Speed sender

ABS (Anti-lock Braking System) and EBS (Electronic Braking System) use sensor signals to detect wheel slip and control braking interventions. Wheel speed sensors measure the rotation of individual wheels; brake pressure sensors provide feedback on pneumatic brake pressure in EBS installations equipped with them. Signal loss can cause warning messages, unwanted intervention at low speeds or deactivation of supporting braking functions. However, a sensor fault code does not prove that the sensing element is faulty: wiring, tone rings and wheel bearings also affect measurement quality.

ABS/EBS sensors are part of the braking system. Depending on the vehicle specification and braking system, they may be fitted in the following locations:

1. At the front axle wheel hubs: wheel speed sensors measure the rotation of the respective wheels. The brake control unit uses these signals to detect impending wheel lock and regulate brake pressure.
2. At the wheel hubs of driven axles, tag axles and pusher axles: wheel speed sensors provide signals for ABS/EBS control at the respective wheel positions. Not every axle or wheel has a separate sensor on every vehicle variant.
3. At the wheel hubs of trailers and semi-trailers: wheel speed sensors transmit wheel rotation data to the towed vehicle's ABS/EBS, allowing it to adjust brake pressure when wheel lock is imminent.
4. In brake modulators or at designated measuring points in pneumatic brake circuits: brake pressure sensors measure the actual brake pressure. The EBS uses this feedback to compare the requested pressure with the pressure actually generated and make adjustments.
5. In the brake pedal module or brake signal transmitter of EBS systems equipped with them: integrated sensors register the driver's braking demand, for example through pedal travel or pressure. The brake control unit uses this signal to determine the required braking effect.

Always identify ABS/EBS sensors using the vehicle's chassis number. Visual comparison or selection based solely on the model name is never sufficient. For example, on MAN TGL, TGS and TGX, Mercedes Atego and Actros, buses such as Lion’s City and Citaro, and vans such as TGE and Sprinter, sensor technology, connections and installation positions vary by vehicle specification. Axle configuration, braking system generation and wheel hub design determine sensor selection.

Passive inductive wheel speed sensors work with a toothed ferromagnetic tone ring and generate an AC voltage whose frequency and amplitude depend on rotational speed. Active Hall-effect or magnetoresistive sensors require a power supply and work with a magnetic encoder ring or a suitable toothed ring, depending on the design. Two connecting wires therefore do not automatically mean that a sensor is passive.

Check the mounting diameter, insertion length, cable routing, connector keying, pin assignment and specified air gap. For brake pressure sensors, the measuring range, permissible overpressure, supply voltage, signal characteristics, thread and seal material are decisive. Not every pressure sensor can be replaced separately; some are integrated into a brake modulator. Use only the pressure values and fit tolerances specified for the relevant variant.

Recurring faults after work on wheel hubs often indicate an incorrect sensor gap, damaged encoder rings or incorrectly installed wheel bearings. Intermittent warnings during steering or suspension compression are consistent with broken wires and strain on connectors. Corrosion beneath the sensor holder can displace the sensing element; metal particles on the sensor head can disrupt the signal.

1. Read fault codes and live data from the brake control unit. Record the affected wheel position, vehicle speed and operating conditions. Distinguish open-circuit faults from plausibility faults.
2. Secure the vehicle safely and support it in accordance with workshop instructions. Check cable sheathing, connector seals, contact pins and retaining clips. Measure the supply voltage and voltage drop in accordance with the wiring diagram.
3. With passive sensors disconnected, measure their resistance using a multimeter and compare it with the manufacturer's specified value. Then check the AC voltage signal with an oscilloscope during controlled rotation. Do not use resistance measurement to assess active sensors.
4. Check active sensors for power supply and current or voltage modulation using a suitable test adapter. Compare wheel speed readings at the same rotational speed. Investigate periodic signal loss for damaged teeth, encoder defects or excessive runout; measure wheel bearing play with a dial indicator.
5. When investigating brake pressure issues, compare the pressure readout with a calibrated pressure gauge at the specified test point. Check several pressure levels and pressure build-up before condemning a brake pressure sensor.

Clean the sensor bore and check the clamping sleeve, if fitted, for corrosion and clamping force. Use only the specified assembly products and adjust the sensor position in accordance with the vehicle instructions: a self-adjusting inductive design requires a different procedure from a bolt-mounted active sensor. Keep cables clear of brake discs, sharp edges and moving suspension components.

Depressurise pneumatic circuits before removing brake pressure sensors. Observe sealing requirements and tightening torques. After the repair, clear the fault codes, carry out the specified calibrations and check live data during a safe functional test. Confirm that fault messages do not return before returning trucks, buses and vans to service.