*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
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.
