The purpose of this test is to evaluate the operation of a digital Antilock Brake System (ABS) wheel speed sensor based on the output voltage and frequency.
Access to individual wheel-speed sensors may be difficult.
All wheel-speed sensors connect to the ABS control module, which is usually located in the engine bay.
Use manufacturer data to identify the wheel speed sensor circuits.
Connect PicoScope to the suspect sensor(s).
Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
Turn the ignition on but do not start the engine.
Start the scope to see live data.
With the suspect wheel(s) raised, rotate it by hand. This will be sufficient to produce an output from a good speed sensor.
With your live waveforms on screen stop the scope..
Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.
PicoScope can be connected to gather waveforms from driven wheels. In this case, the vehicle should be securely fixed onto a wheel free lift in order to drive the wheels to obtain waveforms.
In this example, Channel A is connected to the front left wheel and Channel B to the front right.
These known good waveforms have the following characteristics:
A square wave structure with the voltages switching between two, fixed, levels, respectively at 0.45 V and 1.0 V (note: voltage levels will vary across different manufacturers).
The square wave frequency increases in proportion to the wheel speed.
Wheel speed sensors provide wheel and road speed feedback to ABS and their derivative active vehicle safety systems (i.e. stability and traction control systems etc.).
These systems are designed to provide corrective action (e.g. wheel braking or engine torque limitation) when the vehicle or wheel speed(s) have exceeded their normal operational tolerances, for example, during conditions of wheel slip, oversteer or understeer etc. Wheel speed sensors are critical to the operation of these systems and, therefore, the safe handling of a vehicle.
A digital ABS sensor includes a semiconductor (Hall effect sensor) that acts as either a conductor or an insulator depending on the orientation of any nearby magnetic field. When in the presence of an alternating magnetic field, as provided by a multipole rotor ring mounted in a wheel bearing seal or hub assembly, the sensor switches its output on and off. This action produces a digital square wave which is received by the ABS control module.
Digital wheel speed sensors require power to operate; hence they are referred to as active sensors. These sensors always have a positive supply voltage on one terminal; however, they may have one of two terminal configurations:
An ABS control module expects similar (within a given tolerance) square wave frequencies from all the vehicle’s wheel speed sensors and uses any differences to calculate the timing and scale of its interventions.
If one, or more, wheel speed signals continuously falls outside of normal parameters, the control module may turn the ABS function off. A driver warning light will be illuminated but, as with any electrical fault on ABS, normal hydraulic braking is maintained.
Wheel speed sensors and their multipole rotor rings are exposed to the elements and have to operate under conditions of constant vibration and movement. As such, common faults are:
Selection of component related Diagnostic Trouble Codes (DTCs):
C0000 - Vehicle Speed Information Circuit Malfunction
C0035 - Left Front Wheel Speed Circuit Malfunction
C0036 - Left Front Wheel Speed Sensor Circuit Range/Performance
C0040 - Right Front Wheel Speed Circuit Malfunction
C0041 - Right Front Wheel Speed Sensor Circuit Range/Performance
C0045 - Left Rear Wheel Speed Circuit Malfunction
C0046 - Left Rear Wheel Speed Sensor Circuit Range/Performance
C0050 - Right Rear Wheel Speed Circuit Malfunction
C0051 - Right Rear Wheel Speed Sensor Circuit Range/Performance
C0221 - Right Front Wheel Speed Sensor Circuit Open
C0222 - Right Front Wheel Speed Signal Missing
C0223 - Right Front Wheel Speed Signal Erratic
C0225 - Left Front Wheel Speed Sensor Circuit Open
C0226 - Left Front Wheel Speed Signal Missing
C0227 - Left Front Wheel Speed Signal Erratic
C0229 - Drop Out of Front Wheel Speed Signals
C0235 - Rear Wheel Speed Signal Circuit Open
C0236 - Rear Wheel Speed Signal Circuit Missing
C0237 - Rear Wheel Speed Signal Erratic
C0238 - Wheel Speed Mismatch
C0245 - Wheel Speed Sensor Frequency Error
C0300 - Rear Speed Sensor Malfunction
C0305 - Front Speed Sensor Malfunction
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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