Here you will find useful information and important tips relating to the ABS sensor and wheel speed sensor in vehicles.
Important safety note
The increasing complexity of the traffic situation on our roads is placing high demands on car drivers. Driver assistance systems relieve the burden on the driver and optimise road safety. As a result, state-of-the-art driving assistance systems are now included as standard on virtually all new European vehicles. This also means that workshops are being faced with new challenges.
Nowadays, the vehicle electronics play a key role in all comfort and safety equipment. Optimal interaction between complex electronic systems ensures that the vehicle operates without problems, and this, in turn, increases road safety.
Intelligent communication of data between electronic vehicle systems is supported by sensors. When it comes to driving safety, speed sensors play a particularly important role, and this is reflected by their varied use in a number of different
They are used by the control units in driving assistance systems such as ABS, TCS, ESP, or ACC in order to detect the wheel speed.
The wheel speed information is also provided to other systems (engine, transmission, navigation, and chassis control systems) via data lines by the ABS control unit.
As a result of their varied use, speed sensors contribute directly to the driving dynamics, driving safety, driving comfort, lower fuel consumption, and lower emissions. Wheel speed sensors are often also called ABS sensors as they were used in vehicles for the first time when ABS was introduced.
Wheel speed sensors can be designed as active or passive sensors, depending on how they operate. A clear and precise way of distinguishing or categorising them has not been defined.
The following strategy has therefore proven useful in day-to-day workshop activities:
The wheel speed sensors are positioned directly above the impulse wheel, which is connected to the wheel hub or drive shaft. The pole pin, surrounded by a winding, connects to a permanent magnet whose magnetic effect extends to the pole wheel. The rotation of the impulse wheel and the resulting switching from tooth to tooth space brings about a change in the magnetic flow caused by the pole pin and winding. This changing magnetic field induces a quantifiable, or measurable alternating voltage in the winding (Figure 1).
The frequency and amplitudes of this alternating voltage are in relation to the wheel speed. Inductive passive sensors do not require a separate power supply from the control unit. Since the signal range for signal detection is defined by the control unit, the amplitude level must be within a specific voltage range. Gap (A) between sensor and impulse wheel is provided by way of the axle design (Figure 2).
The active sensor is a proximity sensor with integrated electronics that is supplied with a defined voltage from the ABS control unit. A multipole ring can be used as an impulse wheel while at the same time being integrated in a sealing ring of a wheel bearing. Inserted in this sealing ring are magnets with alternating pole directions (Figure 3). The magneto-resistive resistors integrated in the electronic circuit of the sensor detect an alternating magnetic field when the multipole ring rotates. This sinusoidal signal is converted by the electronics in the sensor into a digital signal (Figure 4). It is then transferred to the control unit as a current signal using the pulse-width modulation method.
The sensor is connected to the control unit via a two-pole electric connecting cable. The sensor signal is also transmitted at the same time over a power supply line. The other line is used as a sensor ground. In addition to magneto-resistive sensor elements, nowadays Hall sensor elements are also fitted that permit a larger air gap and respond to the smallest of changes in the magnetic field. If a steel impulse wheel is installed in a vehicle in place of a multipole ring, a magnet is also affixed to the sensor element. When the impulse wheel turns, the constant magnetic field in the sensor changes. The signal processing and IC are identical to the magneto-resistive sensor.
The following system characteristics can be recognised when wheel speed sensors fail:
Causes of failure:
Generally speaking, a malfunction of the ABS/TCS/ESP braking system occurs before the wheel speed sensors are tested.
After the warning light illuminates, the following options are available for troubleshooting and diagnostics:
Troubleshooting wheel speed sensors can be difficult when it comes to differentiating between active and passive sensors as these sensors cannot always be easily distinguished by appearance. Here, specific manufacturers' specifications and the specifications of the respective vehicle manufacturer must be consulted. Due to their advantageous technical characteristics such as accuracy and low structural size, vehicle manufacturers have mainly installed active wheel sensors since 1998.
When performing diagnostics on complex systems, the best technology alone cannot help repair the vehicle. Only well-trained personnel can prevent the random replacement of system components, avoid malfunctions in workshop processes, and strengthen the trusting relationship with the customer.
Observe the following general repair notes:
Repair work to brake systems may only be carried out by qualified specialists.
When carrying out any repairs on the brake system, you must follow the maintenance and safety instructions of the vehicle manufacturer and the product-specific assembly instructions
Using the following example of "Rear left speed sensor faulty", we will explain the diagnostics work required for an active wheel speed sensor. Your customer has reported a functional problem with the ABS system. The ABS warning light comes on while driving.
In this example, the repair procedure is shown based on a BMW E46. The schematic illustrations, figures, and descriptions are intended solely as explanations of the document text, and cannot be used as the basis for carrying out installation and repair work.
Preparing the diagnostics:
If the signal curve does not show any irregularities, first delete the fault. Once this is done, carry out a test drive with the diagnostic unit connected. In the process we recommend evaluating the parameters at the same time so that the cause can be narrowed down if the fault occurs again.
Due to the connector design, it is very difficult to take a reliable measurement directly at the plug contacts. It may be helpful to produce an adapter from an old identical sensor.
In our example, a defect has been found on the sensor cable. A cable break in the supply cable, caused by mechanical damage, resulted in a loose connection on the connector housing.
Please note the vehicle manufacturer's information when carrying out any testing and diagnostic work. Depending on the manufacturer, additional vehicle-specific testing methods may have to be taken into consideration. Ensuring optimal interaction between humans and technology has become more important than ever. Only workshop specialists with the latest expertise and who know how to optimally use their technical equipment will be ready to tackle the challenges of the future.
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