“ABS light on car” isn’t a phrase that glides nicely off the tongue but, thanks to Google, it’s probably why you’re reading this. People in the U.S. searched that phrase and others like it (e.g. “What is ABS” and “ABS light meaning”) over 40,000 times last year. So ABS brakes, or anti-lock brakes, are something folks wonder about. In this article we’ll look at how it works. We’ll also look why the ABS light on your dash goes on, and why it’s often trigger by a part called the ABS wheel speed sensor.  — Motor City Mitch

MY GEO METRO & THE SCHOOL BUS

On a snowy Michigan morning 25 years ago, I was late for work and tried to make up time by taking side streets. I skipped highway M-59, which I figured would be a snowbound mess. Instead I took a route which cut through a neighborhood behind the industrial park where I worked. I’d just gone over a hill and onto a straightaway (unwisely speeding up) when I saw yellow ahead. It was a school bus, stopping to pick up kids.

Under normal conditions, I could have brought the little Geo Metro I was driving to a safe stop. But that morning wasn’t normal: the street was a runway of snow and ice. When I hit my brakes, my car went into slide mode. As far as I could tell, I didn’t even slow down. The Metro slid straight into the back of the bus and bounced off its huge black bumper.

My car was totaled but, by incredible luck, the bus and everyone in and around it was fine. The bus went on its way and soon an ambulance appeared. I didn’t think I needed a stretcher, but I did. Later, in the hospital ER, looking up at my wife and our 6-month old son, I realized how lucky I was. I hadn’t hurt or killed anyone, and I was alive and intact. My only pain was from the seat belt: I felt like Mike Tyson, circa 1986, had given me a body shot.

What I’d experienced — my car sliding across snow into a potentially fatal crash — was the very thing ABS brakes (which include the ABS light on your dashboard) are designed to avoid.

SO… WHAT IS ABS?

When I was in high school, our driver’s ed teacher told us there were two ways to avoid a car crash. We could perform an “evasive braking maneuver,” as he put it, or an “evasive steering maneuver.” That was it: brake or steer. What ABS lets you do — on a slick road — is both.

So what is ABS? The letters stand for Anti-lock Braking System, although they’re also called anti-lock brakes. Or often just “ABS brakes.” Whatever you call it, it’s a system that keeps your wheels from locking and skidding. And what that means is you can stop faster and steer out of trouble, specially when things are slippery.

The first anti-lock brakes were developed for airplanes in the early decades of aviation. The idea was partly the same as it is for automotive: to stop faster. But cars and trucks have another need, the ability to steer out of trouble rather than skidding into it.

STOPPING, STEERING & SNOW

The image below, from an article on ABS sensors by ScienceABC.com, shows how ABS helps you steer. The top car, with anti-lock brakes, steers clear of the cone. The bottom car, like my old Geo Metro, slides straight into trouble.

what is abs, evasive steering, anti-lock brakes

So, what is ABS doing? The next visual does a nice job of illustrating that. It’s an animation Mazda Australia posted on YouTube a few years back. In it, you see two cars side by side, one with anti-lock brakes and one without. And you can see how the car with ABS can both stop faster and steer.

One thing to note: this animation shows the cars on what looks like a sunny Australian highway. The reality of anti-lock brakes is different. ABS was designed for — and is mainly effective on — roads that are icy, wet, or just plain slippery. So imagine this animated highway covered with snow…

SOCCER REFS & HOW ABS WORKS

My son is a college soccer coach and at his games there are three referees running the show. Two guys work the sidelines, waving flags when the ball goes out, while the third guy works the middle of the field. The guy in the middle is the boss, making decisions, calling penalties, and handing out cards for rough fouls.

This system — sideline helpers with a “boss” in the middle — is more or less how ABS works.

With anti-lock brakes, the helpers are ABS wheel speed sensors on your wheels. As you drive, they measure how fast your wheels are spinning and send the information to a “boss” in the middle of the vehicle.

That boss is a computer called an ABS Control Module, and it does two things. First, it monitors and compares the wheel speeds. Second, when needed, it signals a nearby ABS motor to pulse the brake pressure on an individual wheel. That, in turn, releases brake pressure and lets a locked wheel spin.

abs sensor, what is abs, anti-lock brakes diagram

ABS BRAKES & WHEEL SPEED

Now it may sound strange, but your car wheels often go different speeds. When you turn or take a curve, the wheels on the “inside” of the curve spin slightly more slowly than your “outside” wheels. Your ABS control module knows this and stays out of the picture.

But when you start skidding, it’s another ball game. When a wheel locks, it stops spinning and its speed drops to zero. That’s the kind of change an ABS Control Module is designed for. It signals an ABS motor to release brake pressure to that wheel. This happens quickly and in a rapid-fire way, which can feel like a pulsing in your brakes. The result? Your locked wheel resumes spinning, so you can stop faster and steer. All while your foot is clamped down on the brake pedal.

