Technical information

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Technician Guidelines for Antilock Braking Systems
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malfunctions in any towed trailer’s or dolly’s ABS. Its
location and function are the same as for the powered
unit’s ABS malfunction lamp.
• Trailer and dolly ABSs—built on or after March 1,
2001—have the equipment needed to send an ABS
malfunction signal to the towing vehicle. A towing
trailer must also be able to relay an ABS malfunction
signal from the vehicle it is towing to the vehicle
towing it.
B. How Do ABSs Work?
An ABS consists of several key components: electronic
control unit (ECU), wheel speed sensors, modulator valves, and
exciter rings. Here’s how these components work together:
1.Wheel speed sensors constantly monitor and send
electrical pulses to the ECU at a rate proportional to
the wheel speed.
2.When the pulse rates indicate impending wheel
lockup, the ECU signals the modulator valve(s) to
reduce and/or hold the brake application pressure to
the wheel(s) in question.
3.The ECU then adjusts pressure, seeking one which
gives maximum braking without risking wheel lockup.
4.When the ECU acts to modulate the brake pressure, it
will also (on most vehicles) turn off the retarder (if so
equipped) until the risk of lockup is over.
5.The ECU continually checks itself for proper
operation. If it detects a malfunction/failure in the
electrical/electronic system, it will shut down that part
of the ABS affected by the problem—or the entire
ABS—depending upon the system and the problem.
When this happens, the ABS malfunction lamp lights.
An ABS adjusts brake pressure much faster and more
accurately than can drivers. It’s faster because:
• electronic controls are very fast and
• ABS modulator valves are physically closer to the
brakes than is the driver’s foot brake valve.
It is more effective, too, because an ABS can tailor the brake
pressure to each wheel or set of wheels to provide maximum
Electronic controls allow
an ABS to adjust brake
pressure faster and more
accurately than can
drivers.
An ABS is more effective
on slippery roads
because it tailors the
brake pressure at the
wheel to maximize
vehicle braking and
stability.