Today’s vehicles make use of varied types of sensors, including those measuring engine speed (crankshaft/camshaft position), transmission speed, and wheel speed. Wheel speed sensors are especially reliable but are susceptible to failure due to their location.
Let’s consider some of the most common signs of a bad wheel speed sensor, whether or not it’s safe to drive, and how much the speed sensor is going to cost to replace.
What is a Wheel Speed Sensor?
Whether referred to as a Wheel Speed Sensor (WSS) or ABS Sensor, this electrical device provides feedback regarding the rotational speed of one wheel on a vehicle. It should not be confused with a vehicle’s transmission speed sensor, which performs differing duties.
Wheel speed sensors are generally magnetic in nature, creating measurable “pulses”, as a hub-mounted tone ring rotates before their probes.
The speed at which these pulses are created is directly related to the detected speed of the wheel they concern, as interpreted by a vehicle’s management software. More pulses in a given time means a faster wheel’s registered rotational speed.
There are two individual configurations of a wheel speed sensor.
- The first is a passive sensor, which is analog in nature and does not require a reference voltage in order to fulfill its function.
- The second configuration of a wheel speed sensor is that of an active design. Such sensors do require an input voltage for operation, and employ an encoder-style tone ring.
Most late model vehicles utilize the latter of these two sensors due to its inherent reliability.
The vehicle’s management software compares feedback from one speed sensor with information transmitted by all other wheel speed sensors found on the same vehicle. In this way, the software can judge if one wheel is rotating faster than any others, indicating a slide or impending brake lockup.
Where Is It Located?
Wheel speed sensors can be found mounted within each of a vehicle’s four hubs, their location generally indicated by following the electrical pigtail to its point of mount.
This wiring usually runs down a vehicle’s frame rail, branching off to a wheel-end’s brake hoses or hub assembly.
The wheel speed sensor itself is countersunk into the hub assembly, its outer face residing well within close proximity of a hub-mounted tone ring, if indeed, this inset tone ring is not the very thing that serves to support the sensor to its mount.
A speed sensor would not be an effective speed sensor, at any rate, if it were not arranged in such an orientation as this so as to interface with the inset tone ring.
Bad Wheel Speed Sensor Symptoms
The existence of a guilty wheel speed sensor is most often accompanied by a few other symptoms, some of which might be more noticeable than others. Quick recognition of these symptoms can really help in quicker diagnosis, heading off the problem.
We’ll show a few of the more common symptoms of a guilty wheel speed sensor.
#1. ABS (or Stability Control) Light On.
Far more times than not, defective wheel speed sensor is picked up by the vehicleconnectd control software, resulting in illumination of an ABS warning light. This is because an ABS fault is being stored.
In some cases, manufacturer-specific traction or stability control warnings (ie: Service StabiliTrak, VSA, VSC, Service AdvanceTrac) will be noted as well.
#2. No Active Safety System Operation.
A vehicle’s anti-lock brakes, stability control, and traction control all take feedback from each of a vehicles wheel speed sensors. In most cases, they are disabled or locked out from use, whenever either wheel speed sensor malfunction is detected.
#3. Brake Pedal Pulsating.
In rare instances, a vehicle’s brake pedal may pulsate, as if an ABS modulated brake application were being made, simply driving down the road.
#4. Longer Stopping Distances.
As mentioned earlier, you may notice that it takes longer to stop your vehicle once one or more wheel speed sensors fail, particularly in panic and emergency stops. This is due mainly to the lack of ABS function, which results from the automatic lockout mentioned above.
What Causes a Wheel Speed Sensor to Fail?
A wheel speed sensor may fail for various reasons, some of which tend to be more common than others. The sensor may be irreparably damaged by a debris strike that occurs as a result of hitting any solid material lying in the roadway.
Alternatively, a wheel speed sensor’s signal may be disrupted due to the accumulation of metallic debris in or around the magnetic portion of the sensor.
On top of that, a wheel speed sensor may also be hampered whenever its wiring is damaged, or because of excess corrosion build-up on its connector. Either of these conditions can cause excess resistance within the speed sensor circuit itself, or even an open connection in the most severe cases.
Wheel Speed Sensor Replacement Cost
The exact cost of wheel speed sensor replacement can vary from make and model to make and model of vehicle. In many cases, one manufacturer will charge significantly more for a speed sensor than others.
Alternatively, labor rates for the replacement of a specific speed sensor might be much higher on one model of vehicle than the next.
On average, you can expect to pay between $150 and $300 to have one of your vehicle’s wheel speed sensors replaced. Generally speaking, about 1/3 of this cost will be incurred by buying the sensor itself, and about 2/3 by labor charges involved in effecting the repair.
Is It Safe to Continue Driving?
While it’s possible to continue to drive with a compromised wheel speed sensor, it’s not advisable to do so. Several active safety systems, like anti-lock braking, stability control, and traction control, depend on a car’s speed sensors in order to function properly.
In most instances, these systems will simply be disabled should an individual sensor experience failure.
To knowingly drive months on end with one or more safety systems inoperative due to a mechanical defect is problematic for several reasons. The first of these reasons relates to your safety, your passenger’s safety, and the safety of all other drivers on the road.
You might even have to absorb additional liability should this negligence lead to a crash!
In any case, the first thing you should do is properly diagnose and remedy the underlying cause of a car’s wheel speed sensor fault. That’s just good sense when it comes to preserving your safety.
If you’re unsure of your ability to make these repairs alone, we advise paying a visit to a local trusted service centre.
How a Mechanic Diagnoses a Faulty ABS Sensor
Usually, a scan tool will identify not just which wheel speed sensor is at fault, but it will also indicate the individual wheel-end location.
From there, an oscilloscope or high-end multimeter will be used to troubleshoot feedback from that particular wheel speed sensor or lack thereof.
Or, sometimes a capable bidirectional scan tool can even be used to monitor wheel speed sensor feedback in real time.
In addition to these procedures, the mechanic is expected to carry out a good visual inspection of the suspected wheel speed sensor and its wiring. Oftentimes, especially in the case of a debris strike, damage to the sensor is obvious.
It is also vital to check the air gap between the speed sensor and the sensor’s tone ring.