CDL · Combination Vehicles · 5 min read
Using Antilock Brakes Across Units
- Identify the operational roles and proper positions of shut-off valves across combination units.
- Recognize the identification methods, lamp requirements, and physical components of antilock braking systems (ABS) on trailers and converter dollies.
- Apply safe braking techniques, monitoring habits, and corrective actions across combination vehicles with various ABS configurations.
Shut-Off Valves and Air Line Management Across Trailers
In combination vehicles where one trailer pulls another trailer, managing the flow of air through the entire vehicle combination is essential for proper brake operation. Shut-off valves, which are also referred to as cut-out cocks, are installed in the service air lines at the back of trailers used to tow other trailers. These valves provide a mechanical way to manage the connection between units.
The primary purpose of these shut-off valves is to allow air lines to be closed off when another trailer is not being towed. When pulling multiple units, compressed air must flow freely from the towing vehicle all the way to the rearmost unit. If a trailer at the very end of your combination is not towing another unit behind it, leaving those air lines open would allow air to escape into the atmosphere. The cut-out cocks make it possible to secure the air system completely at the rear of your combination.
For trailers with shut-off valves, check that all valves are open except those at the back of the last trailer, which must be closed. Close the shut-off valves at the back of the last trailer, as the manual directs.
Common mistakes
- Leaving the shut-off valves at the back of the rearmost trailer open, which allows necessary air pressure to escape from the supply or service lines.
- Altering braking technique when operating a mixed ABS configuration, such as pumping the pedal when only the tractor or only the trailer has ABS, rather than braking normally.
- Assuming an ABS malfunction causes a total loss of brakes, when in fact the vehicle retains regular braking capability and should be driven normally until serviced soon.
- Driving faster, following more closely, or driving less carefully because the vehicle has ABS.
Key points
- Shut-off valves (cut-out cocks) are located in the service and supply lines at the back of trailers towing other trailers to close lines when not towing another trailer; all shut-off valves must be open except those at the back of the last trailer, which must be closed.
- All trailers and converter dollies built on or after March 1, 1998, are required to have ABS, while many built before that date were equipped voluntarily.
- Look for a yellow ABS malfunction lamp on the left side of a trailer, at either the front or rear corner; on older units, check underneath for an ECU and wheel speed sensor wires if ABS is difficult to identify.
- To identify ABS on pre-March 1, 1998 vehicles, look underneath the vehicle for an ECU and wheel speed sensor wires running from the back of the brakes.
- Drivers must brake the same way they always have and apply only the braking force necessary to stop safely and maintain vehicle control, regardless of whether ABS is on the tractor, trailer, or both.
- As you slow down, monitor your tractor and trailer, and back off the brakes if safe to do so to stay in control; if a tractor jackknife starts, let up on the brakes if safe to do so until control is regained.
- An ABS malfunction does not cause a total loss of brakes, regular braking remains functional, you should drive normally, get it serviced soon, and never drive faster, follow closer, or drive less carefully.
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