Why is my ABS warning light on? And what is a wheel speed sensor?

One of the warning lights in your car is the ABS warning light. ABS stands for anti-lock braking system, and it ensures you have complete control of your car in emergency braking situations.

What does a glowing ABS warning light mean?

Unless you see leaking brake fluid, the most common reason for the ABS warning light is a faulty wheel speed sensor. In fact, some car brands have this as a recurring problem.

There are other reasons for failed ABS systems too, albeit less prevalent.

Some refer to the wheel speed sensor as the ABS sensor, but since it is also related to the ESC system of your car, let’s just call it the wheel speed sensor.

So, what is a wheel speed sensor?

As the name implies, it is a sensor that detects wheel speed. All four wheels of your car are equipped with a wheel speed sensor.

They are located behind your brakes:

And the whole system looks something like this:

There is a toothed ring and a sensor. The teeth cut through the magnetic flux of the sensor and you can get the instantaneous speed of the rotating wheel.

There are two types wheel speed sensors – passive and active. The passive works via induced current while the active only works with a supplied current.

They differ in signals and direction sensing, but the basic difference we need to know is that the passive-type only works for ABS, while the active-type works for both ABS and ESC.

What does the wheel speed sensor have to do with ABS and ESC?

For the ABS and ESC to work, the car needs to know how fast each wheel is rotating in real time to apply the correct braking pressure.

In Subaru AWD cars with no LSD, the wheel speed sensor tells the AWD system to brake an individual slipping wheel, almost replicating what an LSD does.

How can the wheel speed sensor become damaged/broken?

The failure of the wheel speed sensor can be either mechanical or electrical.

Mechanically,

• the toothed ring could crack
• the teeth could be damaged
• the teeth could be missing
• there could be metal particle contamination

These would mess up the signal sent to the ecu (remember, this system works with magnets).

Electrically, there could be:

• shorted sensor coils (replace sensor)
• weak sensor magnets (replace sensor)
• wiring problems (usually cause by accidents)
• a blown fuse (sometimes it's that simple)

An OBD2 diagnostic tool will help the mechanic detect which wheel speed sensor is faulty. Depending on how seized things are, the job can take quite a while.

Can I drive with a broken wheel speed sensor?

Unless you see leaking braking fluid, yes you can. But drive slowly, especially if it’s a RWD car.

When the ABS and ESC lights are on, this means these systems are not working/turned off. You are basically driving a Perodua Axia E from 2017. You can still drive slowly to your trusted service centre to get things sorted out.

Are wheel speed sensors expensive to fix?

The sensor itself doesn’t cost that much. An individual sensor, even for a BMW, would cost between RM100 to RM300. It's a case of cheap part, but expensive labour. The job can be quite laborious depending on the situation. A damaged toothed ring is rather difficult to access.

Summary

So, ABS issues are usually related with a faulty wheel speed sensor. Unless the brake system is leaking, that's what the ABS light usually indicates.

Some car makes are more prone to this issue than others, and the sensor should not cost too much to replace. You can still drive slowly to your trusted service centre to get things fixed. 

Besides the wheel speed sensor, there could be other causes too like a warped brake rotor, a weak battery (most electricals fail with a weak battery), or a faulty ABS control module (least likely).