solenoids, sensors and controller. | Needs fluid, mechanical and electronic diagnosis. |
Crown wheel and drive pinion
The pinion turns the larger crown wheel or ring gear, establishing the final-drive ratio. Hypoid designs offset the pinion below or above the crown-wheel centreline, improving packaging and tooth overlap but increasing sliding friction.
Manufacturers lap these gears together and mark a set or mounting-depth correction. Replace them as the specified pair; a used gear married to an unrelated mate rarely produces a stable, quiet contact.
Carrier, side gears and pinions
The carrier supports both the reduction gear and differential mechanism
The crown wheel bolts to the carrier. Inside, side gears spline to the output shafts while two or more small pinions rotate on a cross shaft. Straight-line travel makes the assembly rotate together; cornering makes the pinions walk around the side gears.
Cross-shaft wear, damaged thrust washers and worn side-gear splines can add rotational lash even when crown-wheel backlash is correct.
Bearings and their adjustments
Taper roller bearings commonly support pinion and carrier loads in radial and axial directions. Pinion preload may be established by selective shims or a collapsible spacer. Carrier bearings can use threaded adjusters or shims to set both preload and crown-wheel position.
New and used bearing preload limits often differ. Seal drag and cold lubricant must be treated as the procedure states when measuring rotating torque.
Depth, backlash and tooth contact
| Adjustment | What it controls | Measurement | Risk if wrong |
|---|---|---|---|
| Pinion depth | Mesh position from face to root. | Depth gauge/master tools plus gear marking. | Drive/coast whine and concentrated tooth load. |
| Pinion-bearing preload | Bearing rolling resistance and rigidity. | Rotating torque after prescribed assembly steps. | Heat/seizure if high; movement/noise if low. |
| Crown-wheel backlash | Running clearance between teeth. | Dial indicator at several positions. | Binding when hot or impact loading. |
| Carrier-bearing preload | Carrier support under torque. | Shim/adjuster calculation and procedure. | Case movement and changing contact pattern. |
| Crown-wheel runout | Consistency of mesh around rotation. | Dial indicator on clean reference face. | Backlash variation and cyclic noise. |
| Contact pattern | Actual loaded area on drive/coast flanks. | Marking compound with controlled resistance. | Noise, heat and premature pitting. |
Ratios and matched drivetrains
Divide crown-wheel teeth by pinion teeth to calculate the nominal ratio. The identity stamped on the gear or axle data is preferable because tooth counts are not always visible without strip-down.
On permanent or selectable four-wheel drive, front and rear final-drive ratios must match the vehicle design. Tyre rolling circumference also matters. A small mismatch can load the centre coupling continuously.
Electronic locking and active components
A locker may use a motor, solenoid, pneumatic diaphragm or cable to move a selector. Active clutch units can have pressure pumps, temperature sensors and dedicated oil circuits. A command fault is not proof the internal gears have failed.
Read codes and live data, inspect supply and actuator movement, and follow calibration or oil-fill routines. Never energise an exposed selector where fingers can enter it.
Part identification
Use the complete VIN and axle, transmission or transaxle identification plate. Check model-year change points, ratio, differential family, flange, spline count, ABS/speed pickup and whether parts are supplied as a matched kit.
Compare bearing manufacturer numbers only after confirming tolerances and revisions. A dimensionally similar bearing can have a different internal clearance, radius or load specification.
Noise diagnosis by operating condition
| Observation | Differential possibility | Important alternative | Useful check |
|---|---|---|---|
| Whine on drive only | Drive-side contact, depth or worn gear teeth. | Transmission gear or propshaft resonance. | Load reversal at the same road speed. |
| Whine on overrun | Coast-side contact or pinion support. | Gearbox bearing. | Compare coast, neutral and light throttle safely. |
| Rumble rising with speed | Carrier/pinion bearing. | Wheel bearing or tyre. | Lift inspection and chassis listening tools. |
| Clunk on take-up | Excess gear or internal side-gear lash. | Joint, mount or spline wear. | Measure each driveline section separately. |
| Chatter in tight turns | Limited-slip clutch/oil issue. | Tyre mismatch or CV joint. | Fluid identity and controlled figure-eight test. |
| Leak at flange or shafts | Seal surface, bearing play or breather pressure. | Oil tracking from above. | Clean, dye if approved, and trace highest wet point. |
Oil inspection before dismantling
Record the fill level before draining. Inspect oil for metallic sparkle, flakes, burnt odour, water emulsion and friction material. A magnetic plug normally carries a light paste, but chips or tooth fragments require internal investigation.
Strain the drained oil if diagnosis needs debris recovery. Keep samples clean and labelled rather than contaminating them in an open drain pan.
Lubricants and approvals
Use the precise viscosity and performance approval stated for the axle, gearbox or active coupling. GL classifications are not automatically interchangeable: extreme-pressure additive chemistry can be unsuitable for some synchronisers or yellow-metal components in a shared transaxle.
Clutch-type limited-slip units may require an oil carrying the approval or a measured modifier. Too much modifier can reduce locking response. Do not add it to helical or open units without instruction.
