Differential & Final Drive

Possibly, and the test is which speed it follows. A noise that rises and falls with road speed regardless of what the motor is doing points at the final drive, the driveshafts or the wheel bearings. A noise tied to motor speed points elsewhere. Narrow it further by loading and unloading the drivetrain: a whine that changes between gentle acceleration and coasting at the same road speed is a gear mesh problem, while one that stays constant is more likely a bearing.

Because backlash is set by selection rather than adjustment. Manufacturing tolerances mean each carrier, side gear and housing combination ends up slightly different, so the factory measures the assembly and fits the graded washer that brings side gear clearance into specification. A rebuild follows the same procedure: assemble dry, measure, calculate, then fit the grade the measurement demands. That is why this category lists each grade as its own reference rather than selling an assortment, most of which would never be used.

Only if nothing has changed, and after a rebuild something usually has. New bearings sit fractionally differently from worn ones, a replacement gear has its own tolerances, and a housing that has been apart may not settle identically. Record the original thicknesses as a starting point, then measure and confirm. Reusing old grades because they were in there before is the most common reason a rebuilt final drive whines within a few thousand miles.

Look at the drain plug magnet and the tooth faces. Fine grey paste on the magnet is normal wear. Metal flakes, chipped teeth, pitted or scored contact patterns, or a bearing that has been running with debris through it, all argue for a complete assembly, because contamination will have reached every surface. Where the gears are clean and the fault is a worn thrust washer, an O ring or a bearing, a rebuild is the better value, and the graded parts to do it properly are listed here.

Accumulated free play, and it can come from several places along the drivetrain. Inside the differential, worn side gear thrust washers allow the side gears to move away from the pinions before the mesh loads up, which you feel as a clunk when torque reverses. Worn driveshaft joints and tired powertrain mounts produce a similar noise. Check the mounts and the joints first because they are easier to inspect, then measure side gear clearance if the noise remains unexplained.

Yes, wherever one motor drives two wheels. A single speed reduction gearbox contains a differential exactly like a combustion car, because the outer wheel must still turn faster than the inner through a corner. What is missing is the multi speed gearbox in front of it. Vehicles with a motor per wheel or per axle side can dispense with a mechanical differential and manage the split electronically, but the common arrangement of one motor per axle still uses a conventional carrier.

The arithmetic here favours rebuilding. Thrust washers, adjustment shims and O rings are inexpensive enough that a full set of small parts adds little to a strip that has to happen anyway. Carrier bearings are more. Driven gears and pinions are a serious purchase, and a complete final drive assembly is the top of the category. The live spread appears in the stats strip at the head of this page. What usually decides the bill is labour rather than parts, since setting backlash and preload properly takes time and a dial gauge.

Only with the right measuring equipment and patience. The job needs a dial indicator to read backlash, a means of measuring bearing preload as rolling torque, a press for the carrier bearings and the correct torque figures throughout. Without those, the assembly can go together feeling perfectly good and destroy itself within a few thousand miles. Where the tools are unavailable, order the parts by OE number and hand the assembly to a specialist. The measuring, rather than the spannering, is the skill.

Whatever the manufacturer specifies, and on electric drive units that is often a dedicated fluid rather than a conventional gear oil, because it may also cool the motor and must be compatible with the insulation and the seals. Quantity matters as much as grade, since these housings are filled to a level rather than a capacity in many designs. Confirm both before starting, and refill through the correct plug. Lubricants are listed in their own category on the site.

Almost always a preload or backlash error. Too little bearing preload lets the carrier move under load and the mesh pattern wanders. Too much preload overheats the bearings. Backlash outside specification puts the contact patch in the wrong place on the tooth, which whines under drive, on the overrun, or both, depending on which way it is wrong. The cure is to strip, measure properly, and fit the correct graded shim and thrust washers rather than adjusting by feel.

Rarely, and never by appearance. Final drive ratios differ between drivetrains sharing one body, carrier dimensions follow the housing, spline counts follow the driveshafts, and on electric drive units the whole assembly is integrated with the reduction gearing. Fitting a different ratio also confuses the vehicle's speed calculations. Match the OE number from your own unit, and where the reference brings back nothing, send it through the quote form so the factory catalogue is checked directly.

Ordering works through the enquiry route. Gear sets, side gears and adjustment shims move in small numbers and are supplied in graded sizes, so many carry no standing figure. Give the number and, where the part is graded, the letter or thickness marking from the old component. Crown wheel and pinion are matched at the factory and are supplied as a pair, so ask for the set rather than one half of it. If the drain plug magnet showed metal flakes rather than a fine paste, mention that too, because the bearings will need looking at while the unit is apart.