Anti-lock Braking Systems

//link to PDF  Download a three-page flyer on understanding your vehicle’s ABS.


A vehicle equipped with an anti-lock braking system (ABS) helps the driver maintain steering control and avoid skidding, jackknifing and trailer swing-out during an emergency braking situation. When the brakes are applied, the ABS senses when a wheel is about to lock and then releases just enough brake force to get the wheel rolling again while still maintaining as much braking force on the wheel as possible. It may repeat this sequence several times a second and it may control each wheel differently.

Finding/Identifying ABS Lamps

Know where the ABS lamp is on every vehicle you operate and how to check that it’s working properly. A yellow colored lamp, marked with the letters “ABS,” is located on the instrument panel of trucks, buses and truck tractors (dash-mounted), and located on the exterior of trailers, near the red side marker lamp on the left rear side (trailer-mounted). Converter dollies also have the lamp located on their left side. If the vehicle you drive was built after March 1, 2001, and it is equipped to tow a trailer, it will actually have two ABS lamps on the dash – one for the tractor (or truck) and one for the vehicles being towed.

How ABS Works

When power is supplied to the trailer ABS unit, by the ignition switch being turned on (for ABS required power units) or by applying the brake pedal (for non-ABS required units), the lamp on the trailer momentarily turns on to indicate the ABS is working. During this time a self-test of the ABS is automatically conducted. When an ABS malfunction is detected during this test, the lamp stays on; otherwise, the lamp turns off after a few seconds. If the lamp does not come on during the self-test, there could be a problem with the lamp bulb, the wiring or the ABS controller.

If the light stays on during the self-test or comes on as the vehicle is being driven, there is an ABS malfunction. Make sure any problems found are fixed as quickly as possible. Remember, if the lamp does not work, the driver will never know if the ABS stops working.

Verify Your ABS is Working

If your vehicle or combination includes any units that were required to be anti-lock brake system (ABS) equipped at the time of manufacture, those unit(s) must have a functioning ABS malfunction lamp(s) that turns on and then off when power is supplied to the unit. If ABS lamps do not illuminate at all, you likely have a violation. If ABS lamps illuminate and stay on, you also have a violation. Check our field references to see if your vehicle requires a functioning anti-lock brake system (ABS).

  • United States: View a printable PDF on understanding your vehicle’s ABS.
  • Canada: View a printable PDF on understanding your vehicle’s ABS.