2005 Model Commercial Driver’s License Manual ABS is a computerized system that keeps your  wheels from locking up during hard brake  applications.  ABS is an addition to your normal brakes. It does  not decrease or increase your normal braking  capability. ABS only activates when wheels are  about to lock up.  ABS does not necessarily shorten your stopping  distance, but it does help you keep the vehicle  under control during hard braking.  As a system check on newer vehicles, the  malfunction lamp comes on at start-up for a bulb  check, and then goes out quickly. On older  systems, the lamp could stay on until you are  driving over five mph.  If the lamp stays on after the bulb check, or goes  on once you are under way, you may have lost  ABS control.  In the case of towed units manufactured before it  was required by the Department of Transportation,  it may be difficult to tell if the unit is equipped with  ABS. Look under the vehicle for the ECU and  wheel speed sensor wires coming from the back of  the brakes. When you brake hard on slippery surfaces in a  vehicle without ABS, your wheels may lock up.  When your steering wheels lock up, you lose  steering control. When your other wheels lock up,  you may skid, jackknife, or even spin the vehicle.  ABS helps you avoid wheel lock up and maintain  control. You may or may not be able to stop faster  with ABS, but you should be able to steer around  an obstacle while braking, and avoid skids caused  by over braking.  Having ABS on only the tractor, only the trailer, or  even on only one axle, still gives you more control  over the vehicle during braking. Brake normally.  When only the tractor has ABS, you should be able  to maintain steering control, and there is less  chance of jackknifing. But keep your eye on the  trailer and let up on the brakes (if you can safely do  so) if it begins to swing out.  When only the trailer has ABS, the trailer is less  likely to swing out, but if you lose steering control or  start a tractor jackknife, let up on the brakes (if you  can safely do so) until you regain control.  When you drive a vehicle with ABS, you should  brake as you always have. In other words:  Sensors detect potential wheel lock up. An  electronic control unit (ECU) will then decrease  brake pressure to avoid wheel lockup.  Brake pressure is adjusted to provide the maximum  braking without danger of lockup.  ABS works far faster than the driver can respond to  potential wheel lockup. At all other times the brake  system will operate normally.  The Department of Transportation requires that  ABS be on:  Many commercial vehicles built before these dates  have been voluntarily equipped with ABS.  Tractors, trucks, and buses will have yellow ABS  malfunction lamps on the instrument panel.  Trailers will have yellow ABS malfunction lamps on  the left side, either on the front or rear corner. Dollies manufactured on or after March 1, 1998, are  required to have a lamp on the left side.  • Use only the braking force necessary to stop  safely and stay in control.  • Brake the same way, regardless of whether you  have ABS on the bus, tractor, the trailer, or both.  • Truck tractors with air brakes built on or after  March 1, 1997.  • Other air brake vehicles, (trucks, buses, trailers,  and converter dollies) built on or after March 1,  1998.  • Hydraulically braked trucks and buses with a  gross vehicle weight rating of 10,000 lbs or more  built on or after March 1, 1999.  2.18 – Antilock Braking Systems (ABS)  2.18.1 – How Antilock Braking Systems  Work 2.18.2 – Vehicles Required to Have Antilock  Braking Systems  2.18.3 – How to Know If Your Vehicle Is  Equipped with ABS  2.18.6 – Braking with ABS  2.18.5 – ABS on the Tractor Only or Only on  the Trailer  2.18.4 – How ABS Helps You  Section 2 – Driving Safely Page 2-33  Page 2-33 Section 2 – Driving Safely