ABS Wheel Speed Sensor Problem
Updated Friday Jul 07, 2017
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.
A wheel speed sensor 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 wheel speed sensor, measures wheel rotation for individual wheels.
It is located a toothed gear near the brake mechanism/wheel hub assembly. At the other end of the sensor is a signal wire, which leads back to the vehicle?s electronic control unit (ECU), which is the central computer that monitors all electronic activity in the vehicle.
Common causes that cause the ABS to fail are:
- ABS Defective
- ABS Gets Dirt and Fails to read wheel rotations
- Loose or broken ABS wheel speed sensor wire
What is a Wheel Speed Sensor?
The wheel speed sensor, (also called an ABS wheel sensor) is a component of the vehicle anti-lock braking system (ABS). The ABS prevents your vehicle from sliding or slipping when you come to a stop, which is especially useful on slippery surfaces such as wet or snowy roads. It's always reporting the rotational speed of the wheels to the ABS control module, part of the vehicle's computer. When the driver applies the brakes, the ABS control module reads the data given to it from the speed sensor (how fast the wheels are spinning) and sends the correct amount of braking pressure to each wheel, allowing the vehicle to stop in a controlled manner, preventing the wheels from locking up and the car from skidding or sliding across the road.
Where is the wheel speed sensor located?
For every tire that has a rotor, there will be a wheel speed sensor. In a system with four sensors, a sensor can be found at each wheel near 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
- Four sensor, 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 lock up, the ABS will engage for that end of the car.
Symptoms of a Malfunctioning Wheel Speed Sensor
It is important to keep in mind that if your ABS is disabled, as will be the case with a defective wheel speed sensor, that your wheels may slide or slip when you apply the brakes to stop the car, especially on slippery surfaces. You will still be able to stop the car but it will be more difficult to do, especially at slower speeds.
Troubleshooting a Defective Wheel Speed Sensor
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. Comprehensive diagnostics should be conducted to rule out other possibilities before replacing the sensor.
The first thing to check for is whether or not the wheel speed sensor is dirty. Road debris, dirt, and grease can prevent the sensor 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 the problem is not with dirt and debris or accidental damage, you can use an ABS sensor pinpoint tester and the scan tool to diagnose the cause of the problem. To do this, disconnect the ABS sensor first. Hook up the black clip to ground on the vehicle. The 2 green wires hook up to the sensor itself. Spin the wheel and look for rotation on the pinpoint tester. If there is the rotation, then the sensor is good. If there is no rotation, check for the reverse polarity of the green wires. Fix them if necessary and re-check for rotation. If there still is no rotation, then the sensor is likely bad.
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. Hook up the green wires to the connector on the vehicle side. Turn on the pinpoint sensor, go to ?simulate sensor? mode and choose the vehicle'?s type of ABS system. Then, use the scan tool - check the wheel speed being detected from the sensor. Inspect the clip, connectors, and wiring harness and try to find a break or bad connection. Replace as necessary.
If cleaning the ABS sensor does not fix your problem it is time to scan the fault codes from the ABS control unit. To do this you will need a diagnostic scanner that you can connect to the OBD-II port under the dashboard. There are several OBD II scanners that can diagnose ABS faults such as:
- Launch Creader - Popular Diagnostic Scanner for DIY repair
- Autel AL619 ABS/Airbag Scan Tool - Least Expensive
- Autel MaxiDAS DS708 - Profesional Level Diagnostic Scanner
Wheel Speed Sensor Replacement Cost
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. People 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.