6
Independent Regulation
The most obvious control concept is independent wheel
control. In this case a single sensor controls a valve that
operates the brakes at one wheel site. Individual control
makes the best trade off between stability and stopping
distance. However, these systemshavegreatercomplexity
and higher cost with potentially lower reliability than less
complexsystems. Inmany cases it is necessary to control
a single valve with inputs from two sensors.
Select Low
Select low systems monitor several wheels and controls
them with a single valve. Control is based on the wheel
that is at the lowest speed. Select low systems are very
stable but sacrifice stopping distance on split coefficient
surfaces. Modifiedselectlow systems incorporate a delay
before releasing to reduce the bias slightly away from the
low speed wheel.
Select High
Selecthighsystemsalso monitorseveralwheelsandcontrol
them with a single valve. Control is based on the wheel
whichis at the highestspeed. Modifiedselecthigh systems
activate a release before the low speed wheel becomes
severelylocked. Selecthighsystemsgenerallyhave good
stopping distances at the expense of stability. These
systemsmay alsohavean increasedriskoftire flatspotting.
Select Smart
SelectSmartsystemsoperateasselectlowsystemswhen
there is little difference in traction between wheel control
sites. Theyoperateasselecthighsystemswhenthereisa
significant difference in traction between sites. These
systems offer many of the advantages of individual control
systems while using a simpler design and fewer
components.
Bendix®A-18™Trailer ABS Control Strategy
Select Smart is used for the most common applications.
Select low is used on the standard system 4S/2M Axle
Control configuration and is available as an option on the
basic system.
TRAILERABS CONFIGURATIONS
Application Recommendations
Refer to the chart on page 8 to determine a recommended
ABS installation for your application.
Basic System (2S/1M)
The basic system includes two speed sensors and one
modulator valve for direct control of one axle and indirect
control of an additional axle. Other features of the basic
system include one ECU connector and a single-pin
diagnostic lead.
Standard System (2S/1M, 2S/2M and 4S/2M)
Bendix’s standard systems offer either two or four speed
sensorsand up totwomodulatorvalves. Standardsystems
can directly control one or two axles and allow full
diagnostics via J1587. The standard system may be
configured in one of four ways to function as follows:
• 2S/1M—This configuration uses two sensors and one
modulatorvalvetodirectly controloneaxleandindirectly
control an additional axle.
• 2S/2M—Thisconfiguration usestwospeed sensorsand
two modulator valves for direct control of one axle and
indirect control of up to three additional axles.
• 4S/2M—This configuration uses four speed sensors
andtwomodulatorvalves for direct control of two axles
and indirect control of up to two additional axles.
• 4S/2M Axle Control—Special configuration for full
trailers and widely spaced axles. This configuration
uses the select low strategy.
Sensor Placement
When more than one wheel is controlled by a single valve,
sensorsshould be mounted at the axlewhichtends to lock
first. For spring suspensions this is usually the forward
axle. Forairsuspensionsthisisusuallytherearmostaxle.
Lift Axles
• Gen-4™ABS: Sensor inputs “C” and “D” of the control
unit may be used for lift axle wheel speed sensing.
• Gen-5™ABS:Direct liftaxle control isnotavailable. Use
indirect control.
Reading Configuration Codes
On Basic systems, the jumper method must be used to
access the configuration. On Standard systems, access
to configuration information can be achieved by any of the
three methods:
• ServiceRanger diagnostic software on a PC
• Hand-held tester
• Jumper method.
For more information on accessing configuration codes,
refer toAccessing Codes on page 30.
