Wheel speed sen-sors are an in-tegral part of act ive safety systems. They measure the ro-tational speed of the vehicles wheels via magnetic fields in a contact-free manner. Wheel speed sensors are clas-sified as either active or passive, de-pending on how they operate. The classification may seem counterintui-tive, but heres how it works:

A sensor that operates and gener-ates a usable signal without an addi-tional supply voltage applied is called a passive sensor. Early, variable-re-luctance wheel speed sensors were passive, and well discuss their oper-ation first.

A sensor that requires a supply volt-age to operate and generate an output signal is called an active sensor. There are several advantages to this design, which explains why it has been adopted for use on most late-model ABS, trac-tion and stability control systems.

A passive wheel speed sensor is po-sitioned directly adjacent to a toothed impulse or tone ring. The tone ring is connected to the wheel hub or drive-shaft. Inside the sensor, a pole pin is surrounded by a winding and connect-ed to a permanent magnet. The mag-netic effect produced by the magnet extends to the tone ring. The rotation of the tone ring causes its alternating teeth spacing to pass near the sen-sor. This brings about a change in the magnetic field. The changing magnet-ic field induces an alternating voltage in the sensor winding.

The frequency and amplitude of this alternating voltage signal change in direct relation to wheel speed. The faster the wheels rotation, the great-er the frequency and amplitude of the speed sensors signal. All of this takes place without the need of an outside voltage supply from a control unit. The sensor has just two wiresground and signal. A number of fac-

tors can affect the quality of a passive speed sensors signal, however.

First, a passive speed sensor is in-effective at generating a usable signal at slower vehicle speeds. The control unit may require the sensor signal to be above a certain amplitude and the signals amplitude may be just too low for the control unit to deal with it at low speeds. Consequently, a passive sensors signal may not come on line as far as the control unit is concerned until vehicle speed is 5 mph or greater.

Second, because the sensor relies on the oscillating magnetic field cre-ated by the sensor magnet and the tone ring, the physical relationship between the two is very important. If

the gap between the sensor tip and the tone rings teeth is too great, the sensors ability to produce a usable signal will be compromised.

Although the sensor gap can be ad-justed on some vehicles, most cant be adjusted. A nonferrous (brass) feeler gauge must be used to measure the gap, if a spec is available. If the gap is out of spec, and no adjustment is possible, the tone ring or wheel speed sensor probably requires replacement.

Dirt may accumulate on the sensor, which inhibits its ability to generate a

signal. Metallic filings can collect on the sensors magnet, resulting in an erratic signal or a loss of signal. Rust between the wheel speed sensor and the knuckle or hub can disturb the air gap between the sensor and the tone ring. Corrosion can get in between the components and damage the tone ring. A warped tone ring will cause the sensor gap to vary, causing the sensor output to vary in amplitude as the tone ring rotates. If a tooth is missing or out of alignment, it can also affect the quality of the signal. Even a small chip in the tone ring can affect the sensor signal. Never use a pry bar or hammer to remove a half-shaft from a steering knuckle.

One of the biggest shortcomings of a system equipped with passive speed sensors (and a frequent custom-er complaint) is false ABS activation. By design, the sensor signal weakens as vehicle speed decreases, and most false ABS activation events take place at slow vehicle speeds. Even if the sensor gap is correctly adjusted, worn or loose wheel bearings or other fac-tors may cause erratic sensor output signals and false ABS activation.

The ABS control unit compares the signal amplitude and frequency from all of the wheel speed sensors on the vehicle to determine if ABS activation is necessary. Any small difference be-tween them may be interpreted as a wheel thats slowing too rapidly, which can trigger ABS activation, particularly at speeds below 10 mph, despite the fact that the wheel is not locking up. To find the source of the erratic, in-consistent wheel speed sensor signals, check for tone ring movement caused by worn wheel bearings, and inspect the other components previously men-tioned. Worn or damaged components must be replaced.