It’s an automated form of threshold braking — a racing technique of stopping without locking wheels or de-stabilizing the vehicle. The following video does a nice job explaining how all of this works and what it means when your ABS light is on.

THE ABS SENSOR & RELUCTOR

The key to making this work is measuring the speed of all your wheels as you drive. How do you do that? The ABS solution is simple and brilliant. First, there’s a metal disk called a reluctor ring, attached to each wheel. It has magnetic teeth along its edge that spin along with your wheel as you drive.

Second, sitting close to those spinning teeth, is an electro-magnetic device called a variable reluctance sensor. (This is the ABS wheel speed sensor mentioned earlier.) It measures tiny changes in magnetism as the metal teeth fly by: when a tooth is close to the ABS sensor, magnetism goes up; when a tooth is further away, magnetism goes down. Then the ABS sensor sends those measurements as electric signals to your ABS control module.

And that’s how your car knows the speed of all four wheels. As each wheel changes speed, so does the electric signal. In effect, you have a little generator on each wheel, creating and sending electric signals to your ABS control module.

Here’s how it looks… (Note that in this diagram, from Apec Braking, the ABS control module is called an ECU.)

 


ABS wheel speed sensor, ABS sensor, reluctor ring

 

And now, armed with this knowledge, we’re ready to deal with the pesky ABS light on the dashboard.

THE ABS LIGHT

The ABS light on my current car, a Toyota, is simply the three letters: ABS. All in caps. But if I were in Canada, it’d be the letters ABS surrounded by a circle and flanked by what look like parentheses. (See the first image in the next section below.)

So what your ABS light actually looks like depends on what car or truck you have and also what country you’re in. For example, you might see just the word ANTILOCK, all in caps. Or you might see a variation of that: the word ANTI above the word LOCK. Or you could see the symbol of a skidding car, inside a circle and parentheses. All of these, in different vehicles, are ABS lights.

To find out for sure, you can check the warning lights section of your owner’s manual.

IS ONLY YOUR ABS LIGHT ON?

Okay, so let’s say the ABS light on your dash is on and it’s staying on. Are any other lights on? If there are, that’ll guide what to do. Here are common warning lights and what they mean…


abs light on car

First is the ABS light, as mentioned above. If you see only an ABS light, then your brakes should work fine as conventional brakes. However, you won’t have the anti-lock brakes feature. So if you brake on an icy road, you may skid.


brake system warning light

Next is the brake warning light. If you see this — or BRAKE in all caps — you may have a brake malfunction. Driving could be dangerous. If your parking brake is off, and you still see this, get your brakes checked pronto!


parking brake light

Third is the parking brake light.  If you see this, your parking brake’s probably on. But if you see this with a wrench symbol or an exclamation mark, something’s wrong with your electric parking brake.


brake pad warning light

Next up, worn brake pads. If you see this, it means you may need a brake job to replace some worn-down brake pads. And if you need to replace pads, make sure to replace the brake clips holding them in place. The clips wear out too.


tire pressure warning light

Finally, the tire pressure light. I’m adding it because, to me, it looks a bit like the brake warning light. It generally kicks on when a tire drops 25% or more in pressure. And it could could mean you have a flat.

SO IT’S ON… NOW WHAT?


So what do you do if your ABS light is the only light you’re seeing? First, you can cycle your ignition. Turn your key from “on” to “off” and then back on again without starting the car. See if that makes your ABS light go off.

Next, you can check your emergency brake. Make sure sure it’s all the way down. Try adjusting it a few times to see if that turns the ABS light off.

Next, you can check your brake fluid level. The video below, posted by Advance Auto Parts, shows how:


If your brake fluid level’s down, it could mean other things. Your brakes could be leaking. Or your brake pads could be worn down — so you might need a brake job. But if your ABS light is still on, the best way to find out what’s going on is with a scan tool.

FINDING A SCAN TOOL

If I were an engineer, I could probably tell you how many computers my Toyota has. My guess? It’s well north of forty. The image below, from the Clemson Vehicular Electronics Laboratory, shows computers now common on many cars. Below the back wheel is the callout for anti-lock brakes.

computers in cars, ECU, electronic control units, ECM, electronic control modules

These computers, known as electronic control units or modules (ECUs or ECMs), monitor things as you drive. And, if something goes wrong, they can diagnose the problem. They do so with codes — diagnostic trouble codes known as OBD-II PIDs.

So, find the trouble codes in your car and you’ll know what’s wrong.

One option is to have your dealer or local repair shop do a scan. But another option is to get a scan tool and look at the codes yourself. If you search the Amazon.com for ‘automotive scan tool’ and sort by average customer review, you’ll see lots of options.

