Wheel Speed Sensor (magnetoresistive)

The purpose of this test is to evaluate the operation of an Antilock Braking System (ABS) magnetoresistive wheel speed sensor based upon its output voltage and frequency.

How to perform the test

View connection guidance notes.

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1. Use manufacturer’s data to identify the output signal circuit.       
2. Connect PicoScope Channel A to the output signal circuit.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Turn on the ignition, engine off.
6. Safely raise and rotate by hand the suspect road wheel.
7. With your waveforms on screen stop the scope.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• A square waveform with peak to peak voltages around 200 mV (when AC coupled).
• The switching frequency, between the peaks and the troughs, increases with increasing wheel speed.
• There are no signal drop outs and the width of each cycle changes progressively with changing wheel speed (there are no suddenly much shorter or much longer waveform cycles).

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, ABS or wheel speed sensor (magnetoresistive).

Further guidance

Wheel speed sensors provide wheel and road speed feedback to ABS and derivative active vehicle safety systems (i.e. stability and traction control etc.).

These systems are designed to provide corrective action (e.g. wheel braking or engine torque limitation) when the vehicle chassis or wheel speeds have exceeded their normal operational tolerances, for example, during conditions of wheel slip, oversteer or understeer etc. Wheel speed sensors are critical to the operation of these systems and, therefore, the safe handling of a vehicle.

A magnetoresistive wheel speed sensor includes a semiconductor, a Magneto Resistive Element (MRE), that acts as either a conductor or an insulator depending on the orientation of any nearby magnetic field. When in the presence of an alternating magnetic field, as provided by a multipole rotor ring mounted in a wheel bearing seal or hub assembly, the sensor switches its output on and off. This action produces a digital square wave which is received by the ABS control module.

Magnetoresistive sensors require power to operate; hence they are referred to as active sensors. These sensors usually have two terminals with a constant earth reference on one and a combined supply (often at battery voltage) and signal circuit on the other. The combined supply voltage and signal waveform can be measured in two possible PicoScope modes: with the DC offset from the supply voltage present (DC coupled mode) or AC coupled to show only the changing output signal (as in the example above).

It is not easily possible to distinguish two wire passive (inductive) and active (Hall or magnetoresistive) wheel speed sensors by their external appearance. Diagnostically, this is inconvenient as active wheel speed sensors must never be subjected to resistance tests: this can damage these units, with the only remedy being the acquisition of a new replacement.

Therefore, you must always either consult manufacturer’s data to identify the fitted type prior to diagnostic testing or carry out a speculative PicoScope check to identify the sensor from its output voltage characteristics.

You can check for a positive supply voltage at one of the sensor connector terminals to determine if you have an active sensor type. However, if the supply voltage is missing due to a fault and you then assume the sensor must be passive and perform a resistance check, you can damage a perfectly good active sensor. This will give you two faults.

An ABS control module expects similar (within a given tolerance) oscillation frequencies from all the vehicle’s wheel speed sensors and uses any differences to calculate the timing and scale of its interventions.

If one, or more, wheel speed signals continuously fall outside of normal parameters the control module may turn the ABS function off (along with associated traction and stability systems). A driver warning light will be illuminated but, as with any electrical fault on ABS, normal hydraulic braking is maintained.

Wheel speed sensors and their pulse rings are exposed to the atmosphere and have to operate under conditions of constant vibration and movement. As such, common faults are:

• Sensor signal failures, caused by chafed or fractured circuit wiring, sensor or connector corrosion, or incorrect sensor fitment.
• Pulse wheel related problems arising from corroded or damaged teeth, incorrect pulse ring fitment (misalignment), contamination from a build-up of ferrous material, or an excessive air gap between the sensor and the pulse wheel.

Wheel speed sensor circuits and connectors are also prone to the atmosphere and possible electrical failures, such as open or short circuits or high circuit resistances.

Active wheel speed sensors are susceptible to damage from incorrect testing methods; in particular, the application of resistance testing.

Symptoms of ABS sensor related faults

• ABS warning light (and other associated warning lights) illumination.
• Diagnostic Trouble Codes (DTCs).
• Brake pedal pulsation or vibration.
• ABS inactive.

GT847