Operating temperature, load and lubrication level
Towing, sustained speed, large tyres and low lubricant raise tooth and bearing temperature. Overfilling can aerate oil and push it through a breather; underfilling starves the pinion bearings or crown wheel on slopes.
Fill with the vehicle at the specified level attitude. Active units may need a scan-tool pump routine or separate chamber fill.
Safe removal and strip-down
Chock and support the vehicle on rated points, then support the axle or transmission before mounts are released. Mark shaft/flange relationships only where the procedure allows. Drain oil into a suitable container and protect open housings from dirt.
Measure backlash and rotating torque first. Mark bearing caps by side and orientation; they are machined with the housing. Keep shims, cups and adjusters labelled. Never swap cap positions.
Bearing and gear removal
Use bearing separators and pullers on the specified races. Press only through the ring being fitted so rolling elements do not transmit installation force. Heating bearings or gears is permissible only to the manufacturer's controlled temperature; flame creates local hardness changes.
A crown wheel may use left-hand threads or one-use bolts. Confirm thread direction and locking method before force is applied.
Inspection and measurement
Degrease safely and inspect gear flanks for pitting, scoring, spalling and cracked tooth roots. Measure journals, shaft straightness, cross-shaft fit, thrust clearance and housing bores. Check carrier and flange runout before blaming new gears.
Replace a gear set after tooth fracture or severe pitting. Polishing a damaged flank changes its form and cannot restore the hardened surface.
Assembly controls
Clean every mating face and oil drilling. Install races fully seated, use new specified fasteners and apply the exact lubricant or locking compound. Tighten the crown wheel progressively and verify runout.
Establish pinion depth and preload, then carrier preload and backlash. Record readings around the gear, paint both drive and coast patterns, and adjust systematically. One acceptable average does not excuse a tight sector.
Crush sleeves and pinion nuts
A collapsible spacer shortens gradually as the pinion nut is tightened. If preload is exceeded, loosening the nut does not restore the sleeve; replace it and repeat. Holding the flange safely is essential.
Do not use impact-tool time or nut position as a preload measurement. Follow rotating-torque and final staking/locking instructions.
Seals, breathers and mating flanges
A new seal will leak on a grooved, rusty or loose flange. Inspect the running surface and bearing play. Fit the seal squarely at the specified depth and lubricate its lip with the approved medium.
Clean the breather and route its hose correctly. A blocked breather raises pressure; a damaged low breather admits water during wading.
Commissioning and bedding-in
Rotate the assembly by hand before shafts are connected and confirm no bind. Fill, run without shock loading, recheck for leaks and monitor housing temperature. New gear sets may require gentle heat cycles and an early oil change, especially before towing.
Follow the gear or vehicle manufacturer's bedding instructions. Stop for new whine, binding, smoke or rapid heat rise.
Fault urgency and UK roadworthiness
Growing whine, heavy clunk, oil loss, seizure tendency or metal fragments can precede loss of drive or wheel lock. Do not continue driving with severe noise, visible gear oil on brakes/tyres, a loose flange or an axle that binds.
UK MOT inspection can identify insecure drivetrain parts, serious leaks and defects affecting safe operation, but a current certificate does not validate internal setup. Repair to manufacturer limits whenever evidence demands it.
Common mistakes
- Ordering from the housing shape without confirming axle code and ratio.
- Mixing an unmatched crown wheel and pinion.
- Discarding pre-strip backlash, preload and contact evidence.
- Reusing a collapsed spacer after preload has been exceeded.
- Swapping bearing caps, races or selective shims from side to side.
- Setting backlash at one point without checking runout around the gear.
- Using generic gear oil or friction modifier without the required approval.
- Blaming the differential before excluding tyres, hubs, joints and mounts.
Practical differential-parts FAQs
Q: Can a crown wheel be replaced without its pinion?
A: Normally no; they are supplied and lapped as a matched set.
Q: How is final-drive ratio calculated?
A: Divide crown-wheel tooth count by pinion tooth count, then verify the axle identity.
Q: Can front and rear axle ratios differ?
A: Only if the vehicle was engineered that way; an unintended mismatch can cause wind-up.
Q: Does a drive whine prove the gears are worn?
A: No; depth, preload, bearings and other driveline parts can cause similar noise.
Q: Is all GL-5 oil suitable for every differential?
A: No; use the exact viscosity and manufacturer approval.
Q: Does every limited-slip unit need modifier?
A: No; add only the approved type and quantity for that design.
Q: Can a crush sleeve be loosened and reused?
A: Not after over-compression; replace it and reset preload.
Q: Why measure backlash before strip-down?
A: It preserves diagnostic evidence and a setup reference.
Q: Can bearing caps change sides?
A: No; retain their original side and orientation.
Q: What does metal on the drain magnet mean?
A: Fine paste can be normal, but flakes, chips or fragments require investigation.
Q: Can a leaking seal alone be replaced?
A: Only after checking flange condition, bearing play and breather pressure.
Q: Must new gears be bedded in?
A: Follow the supplier's heat-cycle, load and oil-change instructions.
Q: When is the vehicle unsafe to drive?
A: Stop for binding, severe noise, rapid heat, major play, fragments or oil reaching brakes/tyres.
