Professional Driver's Handbook

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Antilock brake system (ABS)

An ABS is an electronic system that monitors wheel speed at all times and controls wheel speed while braking. If it detects a wheel locking up during a brake application, the system releases brake pressure to that wheel only. This keeps the wheel from skidding and increases vehicle stability and control during emergency stops and in adverse situations such as on wet or icy roads, through curves or during lane changes. The air brake system remains the same with the addition of the ABS components.

ABS indicator lights

ABS indicator lights are amber-coloured and marked with the letters ABS. They are there to tell you that the ABS is working properly. Know where the ABS indicator light is on every vehicle you operate and how to check that it’s working properly. The lights are located on the instrument panel of trucks. Buses and truck tractors are dash mounted and on the exterior of trailers near the red side marker on the left rear side of the trailer. Converter dollies have a light located on their left side. If the vehicle you drive was built after March 1, 2001, and is equipped to tow a trailer, it will have two ABS lights on the dash, one for the towing vehicle and one for the trailers.

How ABS indicator lights work

When ABS is working properly, each time the vehicle ignition switch is turned on and kept in the on position the ABS indicator lights will turn on for a few seconds then turn off. One exception is for towing vehicles equipped with constant power supply (even with key off) through the trailer electrical cord, which may be required for safety or security on certain trailers or loads, such as gasoline tankers, ammunition loads and others. In this case, the ABS indicator light check can be accomplished by disconnecting and reconnecting the power connector (while following appropriate precautions for the powered systems).

When power is supplied to the ABS controller the ABS indicator lights momentarily turn on as the ABS automatically conducts a self test. If the light does not come on at all there could be a problem with the indicator light, the wiring or the ABS controller. When an ABS malfunction is detected the ABS indicator light stays on. If no malfunctions are detected the light turns off after a few seconds.

ABS on trailers also receive backup power through the brake light circuit. In some cases, certain types of malfunctions can cause the ABS indicator light to turn on each time the brake is applied. While the ABS may be partially working, in this condition it is not working properly. Whenever the ABS indicator light turns on when you are driving there is an ABS malfunction present that requires repair. Make sure you have the ABS problem fixed as quickly as possible. Remember, if the light does not work you will never know if the ABS stops working.

Know your ABS is working - check it during your pre-trip!

Knowing your ABS is working can give you peace of mind and it can help you when you need it. Always confirm your ABS is working – see that your ABS light(s) turn on then off when power is applied during your pre-trip inspection. Failure to ensure the ABS system functions as prescribed by the manufacturer will result in a defect being identified on a CVSA road-side inspection.


Rev: 2017