Intermittent ABS Lamp After Rear Brake Reline
A 2018 6x2 tractor unit with 486,000 km comes in after a rear brake reline at another workshop. The driver reports the yellow ABS warning lamp illuminating intermittently above 60 km/h, especially on right-hand bends, and the lamp resets after an ignition cycle. A fault code for the right rear wheel-speed sensor is stored as 'erratic signal'. The sensor was replaced once already; the fault returned within 40 km. You measure sensor resistance at 1.18 kΩ, supply voltage 12.4 V with ignition on, and AC output of 0.55 V when the wheel is spun at about 1 rev/s. The tone ring shows 0.45 mm lateral runout and has metallic paste smeared across two teeth. Hub end float is 0.12 mm, and the wheel bearing emits a faint rumble when loaded. The workshop schedule is full and the vehicle is booked for a 300 km delivery in two hours.
1. Using the measurements, explain why the fault is unlikely to be caused by the sensor alone and identify the most probable root cause.
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The sensor is electrically alive: 1.18 kΩ is a plausible resistance, 12.4 V supply is present, and 0.55 V AC at 1 rev/s shows it can generate a signal. A dead sensor or open circuit would not produce those readings. The clue is that the fault appears at speed and in bends, which points to a varying air gap or a disturbed magnetic field rather than a failed coil.
The tone ring has 0.45 mm lateral runout and metallic paste bridging teeth, both of which distort the signal. Hub end float of 0.12 mm with a loaded bearing rumble means the hub is moving and changing the sensor gap dynamically. That fits an 'erratic signal' that resets after ignition. Replacing the sensor again treats the symptom. The probable root cause is a worn or incorrectly seated hub bearing/hub assembly causing tone-ring runout and debris, likely disturbed or exposed during the brake reline.
2. State whether you would release the vehicle for the delivery and outline the minimum repair and verification steps.
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I would not release the vehicle. An intermittent ABS fault on a heavy tractor can disable wheel-speed information when it matters, affecting ABS and possibly traction or stability functions. The delivery deadline does not justify the risk.
Minimum repair: replace or correctly seat the affected hub bearing/hub assembly, or replace the tone ring if the hub itself is serviceable; clean all metallic debris from the sensor and tone ring; set the sensor air gap to specification; check the opposite side for the same condition. Verification should include a dial-gauge runout check, an oscilloscope waveform at low and road speed, live wheel-speed data on all wheels, clearing codes, and a road test above 60 km/h including bends. The ABS lamp must stay off and all wheel speeds must track consistently. If parts are not available, substitute the vehicle and document the decision.
3. If the fault persisted after hub replacement, what would you check next and why?
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I would move upstream from the wheel end. First, inspect the harness from the sensor to the chassis for chafe, stretched conductors, and corrosion in connector pins; an intermittent open circuit can mimic a wheel-speed fault. Second, check the sensor mounting face for distortion, paint, or debris that changes the gap. Third, compare the waveform with a known-good wheel: amplitude, frequency, and missing pulses. Fourth, check axle end float and bearing seating again, because a incorrectly pressed bearing can still move. Finally, if the signal is clean at the sensor but erratic in live data, suspect the ABS ECU or its connector. The reasoning is to prove where the signal is lost before replacing more parts.