One of the most common problems that can trigger the ABS warning light to come on is a faulty ABS wheel speed sensor.
An ABS wheel speed sensor counts wheel revolutions. It reads the wheel rotation and measures it in revolutions per minute (RPMs). It should not be confused with the gauge on the dashboard that displays the engine RPM via the tachometer.
The most common sign of a faulty wheel speed sensor includes the ABS light coming on but other symptoms include:
- Traction Control Light On
- Stability Control Light On
- Transmission Won't Shift Gears
- Vehicle in Limp Mode
The most common problem that causes an ABS sensor to malfunction is dirt at the tip of the sensor. Other issues that trigger an ABS warning light to come on include:
- ABS Sensor Defective
- Loose or broken ABS wheel speed sensor wire
- Defective ABS relay or a blown fuse
- Bad ABS Module
- Burned ABS motor
- Damaged wires
- Low battery voltage
One or more wheel speed sensors (ABS sensors) may fail to render the ABS inoperable and triggering multiple warning lights on the dashboard. Some things that you may notice are:
- ABS system disabled, as well as disabled electronic stability control (ESC) or traction control systems, if the vehicle is equipped with them, as these systems rely on the same sensors as the ABS system.
- ABS warning light illuminated on the dashboard.
- Occasionally, the speedometer will stop working and the check engine light will illuminate.
Where is the ABS sensor located?
In a system with four sensors, a sensor can be found at each wheel behind the rotor.
In a system with three sensors, in addition to a sensor located at each front wheel, the third sensor for the rear is located in the rear axle, typically.
The type of ABS system a vehicle has will indicate how many sensors are on the vehicle. Older passenger cars will have alternative systems, but these listed below are the four basic types.
- Four sensor ABS: one sensor for all four wheels, most common setup.
- Four sensors, three-channel ABS: one sensor for all four wheels, but there is one channel controlling the rear wheels.
- Three sensors, three-channel ABS: one sensor on each front wheel, but only one sensor for both of the rear wheels - in this case, the sensor is located in the rear axle. In this system, both rear wheels are monitored together - they both have to lock up before the ABS is engaged.
- Two-channel, four-sensor ABS: there is a sensor at each wheel, but there is one channel for each end of the car - one at the front, one at the rear. If the speed sensor at any wheel detects wheel lockup, the ABS will engage for that end of the car.
Troubleshooting an ABS Wheel Speed Sensor
To diagnose ABS problem you will need an OBD-II scanner that can read codes from the ABS module. Below are a few examples of multi-system OBD II scanners that can diagnose ABS faults on many makes and models:
- Popular Scanner
- Inexpensive ABS Scanner
- Professional Level
It is important to check all speed sensors for functionality as it is possible only one sensor may need replacing. Also, it is possible that the sensor may not need to be replaced at all and cleaning the sensor is all that is needed. If the ABS or stability control lights remain on after cleaning the sensor comprehensive diagnostics should be conducted.
Keep in mind that road debris, dirt, and grease at the tip of the ABS sensor can prevent the ABS system from operating properly. If this is the case, you can typically clean the sensor and reinstall it instead of replacing it.
Secondly, wheel speed sensors can be accidentally and unknowingly damaged in other ways, especially if other automotive repair work has been done in the wheel area, such as brake work. If this is the case, the vehicle should be brought back to the repair shop that performed the repairs to be inspected for damage.
If cleaning the ABS sensor does not fix your problem it is time to scan the fault codes from the ABS control unit using a scanner that can read ABS codes.
A common problem with a malfunctioning wheel speed sensor is with the wiring harness that is connected to the sensor, which transmits the speed data to the ABS control unit. First, visually inspect the ABS sensor harness.
Replacing a wheel ABS sensor on most vehicles is a very easy procedure that most DIYers can complete in about 30 to 60 minutes. Watch the video below for an example on how to change a wheel ABS sensor on a car.
- Jack up the vehicle. Set the parking brakes. Secure the vehicle with jack stands
- Remove the wheel that has a bad ABS sensor.
- Locate the ABS sensor which is mounted on the wheel hub.
- Remove the bolt that holds the ABS sensor in place. There is only one bolt. It may be rusted. If that's the case, spray penetrating oil and wait 15 minutes.
- Remove the sensor from the wheel hub.
- Disconnect the wire harness.
- Install new ABS sensor in reverse order.
Depending on where the customer resides and what kind of vehicle they drive, a wheel speed sensor replacement can cost anywhere from under $200 to between $300-400, including parts and labor.
Labor rates will vary among repair shops, and the cost of replacing the speed sensor or any additional wiring will depend on the type of vehicle.
Car owners will save between $100-150 replacing the part themselves, although there work involved and some portions of the repair require special tools, which could factor into the cost as well if someone were to repair it at home.