ABS Lamp On After Wheel Speed Sensor Replacement
A technician replaced a right front wheel speed sensor on a transit bus with a Bendix ABS-6 system after a wheel seal leak contaminated the original sensor. The ABS lamp is still on after the repair, and the blink code indicates a right front sensor fault. The new sensor was pushed fully into the mounting bushing until it contacted the tone ring, then backed off per the OEM procedure. The tone ring was cleaned. The connector was reseated and the harness visually inspected. Battery voltage is 12.6 V key-off and 13.9 V running. The bus has 245,000 miles.
1. The sensor was installed per procedure and the tone ring cleaned, yet the fault persists. What are the likely causes and how would you isolate them?
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A persistent right-front sensor fault after replacement usually falls into four buckets:
- Air gap / sensor seating - sensor pushed in too far or not seated in the bushing; the tone ring may have been damaged during the seal repair. Verify air gap with a feeler gauge and inspect the tone ring for cracks, missing teeth, or runout.
- Harness fault - a chafed wire or corroded connector pin at the wheel well. Wiggle-test the harness while watching live wheel speed on the scan tool; a dropout confirms an intermittent.
- Tone ring runout or contamination - even a cleaned ring can have metal debris embedded or be bent. Check runout with a dial indicator.
- ECU not cleared / wrong sensor part number - verify the sensor matches the ECU calibration and clear codes after repair.
Isolation sequence: scan-tool live wheel speed at low speed → wiggle test → air gap and runout measurement → resistance check of sensor and harness → substitute a known-good sensor.
2. Live wheel speed on the right front reads 0 mph while the other three read 4 mph during a slow roll. What does that narrow the fault to, and what is the next step?
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Zero mph on one wheel while others read correctly means the ECU is not receiving a usable signal from that sensor - the fault is at the sensor, tone ring, or the wiring between them, not the ECU or hydraulics.
Next steps, in order:
- Check sensor resistance against spec and compare to a known-good sensor.
- Inspect the connector pins for push-back or corrosion; back-probe while rolling.
- Verify tone ring - a ring with missing teeth or excessive runout will produce a dropout at low speed even if it looks clean.
- Check air gap - too large a gap kills the signal at low speed first.
If the sensor and ring are good but the ECU still sees 0, run a continuity/voltage-drop test on both signal wires back to the ECU; an open or shorted harness will mimic a failed sensor. Replace the failed component and clear codes, then verify live speed on all four wheels.