Brake on through

Most tractors today come equipped with an Automatic Traction Control (ATC) system and the functionality is optional on most all of today’s trucks. While active, the system monitors the drive axle sensors (4S/4M – 4 sensors/4 modulators or 6S/6M 6 sensors/6 modulators). When the outer end spins under acceleration, the system will apply that brake, moving power to the other outer ends. If the condition continues, engine torque will drop to reduce wheel spin to keep from overheating the brakes. There is also an illuminated ATC light in the dash, indicating a spinning outer end, with active ATC and a mismatch between outer end speeds.

The system either includes or has an optional switch in the dash labelled as ATC or TC. When in existence the system default is on, the switch is for override at low speeds. If the chassis comes with optional mechanical locking axles (lockers), and has the ATC switch, the system should be turned off when the locks are engaged, to eliminate a conflict. If both are engaged, brakes will be applied to the spinning drive axles and the engine will de-rate. For this reason it is imperative to have a switch that turns ATC off with locking axles. Once the chassis gets to road speeds, the ATC function will automatically reengage.

The chassis might come with a Mud and Snow switch. When engaged this feature allows a permissible amount of outer end spin during acceleration. This is designed to increase the available traction in inclement or off-road conditions. When the spin event occurs, usually the ATC light blinks on/off.

The ABS Off-Road switch desensitizes the antilock brake system. This switch is, as noted, used for improved off-road stopping. It permits a longer brake lockup period, which allows the tire to build up material (wedge) in front of the tires, so that they have to ride up over the material, which makes stopping more effective in off road conditions. This can be activated at slower speeds, usually less than 25 MPH. At speeds above the programmed threshold the feature is locked out. This function is not allowed with Automatic Traction Control, so you must make a decision about which system is most important in the customers application.

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.

Turn! Turn! Turn!

What is the purpose of a lubrication pump in a tandem drive axle set? There is a theory that this pump sends lube to the outer ends, this is not true. It has been reported that this pump is required on steep inclines, also not true. Yet another opinion is that over the road chassis do not need the pump. Based upon the information below, we will let you decide about the last one.

The rear axle pump option is located in the forward drive axle. Without the pump the ring gear splashes lubrication up to the gears. As long as the ring gear is spinning forward, it does a fantastic job of accomplishing this. Which is why the pump is not standard. However, without a pump there is no way to ensure lube to the upper gears and bearings in the power divider when the rearmost tandem axle is spinning, and the forward outer ends are not. This condition spins the interaxle components, but not the ring gear which supplies splash lube throughout the housing.

If a driver stops and locks the axles together using the interaxle switch, when the chassis is in inclement weather, then the pump is not required because at least one outer end on forward drive axle will be spinning and the splash from the ring gear action will be sufficient. However, relying on a driver to take this action could be risky.

The pump provides a nice insurance policy for drivers who do not understand when they should lock the axles. Another condition that might warrant the pump is in applications where the truck backs long distances. For example, a dump truck hauling asphalt. Ordering 6S/6M also helps resolve this condition and should always be ordered with ATC.

Not all axles have a pump option. For newer axle designs the Engineers have devised a way to keep the gearing lubricated in all events. When the option exists, the operating principle differs. Some manufacturers operate this pump all the time, others only when there is a difference in forward vs rear axle speed. Some axle manufactures also add an external filter, which requires maintenance.