What module controls wheel speed sensor?

Signs of a Failing ABS Control Module

All new cars sold in the U.S. since 2013 have come with a mandated anti-lock braking system (ABS). The technology reduces wheel lockup during braking by controlling brake pressure to each wheel.

At the center of the system is a computer that controls the system, called the ABS module. The module may also be involved with traction or stability control, hill start assist and even adaptive cruise control.

Signs of a failing ABS module

A faulty ABS module can cause a number of problems. Some of the most common include:

• Illuminated warning lights: A failed ABS module may trigger many warning lights, such as the ABS indicator, red brake light, traction control light and more.
• Loss of antilock braking ability: An ABS module that has gone bad will usually prevent anti-lock braking from working.
• Unintended ABS activation: On the other hand, a bad ABS module may instead activate the anti-lock brakes unintentionally, resulting in random pedal pulsation.
• Traction control can’t be disabled: If the ABS module can’t make proper decisions, the vehicle may go into a default mode where traction control is on all the time.
• Driver assistance functions don’t work: Automatic braking, adaptive cruise control and other assistance features may not work if the ABS module is broken.

How to fix the problem

In some cases, a problematic ABS module may just need to be reprogrammed. Certified mechanics have access to service information that tells them whether a module’s software needs to be updated.

If the module can’t be reprogrammed, it will need to be replaced. This can get pricey. Sometimes, the module can be serviced separately from the ABS hydraulic control unit. In other instances, however, the two are integrated and must be replaced together. On some vehicles, the unit is combined with the brake master cylinder and brake booster as well, making replacement of those items necessary.

Some ABS modules may also need to be programmed or calibrated after replacement. And if the module is paired with the hydraulic control unit or master cylinder, the brake system will need to be bled of air once the job is complete.

How ABS systems work

To further understand the role of the ABS module, it helps to know how anti-lock braking works. All systems have a few essential components: wheel speed sensors, a hydraulic control unit, and the ABS control module.

The wheel speed sensors measure how fast each wheel is turning and transmit the data to the ABS control module.

If the module determines that wheel lockup is about to occur, it activates solenoids within the hydraulic control unit to adjust brake fluid pressure to the slipping wheel or wheels. As a result, the braking force applied to the slipping wheel is reduced, letting it regain traction to prevent lockup.

On most vehicles, the traction control system uses the same components to prevent wheel spin during acceleration.

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

Wheel speed sensor (Hall effect)

The purpose of this test is to evaluate the operation of an Antilock Braking System (ABS) Hall effect wheel speed sensor based upon its output voltage and frequency.

How to perform the test

Access to individual wheel-speed sensors may be difficult.

All wheel-speed sensors connect to the ABS control module, which is usually located in the engine bay.

1. Use manufacturer’s data to identify the wheel speed sensor output circuits.
2. Connect PicoScope Channel A to the sensor signal circuit.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Turn the ignition on but do not start the engine.
5. Start the scope to see live data.
6. With the suspect wheel raised, rotate it by hand. This will be sufficient to produce an output from a good speed sensor.
7. With your waveform on screen stop the scope.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

PicoScope sets Channel A and Channel B : connect both channels to have comparative waveforms.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

• In this example, Channel A is connected to the front left wheel speed sensor and Channel B to the front right.
• A square wave structure with the voltages switching between two, fixed, levels, respectively at 0.45 V and 1.0 V (note: voltage levels will vary across different manufacturers).
• The square wave frequency increases in proportion to the wheel speed.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select ABS or wheel speed sensor (digital / hall effect).

Further guidance

Wheel speed sensors provide wheel and road speed feedback to ABS and derivative active vehicle safety systems (i.e. stability and traction control etc.).

These systems are designed to provide corrective action (e.g. wheel braking or engine torque limitation) when the vehicle chassis or wheel speeds have exceeded their normal operational tolerances, for example, during conditions of wheel slip, oversteer or understeer etc. Wheel speed sensors are critical to the operation of these systems and, therefore, the safe handling of a vehicle.

A Hall effect sensor is a semiconductor device that creates a voltage output in proportion to an applied magnetic field. A Hall effect wheel speed sensor uses this technology to produce a square wave output in response to the magnetic field disturbances caused by a rotating pulse wheel mounted around a hub or driveshaft.

Hall effect wheel speed sensors require power to operate; hence they are referred to as active sensors. These sensors always have a positive supply voltage, usually 5 V, on one terminal; however, they may have one of two terminal configurations:

• Three pin sensors have a dedicated earth reference circuit and a separate output signal circuit.
• Two pin sensors have a combined signal and earth reference circuit, where the earth reference for the sensor floats at the voltage level of the signal.

It is not easily possible to distinguish two wire passive (inductive) and active (Hall or magnetoresistive) wheel speed sensors by their external appearance. Diagnostically, this is inconvenient as active wheel speed sensors must never be subjected to resistance tests: this can damage these units, with the only remedy being the acquisition of a new replacement.

Therefore, you must always either consult manufacturer’s data to identify the fitted type prior to diagnostic testing or carry out a speculative PicoScope check to identify the sensor from its output voltage characteristics.

You can check for a positive supply voltage at one of the sensor connector terminals to determine if you have an active sensor type. However, if the supply voltage is missing due to a fault and you then assume the sensor must be passive and perform a resistance check, you can damage a perfectly good active sensor. This will give you two faults.

An ABS control module expects similar (within a given tolerance) square wave frequencies from all the vehicle’s wheel speed sensors and uses any differences to calculate the timing and scale of its interventions.

If one, or more, wheel speed signals continuously fall outside of normal parameters the control module may turn the ABS function off (along with associated traction and stability systems). A driver warning light will be illuminated but, as with any electrical fault on ABS, normal hydraulic braking is maintained.

Wheel speed sensors and their pulse rings are exposed to the atmosphere and have to operate under conditions of constant vibration and movement. As such, common faults are:

• Sensor signal failures, caused by chafed or fractured circuit wiring, sensor or connector corrosion, or incorrect sensor fitment.
• Pulse wheel related problems arising from corroded or damaged teeth, incorrect pulse ring fitment (misalignment), contamination from a build-up of ferrous material, or an excessive air gap between the sensor and the pulse wheel.

Wheel speed sensor circuits and connectors are also prone to the atmosphere and possible electrical failures, such as open or short circuits or high circuit resistances.

Active wheel speed sensors are susceptible to damage from incorrect testing methods; in particular, the application of resistance testing.

Symptoms of ABS sensor related faults

• ABS warning light (and other associated warning lights) illumination.
• Diagnostic Trouble Codes (DTCs).
• Brake pedal pulsation or vibration.
• ABS inactive.

Diagnostic trouble codes

Selection of component related Diagnostic Trouble Codes (DTCs):