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BRAKE IT DOWN…


The Federal Government has required most tractors to have an Antilock Brake System (ABS) since March of 1997. The legislation extended the requirement to straight trucks and trailers in March of 1998. Prior to this ABS was an option rarely employed.

So, what does this system provide?

ABS reduces wheel lockup, allowing steering control, and shorter stopping distances. On the chassis, if any outer ends (steer or drives, not pushers or tags) nears lockup during a braking event, the system automatically adjusts variable air pressure to that outer end/s, preventing lockup along with slowing the overall speed of the vehicle. This is accomplished using sensors and modulators.


4S/4M is the designation given to 4 sensors and 4 modulators. Two sets are always located on the steer axle, the other two on a single rear axle, or usually the rearmost axle of a tandem. This practice provides sensers to the axle that will lock up first when braking, which is usually the tandem rear-rear, since it’s a bit lighter in weight than the forward-rear. In this tandem configuration the forward-drive axle is controlled using an air valve via the action of the rearmost axle. The right forwardmost action is controlled by the right rearmost, and the same on the left.


In practice, a 4S/4M system on a 6X4 chassis functions much like a 6S/6M system would. The exception would be when the forward drive locks up first, but once the brakes are applied and the chassis weight shifts due to inertia, the rearmost axle would immediately become sensed, so this is a very small window of time. Both 4S/4M and 6S/6M systems meet todays reduced stopping distance laws. If there is any question of application question, sensing all outer ends (6S/6M) is preferred. If utilizing ATC, you definitely want 6S/6M.

The chassis system also automatically applies air to the trailer which utilizes its ABS to function independently from the tractor. The trailer ABS requires electrical power (and related ground wire) from the tractor to operate, utilizing controlled air to stop the trailer tires.


A yellow ABS light located on the left rear corner of the trailer, providing detection of system function. When connected, the light is illuminated while the system goes through a check of the system. If the light stays lit, the system has a fault, plus lighting a dash light informing the driver of the condition. This dash light requirement was enacted in 2001.

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