The following video may help narrow down the list. It’s a review — unsponsored, says the narrator — of the top tools for 2019. And their first choice, BlueDriver, is also the most-reviewed and highest-rated tool on Amazon.

Next is to see how these tools work. The following video of BlueDriver, by the YouTube mechanic ChrisFix, does that very well. While the video is several years old, it’s thorough, clear, and relevant.

HOW TO SCAN ABS CODES

So now you’ve got a scan tool in hand, and your ABS light is on. How do you find the ABS codes?

In the following video, YouTube mechanic Scotty Kilmer gives a brief, clear explanation. He shows how to plug in the tool — in this case a Creader VII+ — and look at ABS codes. 

THE USUAL SUSPECTS & THE ABS SENSOR

A common problem involves the ABS wheel speed sensor we discussed earlier. Since electro-magnetic end is close to your wheel and brakes, it’s exposed to debris and heat. So it might be dirty, not working, or out of position. (If the gap between the sensor and the reluctor ring teeth gets too big or small, the sensor won’t work right.)

Something could be wrong with the ABS control module, or there could be a problem with the wire going to the module.  Or it might be the reluctor ring: the teeth could be dirty, rusty, or worn down.

First, though, let’s check something else — a blown ABS fuse.

DEALING WITH A BAD ABS FUSE

“Automotive fuses,” says Jeremy Laukkonen, “are the gatekeepers and bodyguards of the automotive electronics world.” (His Lifewire article on Car Fuses Explained lays out different fuse types in a clear way.) He also links to a visual guide to fuse types.

So what about fuses for anti-lock brakes? In the video below, mechanics from 2CarPros show how to replace a blown ABS fuse on a 2012 Silverado. They show where it’s located: — in this case, in a fuse box under the hood on the driver’s side. They also test it with a light meter, inspect it, and put in a new one.

Your vehicle probably has more than one fuse box. (My Toyota has three.) Typically, ABS fuses are in a fuse box under your hood, but you can check your owner’s manual to find out for sure.

One more thing: if you replace an ABS fuse and it blows again, there’s probably another problem — like an ABS wheel speed sensor — you need to deal with.

CLEANING AROUND AN ABS SENSOR

If you’re having trouble with an ABS sensor, you can have your dealer or local repair shop check it and, if needed, replace it. Or you can also do things yourself.

In the video below, Matt Waggoner shows how to clean rust away from an ABS sensor on a 2006 Silverado. First, he shows how he measures voltage coming from the sensor wire — he’s getting none. Then, after carefully removing the end of the sensor, he sands away rust from the area around it. Finally, he puts the sensor back in, and it now gets voltage readings. His takeaway? Rust and corrosion lifted the end of the sensor too far away from the teeth of the reluctor wheel. Cleaning things did the job.

But what if cleaning doesn’t fix the sensor problem?

REPLACING AN ABS SENSOR

When it comes to replacing an ABS sensor, you can, of course, visit your dealer or local repair shop.

The next video shows how to do things yourself. Posted by CarsNToys, it shows how a mechanic replaces an ABS wheel speed sensor on a 2010 Subaru Impreza. He does so using only hand tools, with much of his time spent on connecting the sensor to the ABS control module.

If you need a new ABS sensor, Carlson has one for almost every vehicle on the road. You can visit Carlson’s sales support page.

CLEANING OR REPLACING A RELUCTOR RING

What if the teeth of a reluctor ring is dirty, corroded or worn down?

The following video from RockAuto Auto Parts gives a brief description of the issue along with a clear visual of the reluctor.

REPLACING A WHEEL BEARING (WITH ABS INSIDE)

If the the reluctor is built inside, another problem could be a wheel bearing hub assembly.

In the following video, YouTube mechanic ChrisFix scans ABS codes for a 2007 Mountaineer. He finds trouble codes telling him a common fix is to replace the left front wheel bearing. So he installs a new one — a hub assembly that comes with a new ABS sensor.

After removing the old and installing the new, the ABS trouble codes go away. Problem solved!

REPLACING AN ABS CONTROL MODULE

Yet another issue you could be having involves the ABS control module.

In the next video, mechanics at 2CarPros show how to replace the ABS control module on 2000-2006 GMC Yukons and Sierras.

FINAL THOUGHTS

So that’s what anti-lock brakes are, and what the ABS light on your car on your dash is all about. Hopefully, you have an idea of what to do if that light goes on and stays on.

Back in the early 1990s, when my Metro skidded into the back of a school bus, ABS brakes had appeared on a number  of vehicles — just not on my mine. Now, however, they’re mandatory on new cars and trucks. It’s a great innovation, in my view, and one that’s easy to take for granted. So when that light goes on, whether it’s your mechanic or yourself, check things out. And may your next drive on a slippery road be a safe one.

[Carlson, which sponsored this article, makes ABS sensors for almost every vehicle on the road. If you need new ones, you can visit Carlson’s sales support page.]