Equipment Inspections - Trailer ABS

     Non-functioning ABS is viewed upon as a minor defect, and it is an infraction that is ticketed regularly. However, the rules regarding the way ABS works and functions have changed at least three times over the last 15 years. Therefore it becomes challenging to determine which system you have on the trailer you are inspecting, and consequently to determine whether or not it is functioning correctly. In the preceding paragraphs I will attempt to clear up some of the confusion, and hopefully give you a better understanding of the various ABS systems.

     First and foremost, even if the ABS on the trailer is defective, you will still maintain regular braking capabilities, just as if you were pulling a trailer that was not equipped with ABS.

    ABS became a DOT requirement beginning with trailers manufactured March of 1998. This being the case, we have many trailers in the fleet that were manufactured before ABS became standard equipment.

These would include:
  • 456 thru 460
  • 482 thru 490

Trailers Manufactured prior to September 1997 - Front Light ABS
     The ABS indicator light was originally placed on the front of the trailer. With systems where the light is on the front of the trailer, you must be doing at least 7kph before applying the service brakes in order to properly test the ABS. If the light comes on and stays on for greater than 3 seconds when you apply the brakes to a moving vehicle, then the ABS in malfunctioning and is due for service. The ABS would also be due for service if the light fails to come on at all.

     In our fleet, as the
front light systems have been failing, and we’ve been upgrading them to the more modern side-light system. Therefore, the easiest way to determine what system you are dealing with, is to simply look at the position of the light. Front light systems were originally standard on trailers 464, and 5004 thru 5198.


Trailers Manufactured after February 1998 - Side Light ABS
     After Feb of ’98, trailer manufacturers began installing side light ABS systems. Side light systems are more sophisticated in the fact that they do not require a minimum speed to make the system function. Therefore as soon as you are rolling you should be able to apply your service brakes, and then watch the side light come on and then go off, within 3 seconds of your brake application. If the light comes on and stays on, or fails to come on at all when you apply the brakes to a moving vehicle, then the ABS system is not functioning properly and requires service. Please write up any issues on your inspection report.


The ABS light should work as shown below, when the system is functioning properly:

Tractors & Trailers Manufactured after February 2001
     After February of ’01, the trailer ABS system was required by law to send a signal to the tractor. This was accomplished by way of an indicator light on the dash labeled “Trailer ABS”. When you apply your service brakes you will see the trailer ABS light on the dash illuminate for 3 seconds and then go out. If the trailer ABS light illuminates and stays lit for the duration your foot is on the brake, then the ABS system has a malfunction. The trailer abs light on the dash of the tractor, and the side light on the trailer both communicate the same thing, however the pair of them are required to be working as per DOT regulations.

     We only have two trailers in the fleet that are new enough to
require this function to work, 498 & 499. Further more, this function will only work with a tractor that was built after February of ’01, which would include T46 thru T52 , and T-72 thru T79.

     As an aside, this function will work with some of the older trailers in the fleet, as mentioned previously, some of the older ABS systems have been retrofitted, and therefore include the ABS functions of a newer trailer. They however, are not required to do this by law, due to their built date.

TRL ABS light in Tractor

The trailer ABS light on the dash of the tractor should work as shown below, when functioning properly.

Disclaimer: K.A.M. makes no guarantees regarding the accuracy of this information. It is intended for educational and assistance purposes, for the drivers of K.A.M. Trucking Inc.