If DTCs or diagnostic procedures indicate a passive wheel speed sensor failure, visually inspect the affected sensor, its related wiring and connec-tions for problems. A passive wheel

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This early, passive wheel speed sensor design relies on a tone ring spinning close by on a drive axle or brake disc to allow it to generate an analog AC signal that di-minishes in strength when vehicle speed is low. Its exposed location and larger size makes it more vulnerable to damage and deterioration caused by exposure to heat, moisture, salt and other indignities.

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speed sensor can be checked by mea-suring its resistance with a digital mul-timeter (DMM). If the sensor resis-tance is out of specification, it will not produce an accurate signal to the control unit. To verify sensor outputs, also compare them using an oscillo-scope or AC voltmeter.

Some ABS control units send a DC bias voltage to the sensor through the ground wire, so the sensor can be tested by the control unit before the vehicle is moving. If theres a prob-lem, vehicles with this type of system will set wheel speed sensor codes the moment the key is turned on. The ABS control unit is looking at the voltage coming back from the sensor. High resistance or an open circuit can be immediately detected.

Problems with these systems are not always caused by a faulty sen-sor. Problems with the bias voltage as it travels through the harness and connectors may be to blame instead. To measure the circuits bias volt-age, connect the meters ground lead to battery ground, and the positive lead to the signal wire. If the signal is viewed on a scope, the 2.5V bias volt-age will raise above the ground line. The DC bias voltage supplied to the circuit should be present without the need for you to spin the wheel.

With the sensor wiring disconnect-ed, connect the positive test lead to the harness connector leading to the control unit and the negative lead to the connector that leads to the ground for the ABS module. The bias voltage supplied by the ABS module should be between 1.5 and 5V. If the voltage is outside the manufacturers specification, it may indicate a prob-lem with the harness.

Passive and active wheel speed sensors can look similar, but their op-eration is dissimilar. The active, mag-netoresistive wheel speed sensor was designed to combat the weak low-speed signal characteristics of passive sensors. In some designs, the active sensor uses a tone ring and a permanent magnet like the variable-reluctance pas-sive sensor. It may have either two or three wires. One wire delivers a reference voltage from the control

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unit, while a second wire handles the sensor signal. A third, ground wire may also be employed.

Unlike the passive sensors AC out-put signal, changes in the electromag-netic field produced by the toothed tone ring passing the wheel speed

sensor cause the active wheel speed sensor to produce a DC voltage sig-nal. Its a digital high/low voltage, unlike the analog voltage signal pro-duced by a passive sensor.

The active sensor is a proximity sensor with integrated electronics thats supplied with a defined voltage from the ABS control unit. Instead of a tone ring, some active sensor designs use a multipole ring thats integrated into a wheel bearing seal-ing ring as an impulse ring. Inserted in this sealing ring are magnets with alternating pole directions.

The magnetoresistive components integrated in the sensors electronic cir-cuit detect an alternating magnetic field when the multipole ring rotates. This sinusoidal signal is converted by the sensor electronics into a digital signal. The signal is transmitted to the control unit in pulse width-modulated fashion.

In addition to magnetoresis-tive sensor elements, active sensors use Hall sensor elements to permit a larger air gap and to respond to the smallest changes in the magnetic field. If a steel impulse ring is in-

stalled in a vehicle in place of a mul-tipole tone ring, a magnet is also af-fixed to the sensor element. When the impulse ring turns, the magnetic field in the sensor changes.

This type of sensor design permits greater signal accuracy. In fact, an active sensor is able to detect the first edge of the next tooth on the tone ring immediately after powering on. Like the passive sensor design, the control unit uses the frequency of the digital DC signal from the active sensor to calculate the wheel speed. But unlike the passive sensor, its also capable of detecting which direction the wheel is turning.

The elimination of tone rings sim-plifies the design of the related pow-ertrain components, and sensitivity to electromagnetic interference is less pronounced. Slight changes in the air gap between the sensor and magnetic ring have no direct effect on the signal. Active sensors also are less affected by vibrations and temperature fluctuations.

The data from passive and active wheel speed sensors can be observed and graphed with a scan tool. Your scan