ABS speed sensor

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

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.

Static vehicle test

  1. Jack up the road wheel on the hub to be tested and place the vehicle on axle stands.
  2. Plug a BNC test lead into Channel A on the PicoScope.
  3. Place a black clip on the black (negative) plug on the BNC test lead.
  4. Place a red clip on the colored (positive) plug on the BNC test lead.
  5. Disconnect the ABS speed sensor at the connecting socket (or locate the correct wires at the ABS control module) and place the clips onto the sensor's fly-lead.

Press the space bar on the computer to start looking at live readings. Spin the wheel either by hand, or, if it is a driven wheel, by carefully running the engine and selecting a suitable gear.

Road test

For live readings whilst on road test, leave the sensor's fly-lead connected (or make the connections at the ABS control module) and use the Back-pinning Probe or multimeter probes to connect to the two inductive wires. It is also possible to monitor two hubs at the same time by setting up Channel B. Ensure that any test leads are kept away from moving or hot components.

As you will see in the PicoScope settings and in the example waveform on this page, it is essential that the voltage range is set to alternating current (AC) for this test.

Waveform notes

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.

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.

AT003-3

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

    £7.00

  • Multimeter Probes

    £7.00

  • Small Crocodile/Gator Clips

    £5.00

  • PicoScope Battery Clip

    £5.00

  • Large Dolphin/Gator Clips

    £10.00

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

    £48.00

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

    £179.00

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Guided test: Analogue