Wheel Speed Sensor Diagnosis

When anti-lock brakes (ABS) first became common equipment on new vehicles well over two decades ago, it featured a "passive" variable-reluctance wheel speed sensor. Later, the "active" magneto-resistive sensor was introduced, which improved performance by providing a more precise wheel speed reading at very low speeds.

 

The role and importance of an accurate wheel speed reading has continued to grow with the increased popularity of electronic controls for automatic transmissions, traction control, tire pressure monitoring and other vehicle systems.

 

Operation

 

While the appearance of active and passive wheel speed sensors is similar, operation is not.

 

The operation of the variable-reluctance sensor uses a small internal magnet and coil of wire to generate a signal to the Electronic Brake Control Module (EBCM). Operation involves a gear-shaped tone wheel that rotates near the sensor on each wheel. As the tone wheel rotates, a magnetic field fluctuates around the sensor and induces AC voltage into the internal coil windings. AC voltage is sent to the EBCM, which interprets the voltage and frequency as a wheel speed signal input.

 

This type of sensor requires that the tone wheel rotate fast enough in order to generate a usable signal. As wheel rotation slows, the signal strength decreases, resulting in a weak signal at very low speeds.

 

To combat this weakness in signal strength at low speeds, the magneto-resistive wheel speed sensor was designed. (Fig. 11) It uses a tone wheel and a permanent magnet like the variable-reluctance sensor, along with a two-wire connection that consists of a supply circuit and signal circuit connected to the wheel speed sensor. To power the sensor, the EBCM provides 12 volts on the supply circuit.

 

As a toothed ring passes by the wheel speed sensor, changes in the electromagnetic field cause the wheel speed sensor to produce a DC voltage signal. It is a digital high/low toggle rather than an analog voltage like with a passive sensor.

 

The sensor is able to detect the first edge of the next tooth on the tone ring immediately after powering on. The EBCM uses the frequency of the DC signal to calculate the wheel speed.

 

Diagnosis

 

If DTCs or diagnosis procedures indicate a wheel speed sensor failure, visually inspect the sensors, related wiring and connections for problems.

 

A variable-reluctance wheel speed sensor can be checked by measuring its resistance with a digital multimeter (DMM). If the sensor resistance is out of specification, it will not produce an accurate signal to the EBCM. To verify sensor outputs, also compare them using an oscilloscope or AC volt meter.

 

To measure the output of the magneto-resistive wheel speed sensor, start by testing the 12-volt reference circuit from the EBCM. (Fig. 12)

 

Also, DC amperage can be measured across the wheel speed sensor. Slowly turn the wheel and watch the DMM. Amperage should fluctuate from high to low.

 

The digital signal generated by the magneto-resistive sensor also can be viewed using an oscilloscope. Connect the leads as you would for a DMM. A good wheel speed sensor scope waveform should have sharp square corners on the DC signal circuit to the EBCM.

 

Remember, the EBCM sends a 12-volt reference signal to each wheel speed sensor. As the wheel spins, the wheel speed sensor produces a square wave DC signal voltage. The wheel speed sensor increases the signal frequency as the wheel speed increases, but does not increase the signal amplitude.

 

Poor connections and broken wires are some of the leading causes of wheel speed sensor-related failures. Other common causes of wheel speed sensor malfunction are water intrusion and corrosion in the connector to the sensor and damage to the tone ring.

 

For more information on wheel speed sensor diagnosis, visit www.acdelcotechconnect.com and click the Training tab. Log in to the ACDelco Learning Management System to view TechAssist S-BK05-01.01TAS - Active/Hall Wheel Speed Sensor Operation.

 

- Thanks to Mike Militello and Mike DeSander