Speed sender

ABS/EBS sensors monitor wheel speed and provide an essential signal source for the anti-lock braking system, electronically controlled braking system and functions such as ASR, ESP and retarder control. On MAN TGL, TGM, TGA, TGS, TGX and TG3, Mercedes Atego, Axor, Actros, Arocs and Econic, as well as Citaro, Intouro, Tourismo and Neoplan buses, these wheel speed sensors work in conjunction with the EBS control unit and modulators. On vans such as the MAN TGE and Mercedes Sprinter, Vito and Vario, they are often linked to hydraulic ABS/ESP units.

The correct ABS/EBS sensor must always be determined using the vehicle chassis number. Visual comparison, cable colour, connector shape or selection based solely on the model name is never sufficient. Axles, braking systems, number of pins, cable lengths, sensor holders and reluctor rings may differ within a single vehicle series. This applies particularly to vehicles with different axle configurations, for example 4x2, 6x2, 6x4 and bus chassis with steered or driven rear axles.

The most common versions are passive inductive ABS sensors and active sensors with a Hall-effect or magnetoresistive measuring principle. A passive inductive sensor generates an alternating voltage as the toothed ring or reluctor ring rotates past the sensor tip; voltage and frequency increase with wheel speed. Active sensors generally receive a supply voltage from the control unit and return a digital signal. They can measure more accurately at low speeds and are sensitive to incorrect polarity, cable damage and an incorrect air gap.

On pneumatically braked trucks and buses with EBS, including those with MAN D08, D20, D26 or D38 engine families and Mercedes OM 906, OM 457, OM 470, OM 471 or OM 473 powertrains, sensors are assigned to the EBS ECU for each wheel position. Depending on the axle version, the sensor reads the reluctor ring in the wheel hub, brake disc assembly, axle hub or differential area. Older MAN F90, F2000 and M2000, and Mercedes SK, LN and Ecoliner vehicles may also have different sensor lengths, mounting bushes and connector connections.

An ABS or EBS warning light on the dashboard, disabled traction control, fault codes for a specific wheel position or a system that only reports a fault at low speed often indicates an abnormal wheel speed signal. Common causes include a broken cable near the steering knuckle or lift axle, corrosion in connectors, moisture ingress, damaged insulation, a worn sensor head or metal particles on the magnetic sensor tip. On drum brakes, rust, mud and deformation of the toothed ring can also cause an irregular signal.

First read out the faults using a compatible diagnostic device and note the wheel position, fault status and whether the fault is intermittent or permanent. Then inspect the wiring harness from the sensor to the chassis connector for tensile stress, abrasion marks and oxidation. For passive inductive sensors, measure the resistance according to the vehicle specification and, while rotating the wheel, use an oscilloscope or AC measurement range to check for a stable sine-wave signal. For active sensors, the supply voltage, earth and digital output signal must be checked; a simple resistance measurement is not reliable for this purpose.

Also compare all four or more wheel speeds via live data while driving slowly. An abnormal value on one wheel may be caused by the sensor, but also by a damaged reluctor ring, incorrect bearing fitment or excessive wheel bearing play. Therefore, check the radial play of the wheel hub and inspect the ring for missing teeth, cracks, corrosion and metal deposits.

Clean the sensor bore and use a suitable installation paste to prevent seizure in the sensor holder. Carefully press the sensor in until it contacts the reluctor ring; on many systems, the sensor automatically adjusts to the correct operating air gap during the first wheel rotations. Never strike the sensor housing or force the connector pins. Route the cable back into the original clips, allowing sufficient movement for steering and suspension travel.

Replace damaged sensor holders, cable clips or corroded connector housings at the same time to avoid recurring EBS faults. Braem can assist with chassis-number-based identification and technically suitable sensor versions, ensuring that the braking system can be correctly cleared, checked and released after installation.