7.3 Antilock Brakes, Automatic Traction Control, and Stability Control
Key Takeaways
- ABS uses wheel-end sensors, a computer, and modulators: when a wheel slows faster than the others, the modulator releases air from that wheel's service chamber to keep it rolling.
- The dash ABS light comes on at ignition as a bulb check and then goes out; a steady light indicates a system problem.
- Trailers with ABS have a warning light on the left side at the rear; if it stays on after its bulb check, there is a fault.
- Automatic traction control uses the ABS sensors plus ATC valves that can apply the brake on a spinning wheel, sending driveline power to the wheels that still have traction.
- Stability control adds monitoring of rollover and yaw (jackknife) and replaces traction control; ABS works only on the service brakes, and drivers should drive as though these systems were not there.
Antilock Brakes, Automatic Traction Control, and Stability Control
Quick Answer: Antilock brakes (ABS) use wheel-speed sensors, a computer, and modulators. If one wheel starts to slow much faster than the others, which means it is about to lock, the modulator releases air from that wheel's service chamber so the wheel keeps turning and you keep steering control. The dash ABS light comes on as a bulb check at start-up and then goes out; a steady light means a fault. A trailer's ABS light is on the left side at the rear. Automatic traction control (ATC) can apply a brake to a spinning wheel. Stability control also watches for rollover and jackknife. Do not rely on any of them: drive as though you did not have them.
Brake management systems
All vehicles being manufactured now have some type of brake management system. The Alberta course does not expect drivers to know the electronics in detail. It does expect you to know how to use these systems and how to recognize when they need service. The student manual adds an important warning: brake management systems are highly effective, but do not rely on them. Drive as though they were not there.
How ABS works
| Component | Job |
|---|---|
| Wheel-speed sensors | Mounted at the axle ends, they send a voltage signal that the computer reads as wheel speed |
| ABS computer | Compares the wheel speeds and spots a wheel that is slowing much faster than the others (about to lock) |
| Modulators | On the computer's command, they release air pressure from the service chamber of the slower-turning wheel so it keeps rolling. They may be mounted separately near the chambers they control, or wing-mounted directly on the relay or quick release valve of the axle group |
The same principle applies to hydraulic and air brake vehicles alike.
What ABS does not do: it works only on the service circuits. When spring brakes apply during a park circuit failure, or when a trailer's emergency brakes apply after a supply line rupture, ABS has no effect (Chapter 5). ABS also cannot create traction that the road does not have. A vehicle driven too fast for the conditions will still slide.
Braking on slippery roads
The way you brake on icy roads differs with and without ABS:
- With ABS: apply the brakes firmly and steadily, and let the system modulate each wheel. Do not pump the pedal, because pumping fights the system and wastes air.
- Without ABS: brake gently enough that the wheels do not lock. If they do lock, ease off to let them roll, then reapply.
Reading the ABS warning lights
Tractor or truck (dash light). Like other system lights, it comes on when the ignition is switched on (a bulb test) and then normally stays off.
- A continuously flashing light indicates the ABS is controlling the service brakes (as described in the Alberta instructor notes).
- A steady light indicates a problem with the system. Have it repaired. Until then the vehicle brakes like one without ABS.
Trailer ABS light. Trailers equipped with ABS have a warning light on the left side, at the rear of the trailer. How its bulb check behaves depends on how the trailer's ABS gets electric power:
| Trailer ABS power source | What a normal bulb check looks like |
|---|---|
| Stop-light circuit only | The light comes on when the service brake is applied, then goes out |
| Constant power through the blue wire (centre pin) of the trailer light cord, as on most modern tractors | The light comes on when the ignition is turned on, then goes out |
In either case, a light that stays on means a system fault. It is good practice to look down the left side of the trailer when you switch on the ignition, and now and then when braking, to check for ABS faults.
Automatic traction control (ATC)
ATC is a variation of ABS. It uses the same wheel-end sensors and computer, but the computer is enhanced to watch for wheel spin as well as wheel lock. The big difference is the ATC valves:
- ABS modulators can only release air from a chamber so a wheel's rotation matches road speed.
- ATC valves have their own separate air delivery, so they can apply the brake on a wheel that is starting to spin. This redirects driveline power to the wheels that still have traction.
You can think of ATC valves as electrically controlled relays. An ATC warning light on the dash shows when ATC is working and when it needs repair.
Stability control
Traction control systems grew out of antilock brakes, and stability control systems are more advanced still. They use the same wheel-speed sensors and ATC valves. Their sensors also monitor rollover and yaw (jackknife), and the computer can handle many more programmed situations. Stability control systems replace and include the functions of traction control systems, and the stability control warning light replaces the ATC light.
Summary
| System | Senses | Can it apply brakes on its own? | Warning light |
|---|---|---|---|
| ABS | Wheel speed (lock-up) | No; it only releases air from a locking wheel's chamber | ABS light: bulb check, then off; steady = fault |
| ATC | Wheel speed (spin and lock) | Yes; it applies the brake on a spinning wheel | ATC light |
| Stability control | Wheel speed plus rollover and yaw | Yes; it builds on the ATC valves | Stability control light (replaces ATC light) |
When the ABS computer detects that one wheel is slowing much faster than the others during a service stop, what happens?
Where is a trailer's ABS warning light, and what does it mean if the light stays on?
What can automatic traction control (ATC) valves do that ABS modulators cannot?