ABS speed sensor

The purpose of this test is to evaluate the correct operation of an inductive style ABS sensor based upon output voltage and frequency in relation to the speed of the road wheel.

How to perform the test

Access to individual wheel speed sensors may be difficult.

All the wheel speed sensors are connected into the ABS control unit which is usually located in the engine bay.

Manufacturer data will be required to identify the speed sensor pins at the control unit multi-plug. Connect PicoScope by back pinning the suspect sensor(s) at the multi-plug.

Select GO on your screen.

Raise the suspect wheel(s) and rotate by hand, this will be sufficient to produce an output from a good speed sensor.

Example waveforms

Waveform notes

The waveform on Channel A shows a relatively steady voltage and frequency with no gaps. It is essentially a snapshot of the reluctor wheel.

Channel B has a suspect pattern with a possible broken tooth on the reluctor wheel.

Road Test

The ABS control unit continuously monitors and compares the 4 individual wheel speed signals.

Connect each wheel to a PicoScope Channel and road test the vehicle to record this comparison.

NOTE; Please be sure to place the screen out of the drivers view or take a second technician to record the results.

ABS 4 Wheels waveform

4 Wheels

Waveform Library

There are 12 examples in the Waveform Library.

Go to the drop down menu bar at the bottom left corner of the Waveform Library window and select "ABS or wheel speed sensor (analog/inductive)".

This example shows a faulty sensor or sensor circuit connection.

Front Hub ABS

Figure 2

ABS Castellated Drive Ring

Figure 3

Technical information

ABS has been a common safety-related fitment to vehicles since the early 1980s with systems from ATE, Bosch and Bendix. The systems all have a similar operational strategy and are all electromechanical.

The ABS ECM (Electronic Control Module) wants to see a continuous stream of sine waves for all four of the vehicle's wheels. If however the brakes are applied and a wheel 'locks', the ECM will lose the signal from the skidding wheel. If the ECM sees this happen, it will immediately release the hydraulic pressure to the wheel in question and very quickly 'pulse' the pressure within the hydraulic system to maximise braking efficiency.

A vehicle fitted with an ABS system has better braking abilities in adverse wet or slippery conditions. Some vehicles may have ABS fitted to only the front wheels of the vehicle.

The ABS sensors can also perform another role when the vehicle is fitted with traction control: instead of looking for a loss of signal from a 'locked' wheel, the traction control ECM looks at the frequencies of the signals to see if any of the wheels are spinning. If a spinning wheel is detected, the engine's power output is reduced until all frequencies from the ABS sensors are the same and traction is regained. Some systems momentarily apply the brake to the spinning wheel to aid traction on the other wheel.

Figure 2 shows a typical front hub ABS set up, where the driveshaft has a series of teeth on it and the speed sensor is mounted close to them.

Figure 3 shows an alternative set up using a castellated drive ring mounted on the wheel bearing. The inductive sensor is the built into a hub cap located on the inside of the vehicle's hub assembly.

There are several different connection methods, depending on whether you wish to look at an individual speed sensor or a pair of sensors. Also, you will need to decide if the sensor can be tested statically (with the vehicle on axle stands) or while on road test. The test point will differ from one system to another. Some may have fly-lead multi-plugs with easy access, and on others the wiring may be routed into the inner wing or bulkhead in such a way that no connections can be made. In this case you may need to locate the ABS electronic control module and probe the wiring there. Technical data will be needed in order to ensure the correct connections are made; not only that, but the right two sensor wires must be connected with the correct polarity if the correct signal is to be displayed on the oscilloscope.

The anti-lock Braking System (ABS) relies upon information coming in from the sensors fitted to the hub assemblies.

If under heavy braking the ABS Electronic Control Module (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 imperative that the sensors are capable of providing a signal to the ABS ECM.

The operation of an ABS sensor is not unlike that of a crank angle sensor, using a small pick-up that is affected by the movement of a phonic wheel moving close to it. The relationship between the phonic wheel and the sensor results in the production of a continuous alternating current (AC) 'sine wave' that can be monitored on an oscilloscope. The sensor, recognisable by its two electrical connections (some may have a coaxial braided outer shield), produces an output that can be monitored and measured on the oscilloscope.

AT003-4

Disclaimer
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.

Suitable accessories

  • Back-pinning Probe Set

    £48.00

  • Flexible Back-pinning Probe

    £4.25

  • Multimeter Probes

    £7.00

  • Small Crocodile/Gator Clips

    £2.75

  • PicoScope Battery Clip

    £2.75

  • Large Dolphin/Gator Clips

    £10.00

  • Premium Test Lead: BNC to 4 mm, 3 m

    £47.00

  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)

    £179.00

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Guided test: Analogue