7
REVISED 5/31/05
ABS OPERATION
Bendix
®
ABS uses wheel speed sensors, ABS pressure
modulator valves, and an ECU to control either four or
six wheels of a vehicle. The EC-60
™
controller monitors
individual wheel turning motion during braking and adjusts
or modulates the brake pressure at the wheel end. When
excessive wheel slip, or wheel lock-up is detected, the
EC-60
™
controller will activate the pressure modulator
valves to automatically reduce the brake pressure at one
or more of the wheel ends. By these actions, the ABS
system helps to maintain the vehicle's lateral stability and
steerability during heavy brake applications and during
braking on slippery surfaces.
Steer Axle Control
Although both wheels of the steer axle have their own
wheel speed sensor and pressure modulator valve, the
EC-60
™
controller blends the applied braking force between
the two steering axle brakes. This Bendix patented brake
application control, called Modified Individual Regulation
(MIR), is designed to help reduce steering wheel pull
during an ABS event on road surfaces with poor traction
(or areas of poor traction, e.g. asphalt road surfaces with
patches of ice).
Single Drive Axle Control (4x2 Vehicle)
For vehicles with a single rear drive axle (4x2), the brakes
are operated independently by the EC-60
™
controller,
based on the individual wheel behavior.
Dual Drive Axle Control (4S/4M Configuration)
For vehicles with dual drive axles (6x4) using a 4S/4M
configuration, one ABS modulator controls both right-side
rear wheels and the other modulator controls both left-
side rear wheels. Both wheels on each side receive equal
brake pressure during an ABS stop. The rear wheel speed
sensors must be installed on the axle with the lightest
load.
Dual Rear Axle Control (6S/6M Configuration)
For vehicles with dual rear axles (6x4, 6x2) using a 6S/6M
configuration, the rear wheels are controlled independently.
Therefore, brake application pressure at each wheel is
adjusted according to the individual wheel behavior on
the road surface.
6x2 Vehicles with 6S/5M Configuration
6x2 vehicles can utilize a 6S/5M configuration, with the
additional axle (a non-driven rear axle) having two sensors,
but only one Pressure Modulator Valve. In this case, the
PMV controls both wheels on the additional axle. The
additional axle wheels would receive equal brake pressure,
based on the wheel that is currently experiencing the most
wheel slip.
Normal Braking
During normal braking, brake pressure is delivered through
the ABS PMV and into the brake chamber. If the ECU does
not detect excessive wheel slip, it will not activate ABS
control, and normal vehicle service braking is applied.
Retarder Brake System Control
On surfaces with low traction, application of the retarder can
lead to high levels of wheel slip at the drive axle wheels,
which can adversely affect vehicle stability.
To prevent this, the EC-60
™
controller switches off the
retarder as soon as a lock-up is detected at one (or more)
of the drive axle wheels.
When the ECU is placed in the ABS off-road mode (on
vehicles equipped with this optional feature), it will switch
off the retarder only when ABS is active on a steer axle
wheel and a drive axle wheel.
Optional ABS Off-Road Mode
On some road conditions, particularly when the driving
surface is soft, the stopping distance with conventional ABS
may be longer than without ABS. This can occur when a
locked wheel on soft ground or loose gravel plows up the road
surface in front of the tire, changing the rolling friction value.
Although vehicle stopping distance with a locked wheel (in
the absence of ABS) may be shorter than corresponding
stopping distance with conventional ABS control, vehicle
steerability and stability would be reduced.
Advanced EC-60
™
controllers have an optional dash switch
initiated modified ABS control mode (know as "offroad
ABS) that more effectively accommodates these soft road
conditions to shorten stopping distance while maintaining
optimal vehicle steerability and stability.
The EC-60
™
controller will perform a PMV chuff test on all
installed modulators in the following order:
• Steer Axle Right PMV
• Steer Axle Left PMV
• Drive Axle Right PMV
• Drive Axle Left PMV
• Additional Axle Right PMV
• Additional Axle Left PMV
• Drive Axle TCV
The pattern will then repeat itself.
With the EC-60 advanced controller, at the completion of
the second round of PMV & TCV chuff tests, the controller
(if configured to do so) will perform a test to cross-check the
trailer PMV operation with the vehicle stop lamps. If the trailer
PMV circuit is mis-wired (including the steer axle TCV), the
PMV will exhaust a large amount of air, or none at all.
NOTICE: If there are any active Diagnostic Trouble
Codes, stop lamp cross-check portion of the chuff test will
not be carried out until all DTCs are fully diagnosed and
corresponding repairs are successfully conducted. The
ESP/ATC dash indicator will also be illuminated when there
are active ABS, ATC or ESP DTCs.
The ECU will not perform the PMV Chuff Test when wheel
speed sensors show that the vehicle is in motion.
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