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Magneto-Resistive Wheel Speed SensorsPosted 8/13/2001By David W. Gilbert
Changing ABS Diagnostic Procedures
One example of an active WSS is used with 1999 and newer DaimlerChrysler AG vehicles equipped with the Teves Mark 20e ABS. The Teves Mark 20e wheel speed sensors use a magneto-resistive principle and internal electronics to accurately measure wheel rotation. While outward appearances of magneto-resistive sensors are similar to the more common variable-reluctance type wheel speed sensors, internal construction and operation of the magneto-resistive sensors are quite different. Automotive technicians should be aware of basic ABS operation and testing procedures for older types of variable reluctance wheel speed sensors. Fundamental operating characteristics of variable-reluctance type sensors are quite simple. The variable reluctance WSS use a small internal magnet and coil of wire to generate a signal to the ABS control module.
Similar to variable reluctance sensors, magneto-resistive sensors use a permanent magnet, tone wheel and two-wire connection. However, that is where the similarities end. The Teves Mark 20e system uses a two-wire harness consisting of a supply circuit and signal circuit connected to the WSS. Magneto-resistive sensors cannot generate a signal voltage on their own, and must have an external power source. To power the sensor, the controller antilock brake (CAB) module provides 12 volts on the supply circuit to the WSS. Inside the WSS is a small integrated circuit containing a magneto-resistive bridge.
Approximate WSS signal output voltage will be either 0.90 volts or 1.65 volts, depending on tone wheel position. The magneto-resistive WSS maintains current flow through the signal circuit for CAB diagnostics and to complete the sensor's ground path. Inside the sensor, a small power supply returns a constant 7 milliamp (mA) current at 0.90 volts on the signal wire when a tone wheel valley is aligned with the WSS. As a tone wheel tooth approaches the WSS, a second power supply inside the sensor boosts the signal up to 14 mA and 1.65 volts.
When the ignition switch is ON, the CAB monitors the wheel speed circuits every 7 milliseconds for proper operation. If ABS fault codes or diagnosis procedures indicate a WSS failure, visually inspect the ABS controller, wheel speed sensors, related wiring and electrical connections for obvious problems. Using a digital multimeter (DMM), check the WSS electrical circuits for opens, shorts and grounds. If the basic circuit checks are OK, carefully back-probe the WSS connector with T-pins and use the DMM to test for voltages.
Connect the leads as you would for a voltmeter to view the magneto-resistive WSS signal with a scope. Adjust the scope settings to read 0.5 volt divisions at a rate of about 20 ms. A good WSS scope waveform should have sharp square corners on the DC signal circuit to the CAB. The example WSS scope waveform shows a good signal for a wheel speed of about 4 mph. Modern ABS systems have become a dynamic part of expanding electronics and safety technology in a variety of vehicle applications. In addition to ABS, future traction control and tire deflation detection systems will require the precise measurement of wheel speed. Magneto-resistive type wheel speed sensors will undoubtedly play a key role in the performance of these enhanced systems.
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Wheel speed sensors (WSS) are vital components in antilock brake systems (ABS) and are familiar to most brake technicians. However, the introduction of “active” wheel speed sensors brings some new changes to ABS electrical diagnostic procedures.
Each wheel and axle assembly is equipped with a gear-shaped tone wheel that rotates near the sensor. As the tone wheel rotates, a magnetic field fluctuates around the sensor and induces alternating current (AC) voltage into the internal coil windings. AC voltage is sent through a two-wire connector and harness to the ABS control module. The ABS controller interprets the AC voltage and frequency from the variable reluctance sensor as a wheel speed signal input.
The major disadvantage of variable reluctance sensors is the decreasing signal strength as wheel rotation slows and approaches lockup. With this type of sensor, the tone wheel must be rotating fast enough to generate a usable signal to the ABS control module.
The CAB interprets the small digital signal to determine the speed of the wheel. Since WSS signal output is not dependent on wheel speed, the sensor can accurately detect wheel rotation down to zero. Magneto-resistive sensors can provide the CAB with precise wheel speed rotation information at low speeds, which accounts for increased ABS performance.
Be sure to have a good ground connection to the negative lead of the DMM when testing. With the ignition switched ON, the DMM should read above 10 volts on the CAB supply circuit to the WSS. Voltage on the WSS signal circuit should be approximately 0.9 or 1.6 volts depending on the position of the tone wheel. To verify WSS operation, slowly rotate the wheel by hand while monitoring voltage with a DMM on the signal circuit. The sensor signal voltage should alternate between about 0.9 and 1.6 volts. The digital signal generated by the magneto-resistive WSS can also be viewed using an oscilloscope.
