12.5 Antilock Braking Systems (ABS): Required Vehicles, Malfunction Lamps, and Braking With and Without ABS

Antilock Braking Systems (ABS)

ABS is a computerized system that keeps your wheels from locking up during hard brake applications. Understanding it correctly is worth several test items, and more importantly it prevents the two dangerous misconceptions that get drivers hurt: believing ABS shortens stopping distance, and believing ABS requires a different pedal technique.

The defining sentence to memorize:

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.

And the second:

ABS does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking.

1. How ABS Works

Wheel-speed sensors detect a wheel that is about to lock up. An electronic control unit (ECU) then decreases brake pressure at that wheel to avoid lockup, and modulates the pressure to provide the maximum braking without danger of lockup. The system cycles far faster than any driver could respond to impending lockup. At all other times the brake system operates completely normally — you would never know ABS was fitted.

2. Vehicles Required to Have ABS

These three dates are exam staples. Learn them as a set.

Many commercial vehicles built before these dates were voluntarily equipped with ABS, so an older unit is not automatically without it.

3. How to Tell Whether Your Vehicle Has ABS

• Tractors, trucks, and buses have yellow ABS malfunction lamps on the instrument panel.
• Trailers have yellow ABS malfunction lamps on the left side, either on the front or rear corner.
• Converter dollies manufactured on or after March 1, 1998 are required to have a lamp on the left side.
• On newer vehicles, the malfunction lamp comes on at start-up as a bulb check and then goes out quickly. On older systems, the lamp may stay on until you are driving over five mph.
• If the lamp stays on after the bulb check, or comes on once you are under way, you may have lost ABS control on one or more wheels.
• For towed units built before ABS was required, it can be hard to tell. Look under the vehicle for the ECU and the wheel-speed sensor wires coming from the back of the brakes.

4. ABS on Only Part of the Combination

Having ABS on only the tractor, only the trailer, or even on only one axle still gives you more control during braking. In every case: brake normally.

5. Braking With ABS

When you drive a vehicle with ABS, brake as you always have:

• Use only the braking force necessary to stop safely and stay in control.
• Brake the same way regardless of whether the ABS is on the bus, the tractor, the trailer, or both.
• As you slow down, monitor your tractor and trailer and back off the brakes (if it is safe to do so) to stay in control.

There is only one exception to this procedure. If you drive a straight truck or combination with working ABS on all axles, then in an emergency stop you can fully apply the brakes. That is the entire scope of the exception — full-system ABS, emergency stop only.

This is also why pumping or stab braking is wrong on a fully ABS-equipped vehicle: the ECU is already modulating pressure hundreds of times faster than your foot can, and interrupting it wastes reservoir air and lengthens the stop.

6. Braking If ABS Is Not Working

Without ABS you still have normal brake functions. Drive and brake as you always have. If the malfunction lamp stays lit after the bulb check or illuminates while under way, you may have lost ABS control on one or more wheels — but the foundation brakes are unaffected. Drive normally and get the system serviced soon.

7. The Six Safety Reminders

The manual closes its ABS discussion with a list of things ABS will not do. Each one is a plausible-sounding distractor on the knowledge test:

1. ABS won't allow you to drive faster, follow more closely, or drive less carefully.
2. ABS won't prevent power or turning skids. It should prevent brake-induced skids and jackknifes, but not skids caused by spinning the drive wheels or going too fast in a turn.
3. ABS won't necessarily shorten stopping distance. It helps maintain control, but not always distance.
4. ABS won't increase or decrease ultimate stopping power. It is an add-on to your normal brakes, not a replacement.
5. ABS won't change the way you normally brake. Under normal conditions the vehicle stops as it always stopped; ABS only engages when a wheel would otherwise have locked from over-braking.
6. ABS won't compensate for bad brakes or poor brake maintenance. Out-of-adjustment pushrods and worn linings defeat any electronics.

The best vehicle safety feature is still a safe driver. Drive so you never need to use your ABS.