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6.3 – Antilock Brake Systems
6.3.1 – Trailers Required to Have ABS
All trailers and converter dollies built on or after March 1, 1998, are required to have ABS. However, many trailers and converter dollies built before this date have been voluntarily equipped with ABS.
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.
In the case of vehicles manufactured before the required date, 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.
6.3.2 – Braking with ABS
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.
ABS helps you avoid wheel lockup. The computer senses impending lockup and reduces the braking pressure to a safe level, and you maintain control.
Having ABS on only the trailer or even on only one axle still gives you more control over the vehicle during braking.
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 gain control.
When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:
- 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 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.
Remember: If your ABS malfunctions, you still have regular brakes. Drive normally, but get the system serviced soon.
ABS will not allow you to drive faster, follow more closely, or drive less carefully.
Multiple-Choice Questions:
Trailers will have yellow ABS malfunction lamps:
- On the left side, either on the front or rear corner.
- On the right side, either on the front or rear corner.
- Above each set of axles
- Underneath the trailer doors at the rear of the trailer
If your ABS malfunctions:
- You have very little braking ability. Only drive with extreme caution until you can get safely off the road.
- You have limited braking ability. Drive cautiously to the nearest service station for repairs
- You must not drive the truck until it can be repaired. Call a tow truck if necessary.
- You still have regular brakes. Drive normally, but get the system serviced soon.
Which of the following statements is true about ABS (anti-lock brake systems):
- It does not decrease or increase your normal braking capability.
- ABS is an addition to your normal brakes
- ABS only activates when wheels are about to lock up.
- All of these are true
When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:
- Brake the same way, regardless of whether you have ABS on the tractor, the trailer, or both.
- Use only the braking force necessary to stop safely and stay in control.
- 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.
- All of these are correct
When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:
- 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 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.
How does ABS help you avoid wheel lockup?
- When the brake drum spins at a different rate than the hub, the air tank relief valve will release some air, reducing pressure on the brakes
- The computer senses impending lockup and reduces the braking pressure to a safe level
- The spring brakes sense a change in the air pressure from the foot brake and take over braking control from the air brakes
- The S-cam senses wheel lockup and reverses slightly to release some pressure on the brakes
Complete!
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