ABS / WSS

WHEEL-SPEED COMPARISON

Compare wheel-speed signals at the same speed to find dropouts, weak amplitude and damaged encoder patterns.

ESTIMATED SETUP5–10 min2–4 channels

WHY BEFORE DISASSEMBLY

CAPTURE THE SYSTEM
AS IT FAILED.

Removing hubs or sensors can change air gaps, debris and harness position. Capturing signals first proves whether the fault follows one wheel and whether it is electrical, target-related or speed-dependent.

Reading the signal

Good waveform

At matched wheel speeds, signals repeat consistently. Active sensors switch according to their voltage- or current-modulated design; passive sensors produce AC whose amplitude changes with speed.

Faulty waveform

A recurring dropout at the same wheel position suggests a target or air-gap problem. Intermittent loss may indicate wiring or supply issues. Unequal frequency can also be caused by turning, tire size or actual wheel-speed differences; account for these before diagnosing a fault.

Common mistakes

AVOID THE
FALSE CLUE.

Testing with the connector unplugged when the fault only appears under load.
Using a generic voltage value instead of the vehicle-specific specification.
Skipping power, ground and reference checks before condemning the component.
Moving the harness or clearing codes before capturing the original evidence.
Replacing a part without repeating the test.

PREPARE THE TEST

✓ Identify active or passive sensor design✓ Confirm safe wheel rotation✓ Use matching scales for compared wheels✓ Route leads away from tires, shafts and suspension

STEP-BY-STEP PROCEDURE

Connect the suspect wheel and a known-good wheel simultaneously at comparable circuit locations.
For active sensors, measure the correct voltage or current-coded output. For passive sensors, use an appropriate AC range.
Rotate both wheels at the same controlled speed or conduct a safely instrumented road test.
Compare amplitude, frequency, edge consistency and dropout at very low speed.
Use a long capture to find a fault that repeats once per wheel revolution.
Inspect the connector, harness, air gap, bearing play and encoder/reluctor before replacing the sensor or hub.

WHAT THIS TEST CANNOT PROVE

Active sensors may encode direction or use current modulation. The correct connection depends on the circuit design.

⚠ SAFETY / CONNECTION CAUTION

Never work near a rotating wheel or driveline without an approved lift procedure and secured leads. Road testing requires a second qualified person or secure data logging.

STOP BEFORE CONDEMNING THE PART

Repeat the capture, verify the circuit under load and use vehicle-specific specifications. Follow with the most direct confirmation test indicated by the evidence.

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