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Colorado CDL Combination Vehicles study materials
Colorado CDL Combination Vehicles study resources

Colorado CDL Combination Vehicles: Antilock Braking Systems (ABS) on Combination Vehicles

Colorado CDL Combination Vehicles · Work through your exam, one topic at a time.

All guide sections

Antilock Braking Systems (ABS) on Combination Vehicles

What this domain tests

This domain tests how antilock braking works on a tractor-trailer, what the dash and trailer ABS lights mean, and what to do when a fault appears. ABS is an add-on to the air-brake system; it does not replace safe speed and following distance.

Core rules

  • ABS prevents wheel lock-up during hard braking by modulating pressure at individual wheels. It helps you keep steering control and reduces jackknife risk, but it may not shorten stopping distance on loose gravel or snow.
  • The tractor ABS lamp should come on at start-up and go out after the self-check. A trailer ABS lamp is usually yellow and may be on the trailer or the dash. If it stays on, trailer ABS has a fault.
  • A trailer ABS fault does not mean the trailer has no brakes. The trailer still has normal air brakes, but the wheels may lock. Report the fault and follow the repair policy.
  • In an emergency, apply firm, steady brake pressure and steer. Do not pump the brakes on an ABS-equipped vehicle; pumping can reduce performance and confuse the system.
  • Keep enough air pressure and check glad-hand connections. A kinked or leaking service line can cause trailer brake problems that are not ABS faults.
  • ABS does not allow higher speed on ice, and it does not correct a bad following distance.

Worked example

After start-up, the tractor ABS lamp goes out, but the yellow trailer ABS lamp stays on after the vehicle reaches about 5 mph. The driver should not treat this as a total brake failure. The trailer brakes still work conventionally, but the trailer ABS is not protecting against wheel lock. The driver notes the fault, checks the trailer ABS connector and reports it for repair; the vehicle should not be dispatched with a known unresolved ABS fault under the carrier's policy.

Traps

Pumping the brake pedal, assuming ABS always stops shorter, ignoring a trailer ABS lamp, and blaming low air pressure on ABS are common errors. Also do not confuse the red low-air warning with the yellow ABS lamp.