You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.
There are several different connection methods, depending on whether you wish to look at an individual speed sensor or multiple sensors. You will also need to decide whether the sensor can be tested statically, with the vehicle on axle stands, or whilst on road test. The test point will depend on the type of system, some having fly-lead multi-plugs with easy access, others with wiring that may be routed into the inner wing or bulkhead so that no connections can be made. In this circumstance you may need to locate the ABS Electronic Control Module (ECM) and probe the wiring there. Technical data will be necessary in order to ensure the correct connections are made.
If initial diagnosis is that the speed sensor has completely failed, a static test may be appropriate. To perform this test, jack up the road wheel on the hub to be tested and place the vehicle on axle stands.
Plug a BNC test lead into Channel A on the scope, place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe onto the test lead with the coloured moulding (positive).
Probe either the desired ABS ECM terminal or a multi-plug connector, and connect the clip to a suitable earth.
Press the space bar on the computer to start looking at live readings and spin the wheel by hand to see if a signal is generated.
For live readings whilst on road test, route the BNC test leads into the cabin area of the vehicle, ensuring that any test leads are kept away from moving or hot components. You can also set up further Channels in the same way to monitor more than one wheel speed sensor at the same time.
Figure 1 shows a ramp-lock multi-plug connect typical of those that connect to an ABS ECM. To remove the multi-plug, lift the locking lever and pull the multi-plug away from the ABS ECM as in Figure 2. The wiring cover can be removed by releasing four locking tabs, as in Figure 3. After checking the manufacturer's data, probe the desired terminals and refit the multi-plug as in Figure 4. This image also shows the earth connection and the second BNC test lead stacked on the first.
In the sample waveform, two connections were made to the ABS ECM so that the two front speed sensors could be monitored. The resultant waveform is shown below. Whilst on road test you will notice the frequencies change as you corner.
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.
|3||Signal, processed, wheel speed rear right|
|4||Signal, DSC/DCT button|
|5||Signal, brake fluid level|
|6||Signal, wheel front right|
|8||Signal, break pad wear sensor|
|14||Signal, F-CAN low|
|18||Voltage supply front right wheel speed sensor|
|19||Voltage supply rear right wheel speed sensor|
|20||Signal, rear left wheel speed sensor|
|21||Voltage supply front left wheel speed sensor|
|22||Signal, front left wheel speed sensor|
|24||Signal, PT-CAN low|
|26||Signal, F_CAN high|
|27||Signal, processed, wheel speed front right|
|28||Signal, processed, wheel speed front left|
|29||Signal, brake pads wear sensor|
|30||Break light switch|
|31||Signal, rear right wheel speed|
|33||Voltage supply rear left wheel speed sensor|
|35||Wake-up signal, terminal 15|
|36||Signal, proccessed, wheel speed rear left|
Our example waveform was taken from a BMW 5 series. Its system is referred to as Dynamic Stability Control (DSC). An example pin configuration is shown below. Refer to manufacturers specific data for correct terminal identification.
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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