Brake sensors

Brake sensors provide the measured values used by ABS (Anti-lock Braking System), EBS (Electronic Braking System) and ECU (Electronic Control Unit) to monitor brake pressure, wheel slip and brake wear. This subcategory mainly includes ABS wheel speed sensors and brake pad wear sensors, supplemented by versions with specific plug connections, cable protection and installation lengths. They are essential for safe brake control on MAN TGL, TGM, TGA, TGS, TGX and TG3, as well as on Mercedes Atego, Axor, Actros, Arocs, Antos and Econic. Buses such as the MAN Lion’s City, Lion’s Coach, Mercedes Citaro and Tourismo, Neoplan coaches and vans such as the MAN TGE, Mercedes Sprinter and Vito also use application-specific brake sensors.

• ABS wheel speed sensors measure the rotational speed of each wheel.
• Brake pad wear sensors monitor the remaining friction material thickness of brake pads.
• Brake pressure sensors measure the hydraulic or pneumatic pressure in the brake circuit.
• Air pressure sensors monitor the pressure in air reservoirs, brake circuits and the compressed air supply.
• Brake pedal position sensors and brake pedal travel sensors register brake pedal operation.
• Brake light switches signal brake pedal application to the ECU and brake lights.
• Brake fluid level sensors monitor the level in the brake fluid reservoir of hydraulic braking systems.
• Axle load sensors measure vehicle or axle load for adjusted brake force distribution.
• Yaw rate sensors, longitudinal acceleration sensors and lateral acceleration sensors provide data for stability and brake control systems.
• On certain versions, temperature sensors monitor the temperature of brakes, brake discs or brake components.

An ABS sensor measures the rotational speed of a wheel via a pulse ring or toothed ring in the wheel hub. Depending on the vehicle series, this may be a passive inductive sensor or an active Hall sensor. Passive versions produce an alternating voltage signal whose amplitude increases with wheel speed; active sensors, on the other hand, require a supply voltage and provide a digital signal. The two types are not interchangeable. The sensor head, nominal air gap to the pulse ring, polarity, cable design and connection to the EBS wiring harness must match exactly.

Brake pad wear sensors monitor the remaining friction material thickness of brake pads in disc brakes and brake linings in drum brakes. In certain systems, an electrical conductor is interrupted once the specified wear limit has been reached; other variants transmit an analogue or multi-stage signal. A faulty sensor can therefore trigger a warning while the brake pads are still sufficiently thick, but it must never be a reason not to mechanically check the actual lining thickness.

Vans with hydraulic braking systems, such as the Mercedes Sprinter, Citan and MAN TGE, often have different connectors and cable routing than pneumatically braked trucks and buses. In heavy-duty EBS applications, axle position, left/right installation and the combination with the brake caliper or wheel hub are also decisive. Sensors must always be identified using the chassis number. Visual comparison or selection based solely on the model name is never sufficient.

An illuminated ABS, EBS or brake wear warning light on the dashboard, disabled traction control, fault codes for a specific wheel position or a speed reading of 0 km/h while driving often indicate a sensor issue. Intermittent faults are frequently caused by broken wiring near the steering knuckle, stone-chip damage, moisture in the connector or corroded contacts. On vehicles with hub bearings, excessive radial wheel bearing play can also alter the air gap and cause an unstable signal.

1. Read fault codes and live data using a suitable diagnostic tester. Compare wheel speeds during a short test drive or while turning the raised wheel.
2. Check the cable sheath, connector locking mechanism, sealing rings and pins for corrosion, tensile stress and oxidation. On passive sensors, measure resistance and compare it with the manufacturer's specification; on active Hall sensors, check the supply voltage, earth and signal.
3. Only remove the sensor after inspecting the pulse ring. Check for rust, missing teeth, metal particles, deformation or a damaged magnetic encoder bearing.
4. Measure bearing and hub play according to specifications. A new sensor will not resolve a fault if the sensor distance is out of tolerance due to worn roller bearings.

Clean the sensor bore carefully without reaming out or enlarging the seat. Install the sensor straight, without applying force to the sensor head, and follow the specified installation position and cable routing. On self-adjusting ABS sensors, the correct air gap is generally set when the sensor lightly touches the pulse ring and retracts while driving; always follow the vehicle-specific procedure. Secure cables in the original clips, away from brake discs, drums, moving steering components and heat sources.

Replace brake pad wear sensors preventively when the cable insulation is brittle, the sensor tip is worn down or brake pads are being renewed. A suitable sensor and fast technical support limit unnecessary downtime, while correct diagnostics prevent EBS valves, brake calipers or wheel bearings from being replaced unnecessarily.