Abs Operation - BENDIX TABS-6 TRAILER ABS MODULE Manual

ABS System
Status Indicators
Trailer-mounted ABS
Indicator Light
Dash-mounted Trailer
ABS Indicator Light (PLC
Detected)*
Dash-mounted Trailer
ABS Indicator Light
(PLC Not Detected)
*Some vehicle manufacturers may illuminate the trailer ABS
indicator lamp at power-up regardless of whether a PLC
signal is detected from the trailer or not. Consult the vehicle
manufacturer's documentation for more details.
FIGURE 8 - TRAILER ABS LAmP START UP SEqUEnCE
Trailer ABS indicator lamp
At power-up without detected faults, the trailer ABS
indicator lamp will turn on for 2.5 seconds as a bulb check
and then turn off. See Figure 8.
If a PLC-ready towing vehicle and trailer are powered at
the same time, the TABS-6 module will also trigger a bulb
check on the dash-mounted trailer ABS indicator lamp.
Modulator Chuff Test at Power-Up
At power-up, the TABS-6 module activates a modulator
chuff test. This electrical and pneumatic ABS modulator
test can help the technician identify problems with
modulator installations and/or wiring.
With brake pressure applied, a properly installed modulator
will cause five rapid audible chuffs of air pressure. If
additional modulators are installed, the TABS-6 module
activates 5 chuffs at the internal modulator (MOD1) then
for each additional modulator in sequence (e.g. MOD1,
then MOD2, and then MOD3). The chuff sequence is
then repeated.
If the modulator is wired incorrectly, the modulator will only
produce one chuff, or no chuff at all. If an issue is detected
during the modulator chuff test, compare the modulator
wiring and plumbing to the TABS-6 module's electrical
system schematic (see page 17) and make repairs.

ABS OPERATION

The TABS-6 module uses wheel speed sensors, modulator
relay valves and an ECU to control trailer wheels by axle
or by side. By monitoring individual wheel turning motion
during braking, and adjusting or pulsing the brake pressure
at each wheel, the TABS-6 module is able to optimize slip
between the tires and the road surface. When excessive
wheel slip, or wheel lock-up, is detected, the ECU will
activate the Pressure Modulator Valves to modulate
Power
Application
0.5 1.5 2.0 2.5 3.0 (sec.)
ON
OFF
ON
OFF
ON
OFF
braking pressure at the wheel ends. The ECU is able to
pump the brakes on individual wheels (or pairs of wheels),
independently, and with greater speed and accuracy than
a driver.
Axle Control
TABS-6 module axle control uses a single modulator relay
valve to control wheels on both sides of a given axle or
axles. In the case of an ABS event on road surfaces with
poor traction (worn, slippery, or loose gravel roads) or areas
of poor traction, (e.g. asphalt road surfaces with patches of
ice), axle control will maintain the wheel that is not slipping
at just under the speed that will lock the wheel. Temporary
periods of wheel lock are permitted on the other wheel that
is experiencing slippage.
Axle control should not be used on 5th wheel dollies or
steerable axles. When braking on even surfaces, an
axle-control system will perform similar to a side control,
two-modulator system. Axle control is available in 2S/1M,
2S/2M and 4S/2M installations, and for Modulator 3
(MOD3) in a 4S/3M installation.
Dolly-Axle Control (Select Low)
TABS-6 module dolly-axle control uses a single ABS
modulator valve to control wheels from both sides of a
given axle or axles. In the case of an unbalanced braking
surface, (e.g. asphalt road surfaces with patches of ice),
dolly axle control will control the low coefficient (slipping)
wheel just under the lock limit. Vehicle stability is assisted
by not allowing the high coefficient wheel (where traction
is still being maintained) to sustain wheel lock.
When braking on even surfaces, a dolly axle control system
will perform similar to side control or axle control system.
Dolly axle control is only available in 2S/1M installations.
Side Control
The TABS-6 module uses a single modulator relay valve
to control one or more wheels on a given vehicle side. In
the case of an unbalanced braking surface, the side control
will individually control wheels on each side just under the
point where they would lock up.
Side control is available in 2S/2M and 4S/2M installations,
and for the internal modulator (MOD1) and Modulator 2
(MOD2) in the 4S/3M installation.
Normal Braking
During normal braking, the TABS-6 module functions as a
standard relay valve. If the ECU does not detect excessive
wheel slip, it will not activate ABS control, and the vehicle
stops with normal braking.
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