8.5 Wheel-End Systems: Preset and Unitized Hubs, ABS Wheel Speed Sensors and Tone Rings, and Wheel Bearing Diagnosis
Key Takeaways
- ConMet PreSet hubs use a precision spacer to set bearing clearance, so the spindle nut is torqued without the conventional back-off.
- ConMet PreSet Plus drive hubs are installed by torquing the spindle nut to 500 lb-ft while rotating the hub, with no back-off.
- Bendix checks a WS-24 wheel speed sensor at 1,200–2,700 ohms, and for at least 0.25 VAC at the ECU with the wheel turned at 1/2 revolution per second.
- If a sensor's output is low, Bendix says to push it back toward the tone (exciter) ring until it touches, then retest before replacing it.
- Excessive wheel bearing end play, beyond RP 618A's 0.001–0.005 in. window, can let the tone ring move away from the sensor and set intermittent ABS codes.
8.5 Wheel-End Systems: Preset and Unitized Hubs, ABS Wheel Speed Sensors and Tone Rings, and Wheel Bearing Diagnosis
The drive axle task list ends at the wheel. It covers diagnosing wheel bearing noise and damage; checking hub seals and axle flange gaskets for leaks; servicing extended-service (preset and unitized) bearing assemblies; and checking, testing, cleaning, adjusting or replacing wheel speed sensors and tone (exciter) rings. Conventional adjustable bearings and the TMC RP 618A procedure were covered in section 8.3.
1. Diagnosing Wheel Bearing Noise and Damage
- Symptom pattern: a growl or roar that follows road speed and does not change with throttle (drive, coast, or float). It usually gets louder when the vehicle sways or turns and loads that wheel end. Differential and pinion noises change with throttle; wheel bearing noise does not.
- Check end play: mount a dial indicator and push-pull the hub along the spindle axis. For conventionally adjusted bearings, TMC RP 618A's target is 0.001–0.005 in. Too much means loose adjustment or worn bearings; zero end play with a hot hub means preload.
- Read the damage:
| Condition | Appearance | Usual Cause |
|---|---|---|
| Fatigue spalling | Flaking pits on races or rollers | Normal end of life, overload, or incorrect adjustment |
| Brinelling | Indentations spaced like the rollers | Impact loads or improper installation |
| Etching/corrosion | Stains and pitting | Water in the lubricant (failed seal, plugged breather) |
| Heat discoloration | Blue or brown races and rollers | Lack of lubrication or preload |
| Abrasive wear | Dull, scratched surfaces | Contaminated oil or grease |
Always find the cause — low hub oil, a leaking seal, a plugged breather, or incorrect adjustment — before replacing bearings.
2. Hub Seals and Axle Flange Gaskets
- Wheel seals: oil on the brake linings or the inside of the wheel means a leaking seal. Check for a plugged axle breather, overfilling, a seal installed cocked (use the seal manufacturer's driver), and loose bearing adjustment that lets the hub wobble on the seal.
- Axle flange joint: the gasket and flange of the axle shaft must fit flat against the wheel hub (Meritor). If Meritor's silicone option is used instead of a gasket, apply a 0.125-inch (3 mm) bead around the hub mating surface and around the edge of each fastener hole. Tighten the stud nuts to specification with the correct hardened washers or lock washers.
3. Extended-Service Hubs: Preset and Unitized Assemblies
Extended-service wheel ends remove the adjustment step that causes so many bearing failures:
- Preset (pre-adjusted) hubs: ConMet's PreSet design uses a precision-machined spacer between the tapered roller bearings to control their spacing, so there is no manual adjustment. The installer torques the spindle nut to the specification and does not back it off.
- ConMet PreSet with a double-nut or jam-nut system: torque the inner nut to 300 lb-ft (do not back off; advance it as needed to install the locking ring), then torque the outer nut to 200 lb-ft and engage any locking device.
- ConMet PreSet Plus (integrated spindle nut): torque the spindle nut to 500 lb-ft on a drive or trailer hub (300 lb-ft on a steer hub) while rotating the hub, with no back-off.
- Unitized hub assemblies: some hubs are sealed units with bearings, seals, and lubricant built in. Meritor notes that its sealed, greased truck hub units need no maintenance, and that disassembling, lubricating, or repairing them voids the warranty. They are replaced as a unit when they fail.
- Key rule: never apply the conventional RP 618A "back-off" to a preset or unitized hub, and never mix preset hub parts with conventional bearings or spacers. Follow the hub maker's torque, locking-device, and end-play verification instructions exactly.
4. ABS Wheel Speed Sensors and Tone Rings
Each ABS-monitored wheel has a tone (exciter) ring on the hub and a wheel speed sensor held in a mounting block by a spring clamping sleeve. The sensor is pushed in until it touches the tone ring. As the ring turns, normal bearing end play pushes the sensor back to a small working gap. The sensor produces an AC signal whose frequency matches wheel speed.
Bendix WS-24 sensor checks:
- Inspect the sensor cable for cuts, pinching, wear, or exposed wires, and the connectors for corrosion and loose pins.
- Resistance: at ambient temperature (0–100°F), resistance across the sensor should be 1,200–2,700 ohms. Outside that range, replace the sensor.
- Output: with the wheel raised and the parking brakes released, turn the wheel by hand at at least 1/2 revolution per second. The AC voltage at the ECU across the sensor terminals should be at least 0.25 VAC.
- Low output: gently push the sensor toward the wheel until it touches the exciter ring, and retest. If output is still low, replace the sensor using a new sensor kit with its clamping sleeve.
- If resistance and voltage are good, the sensor is not the problem. Look for pinched or chafed harness wires, loose pins, or ECU connector problems.
Tone ring problems: a cracked, loose, or damaged ring, missing teeth, or ring runout gives an erratic signal. Excessive wheel bearing end play lets the hub wander away from the sensor and causes intermittent low-signal codes, which is one more reason to hold end play in specification. When replacing a hub or ring, protect the ring from damage and make sure it is fully seated.
Safety: Bendix warns that on vehicles with automatic traction control (ATC), ATC must be disabled before any maintenance where drive-axle wheels are lifted and turning. Otherwise the system can apply brakes or change engine torque unexpectedly.
A technician is installing a new ConMet PreSet Plus hub on a drive axle. How is the spindle nut tightened?
A drive-axle ABS sensor sets an intermittent low-signal code. Its resistance is 1,800 ohms at room temperature, and turning the wheel at 1/2 revolution per second produces 0.12 VAC at the ECU. Following Bendix's procedure, what is the next step?
Technician A says wheel bearing noise usually changes with road speed but not with throttle position, and often grows louder when the vehicle sways onto that wheel. Technician B says excessive wheel bearing end play can let the tone ring move away from the ABS sensor and cause intermittent ABS codes. Who is right?
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