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.
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 ABS control unit multi-plug.
The waveform on Channel A shows a relatively steady voltage and frequency with no gaps.This displays a constant road wheel speed and good reluctor wheel.
Channel B has a suspect pattern with possible damage on the reluctor wheel.
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 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 ECU (Electronic Control Unit) 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 ECU will lose the signal from that wheel. If the ECU 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 ECU looks for increased individual frequencies as an indication of wheels spinning, losing grip. If a spinning wheel is detected, the engine 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 through differential action.
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 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 a wheel free lift) 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 the two sensor wires must be connected with the correct polarity for the correct signal is to be displayed on the oscilloscope.
If the wheel speed signals fall outside of normal parameters then the ABS function may be turned off. A driver warning light will be illuminated and as with any electrical fault on ABS, normal hydraulic braking is maintained.
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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