Demco CR 108, Operator'S Manual

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Full Function Anti-lock Brake System
GENERAL OPERATION OF THE ABS SYSTEM
The Full Function Anti-lock Brake System (FFABS) maintains stability and control during
braking by preventing wheel lock-up. FFABS consists of Sensors and Exciters, Modulating
Relay Valve, and an Electronic Control Unit (ECU) to maximize the braking ability of the
trailer. The ECU monitors wheel speeds, and thus vehicle speed, through the use of
sensors and exciters that are mounted on the hubs of the trailer. When the ECU detects
the speed of a wheel or wheels decreasing rapidly during a braking application, it releases
the air pressure in the brake chamber of the affected wheel (s) via the modulator. This
allows the wheel(s) to begin rotating again, thus avoiding lock-up. As soon as the wheels
begin to rotate again, the ECU reapplies pressure in the affected brake chambers to
maximize braking effectiveness. If the condition that caused the lock up remains, the cycle
is repeated until either the brake application or vehicle is stopped. Operation is totally
automatic and can occur up to six times per second.
FFABS can be installed on a variety of trailer configurations capable of controlling 2, 4 or 6 service/spring
brake chambers. When the system is installed on a multi-axle trailer, it is important the proper axle is
selected for positioning of the ABS wheel end sensors. Several factors are involved in making this decision,
but basically, the axle selected should be the one that carries the least load or is most likely to lock first in
a hard brake application.