Axles and Four-Wheel-Drive Components
Axles and four-wheel-drive systems
Axle assemblies carry the vehicle load and transmit torque to the wheels. Solid axles use axle shafts, wheel bearings, seals, and retainers; some use C-clips at the differential side gears, while others use pressed bearings and retaining plates. Independent suspension uses half-shafts with CV joints and hub bearings. Four-wheel-drive systems add a transfer case, front axle engagement, locking or automatic hubs, and sometimes a viscous coupling or electronic clutch. Part-time 4WD should not be used on dry, high-traction pavement because the front and rear drivelines cannot slip.
Traps
Mismatched tire sizes or wear on a 4WD or AWD vehicle cause different wheel speeds, which can make a transfer case clutch or viscous coupling work continuously and overheat. Different front and rear axle ratios cause the same problem. Binding in tight turns on dry pavement suggests part-time 4WD is engaged or a differential is locked. A 4WD indicator that does not light may be a shift motor, encoder, vacuum actuator, or switch problem-not necessarily a mechanical failure.
Worked example
A vehicle has one new tire and three worn tires. Measure circumference, not just sidewall size. A significant difference forces the transfer case to compensate continuously. Rule of thumb: on a 4WD or AWD vehicle, keep all four tires the same size, brand, and approximate wear; replace in sets when possible.
Service points
Check transfer-case fluid level and type, axle seals for leaks, hub engagement, and axle-shaft boots. Verify that the 4WD system engages and disengages in both directions.