The antilock braking system, or ABS, is a safety feature that became common on new vehicles starting in the 1980s and ‘90s. It was aimed at preventing the wheels from locking up and causing a skid when braking on slick roads, thereby providing shorter stopping distances and better steering control.
If, however, you find the ABS light illuminated, it means the safety system is not functioning, even if your brakes otherwise feel as though they’re working properly.
Why ABS Is Important
If you lock up the wheels while braking (which can easily happen on slick surfaces), the tires lose much of their grip on the road, lengthening stopping distances and severely hindering the ability to steer the car where pointed. Before ABS, drivers were taught to pump the brakes if they were skidding, which allowed the car to slow somewhat when the brakes were on and steer somewhat when the brakes were off.
That’s essentially what ABS does automatically, though the system can do it much more quickly than a person can. Most automatic braking systems can also detect which wheels are locked up and only reduce braking on those wheels, affording even better control.
How ABS Works
With ABS, there is a ring on each wheel hub (or some other component that rotates with the wheel) that looks a bit like a small bicycle sprocket with “teeth” on it. A wheel-speed sensor is mounted above the ring and can detect when each of those teeth passes by, and it feeds that information to a computer that can determine the speed of each wheel at any given time.
If the computer determines that one or more wheels are stopped or going more slowly than the others when the brakes are applied — indicating they’re either locked up or starting to be — the computer will reduce some of the pressure (thus clamping force) on that wheel’s brake, thereby allowing the wheel to turn more easily and regain its grip on the road.
Why the ABS Light Could Be On
There are multiple parts to the ABS, and unfortunately, any of them can fail and cause the warning light to illuminate.
While you’ll probably need to have someone use a special scanning tool to determine where the problem lies, there are some simple things you can probably do yourself before taking that step. In some cases, if you find anything amiss and can correct it, you may be able to solve the problem.
The first thing is to check your tire pressures. If one or more tires are excessively low or high, it will cause that wheel to rotate at a different speed than the others, which can confuse the computer. Next is to check your brake fluid level, which will sometimes get low due to normal wearing of the brakes, though a leak is also possible.
Finally, check for a blown ABS fuse. If you have your owner’s manual, you can probably find which fuses are used for the ABS by looking up “Fuses” in the index, which should send you to a diagram of the fuse block and a listing of the fuses. You may also be able to Google “Which fuse is for ABS on my (Year/Make/Model)?” to find a similar diagram and list. Either way, you can search the list of fuses, but note that there may be more than one that applies to the ABS.
If you find and are able to correct any of these problems, the ABS light may go out on its own after driving a while, but you may also be able to turn it off yourself. Procedures for doing so range from quite simple (among them, disconnecting the battery for a while, which may reset the trouble codes) to some rather complicated, mind-boggling series of steps. If you reset the light, it will come on again if a problem is still detected.
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Most Common Failures
While there are some further checks you can do yourself (they’ll require jacking up the car, removing some or all of the wheels and using a voltmeter — which is beyond our scope here), the best way to narrow down why the ABS light is on is to use a special scanning tool to get diagnostic codes.
Passenger vehicles built in 1996 and later have what’s called an OBD II port (“OBD II” stands for onboard diagnostics, second generation) to which an OBD II scanner can be plugged in to read out diagnostic codes. (Note that some heavy-duty pickups may not have one.)
This is the same procedure used to help determine what running problem the check engine light is catching, but the scanner has to be a special, more expensive one that also reads ABS fault codes. Some auto-parts stores may do this for you for free, but you may need to pay someone to do it. In the latter case, note that you can probably buy one of these special ABS-enabled OBD II code readers for $150 or less, and since they’ll also read regular check-engine-light OBD II codes, they are really handy to have and fairly easy to use.
The most common ABS failure is probably a sensor, and the scanner can usually narrow it down to which one. In some cases, the sensor may not have failed so much as rust or dirt have prevented it from detecting when the ring’s teeth pass by. But there could also be a problem with the ring itself, the wiring that feeds information to the computer, the computer itself or the valves that reduce pressure to the brakes. The scanner can usually pinpoint those problems, as well. Another possibility is a worn wheel bearing (which allows the wheel to wobble, thus causing misalignment between the ring and sensor) or a problem related to the brakes themselves.
In some cases, the ABS light might be joined by a brake warning light and/or a parking-brake warning light. While it may seem as though the brakes work OK (and ABS really only comes into play if the vehicle is skidding), it’s best to have any such warnings checked out as soon as possible.