Check and change ABS and wheel speed sensors

Operating principle
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).

Operating principle
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.

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:

• Read out the fault memory
• Check the supply voltages and signals using a multimeter and oscilloscope
• Visually inspect the wiring and mechanical assemblies

Diagnostic unit

• Read out the fault memory
• Evaluate parameters
• Compare and evaluate signals from individual wheel speed sensors

Multimeter

• Voltmeter (check the voltage supply)

Ohmmeter

• Check the internal resistance (the internal resistance cannot be measured on active sensors)

Oscilloscope

• Signal depiction (evaluation of signal curve)

• Proper documentation in the form of technical data
• A suitable diagnostic unit, multimeter, or oscilloscope
• Sufficient technical know-how of the technician and targeted employee training

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.

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:

• In order to allow correct allocation of the vehicle, it is important that the vehicle documents (registration documents) are included with the job sheet.
• Check the battery voltage. Insufficient voltage supply may cause system failure or result in incorrect measurements or voltage drops.
• Check the system-related fuses. A quick look in the fuse box might eliminate the first cause of the fault.