Rear Hub and Bearing Heat With ABS Light on a Loaded Route Bus
A school bus on a daily route has an ABS light on and the driver reports a hot smell from the right rear after runs. The right rear hub is noticeably hotter than the left after a 20-mile run. The wheel has slight vertical play when rocked at 12 and 6 o'clock with the brake applied, and there is a faint growl that changes with vehicle speed. The ABS light sets a right rear wheel speed sensor code. Tire pressures are equal and the tire shows even wear. The axle is a full-floating rear axle. The technician wants to replace the ABS sensor and send the bus out.
1. Explain why replacing the ABS sensor alone is inadequate and how the hub play and heat relate to the ABS code.
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The heat, vertical play with the brake applied, and speed-related growl point to a worn or failing rear wheel bearing/hub, not a sensor fault. On a full-floating axle the hub rides on the spindle bearings; excess bearing play lets the hub and tone ring move relative to the sensor, which can set a wheel speed code and can also damage the sensor. Heat is generated by a failing bearing or by excessive bearing preload. Replacing the sensor without addressing the bearing leaves a safety-critical wheel-end defect and the code will likely return. The correct path is to inspect and replace/repack and properly adjust the rear wheel bearings per spec, inspect the spindle and tone ring, then clear and recheck the ABS code.
2. Describe the inspection and adjustment procedure for the full-floating rear wheel bearings and how you would verify the repair.
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- Raise and support the axle, remove the wheel, and remove the axle shaft. 2) Remove the locknut/lock ring and outer bearing, then inspect the bearings, cups, spindle, and seal; replace if pitted, discolored, or worn. 3) Pack bearings with the specified grease, install the inner bearing and a new seal, then the outer bearing. 4) Adjust bearing preload/end play per the manufacturer's spec (many full-floating axles use a specified end play or torque-then-back-off procedure); do not guess. 5) Reinstall the axle shaft and torque fasteners. Verification: check for correct end play with a dial indicator, spin for smoothness, road test and recheck hub temperature against the opposite side, confirm the tone ring/sensor gap, and verify the ABS light stays off after a drive cycle.