Wheel Speed Sensor: Testing and Inspection

Wheel Speed Sensor In Place:

Background and Application Information:

ABS wheel speed sensors are usually AC signal generators like this example. ABS wheel speed sensors are usually analog sensors. They are very reliable. These sensors can be found mounted inside a wheel hub or on a front spindle. They are two wire sensors, but their two wires are often encased in a shielded, wire-braid conduit. This is because their signal is very important from a safety standpoint and it is somewhat delicate and susceptible, in electronic terms, to interference from Electromagnetic Interference (EMI) or Radio Frequency (RF).

If EMI or RF distorted this sensor's signal at the wrong time, it could mean ABS failure. The braided shields are there to insure the "electronic communication" between the ABS sensor and the ABS control unit doesn't break down. Diagnostic technicians should take heed as well, and only access the signals at the control unit so as not to risk damage to the wire. Connection to the circuit by backprobing a connector may also be possible.

Connect and Setup the Digital Storage Oscilloscope (DSO):

1. Connect the "COM" probe to the sensor wire labeled (-) or "low" in the wiring diagram.
2. Connect the "CH1" probe to the sensor wire labeled (+) or "high" in the wiring diagram.
3. Set the voltage scaling to 500 mV/div. Alter as needed later.
4. Set the time base to 10 ms/div. Alter as needed later.
5. Select "GND" coupling and position the trace on the division line in the center of the DSO display.
6. After ground position is set, select "AC" input coupling.
7. Select "NORMAL" acquire mode.
8. Set the "TRIGGER" mode to "AUTO."

Exercise the Sensor

If the sensor to be tested is on a drive wheel, raise the wheels off the ground to simulate driving conditions. If the sensor to be tested is not on a drive wheel, use DSO lead extensions to enable viewing the DSO while driving. Raising the wheel with a floor jack or lift and spinning the wheel by hand may also be an option, but driving the vehicle is the best method because weight of the vehicle on the bearing hub may be important to successful diagnosis.

Good ABS Wheel Speed Sensor Pattern (Typical):

Expect This Waveform Result

As the wheel begins to spin, the flat line at the center of the DSO display will start oscillating above and below zero volts, bigger and bigger as wheel speed increases. A waveform very similar to this example should appear. This waveform was logged (saved to software) at about 20 MPH. Unlike some other AC signal generator waveforms, like crankshaft or camshaft position sensors, but very much like VSS, ABS wheel speed sensors make waveform shapes that just about all look alike. Generally, the oscillations (the "ups and downs" in the waveform) will be very close to mirror images of each other above and below the zero level mark (symmetrical).

The amplitude of the wheel speed sensor's AC signal should increase with vehicle speed. The higher the MPH, the taller the waveform. And, as MPH increases, frequency increases, meaning that more oscillations show up on the DSO display. What the diagnostic technician is really looking for is assurance that the critical dimensions of amplitude, frequency and shape are all intact, repeatable, regular and predictable. This means the amplitude of the peaks has to be sufficient, the time between pulses repeatable, and the shapes repeatable and predictable. Ragged peaks or spikes may be caused by the sensor magnets hitting the reluctor ring on the wheel hub, which could be caused by a worn out wheel bearing or bent axle. A missing peak or peaks should alert the diagnostician to a physically damaged reluctor ring.

The 5 Critical Dimensions


IMPORTANT NOTES:

Peak voltages will vary a little from one type of sensor to the next. Also, since an integral part of the sensor is a coil, or winding, their failures can be very temperature or vibration dependent. In most of these cases the waveform will become much smaller or very deformed, and a DTC (Diagnostic Trouble Code) may set at the same time. In general, the most common ABS Wheel Speed Sensor failure by far is where the sensor won't generate a signal at all. This is not too interesting to look at, but when driving the vehicle, if the waveform stays flat, check the sensor and DSO connections, make sure the circuit isn't grounded, make sure the sensor air gap is correct, make sure the proper parts are spinning (reluctor ring present, etc.), then condemn the sensor.


For More Information

Description of Automotive Signals

Diagnosis Using A Labscope

Labscope Quick Reference Appendix