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
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