The purpose of this test is to evaluate the correct operation of a digital ABS wheel speed sensor based on the output voltage and frequency in relation to the speed of the road wheel.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
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.
Manufacturer data is required to identify the sensor pins at the ABS control module multi-plug.
PicoScope can be connected to gather waveforms from 2 or more wheel-speed sensors during a road test. In these conditions, the computer and screen must be out of the driver's view and/or operated by a second technician.
The digital ABS sensor is a simple digital on/off switch which produces a square wave output. It is this output signal that is recognised and processed by the ECM. The characteristic squarewave, shown in the example waveform, switches from zero volts to 1 volt. Other vehicle manufacturers' systems may have a different operating voltage and it will be necessary to refer to specific technical data.
The Anti-lock Braking System (ABS) relies upon information coming in from the sensors fitted to the hub assemblies and transmitted to the vehicles ABS ECM.
If, under heavy braking, the ABS ECM loses a signal from one of the road wheels, it assumes that the wheel has locked and releases that brake momentarily until it sees the signal return. It is therefore important that the sensors are capable of providing a signal to the ABS ECM.
The sensor usually has three connections: a voltage supply, an earth and the output signal. There may also be an additional earthed coaxial cable, whose purpose is to inhibit spurious signals being introduced into the sensor's output. The squarewave, as seen an oscilloscope, will vary in amplitude. The amplitude is, however, not as important as the frequency, as it is the frequency that is monitored, not the voltage. The digital ABS sensors output will produce the same voltage regardless of road speed, unlike an inductive sensor whose voltage will increase proportionate to rotation speed.
A digital ABS sensor includes a semiconductor that acts as either a conductor or an insulator depending on whether it detects a magnetic field. This magnetic field is switched on and off by a rotating wheel that is located within the wheel hub assembly. When a magnetic field passes through the semiconductor, it stops the current through the device. When the field is switched off, the current is reinstated. This action produces a digital square wave which is understood by the ABS ECM.
ABS sensor outputs can also be used to measure wheel spin or slip when the vehicle is accelerating, and this information is then fed into the traction control unit.
The output can also be measured on a multimeter that has a frequency function.
An ABS sensor with an output at a constant zero voltage should first of all be tested to ensure that it has a voltage supply. If the voltage is present and comparable to the other ABS sensor supplies on the vehicle, the sensor should be changed. Similar failed outputs will be seen if the gap between the sensor and the rotating wheel (within the hub) is too large for the magnetic field to have an effect. If the voltage at the sensors output fails to switch and is the same as the input voltage, the sensor has lost its earth return.
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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