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Diagnosing ABS Problems

By on September 1, 2019

Keep It Simple, Keep It Rolling

By: Homer Hogg, Director, technical service

What once was a mystery to the trucking industry, anti-lock braking systems (ABS) are now old technology, yet still very effective. A vehicle equipped with ABS helps the driver maintain steering control and avoid sliding and jackknifing during an emergency braking situation.

According to FMCSA 393.55, “Each truck tractor manufactured on or after March 1, 1997, and each single-unit, air-braked vehicle manufactured on or after March 1, 1998 shall be equipped with an electrical circuit capable of signaling a malfunction that affects the generation or transmission of response or control signals to the vehicle’s anti-lock braking system.”

Though the technology has been used in tractors for over 22 years, I still receive calls from technicians and customers trying to turn off that pesky ABS light. Regardless of the manufacturer, ABS operations are very similar. This helps provide a consistent diagnostic approach to turn the light off by locating and correcting the root cause of the problem.

ABS Communications 101

The majority of problems we see are related to the trailer ABS and for good reasons. One or more of the existing circuits in the truck’s pigtail are used to transmit bilateral communications from the tractor to the trailer. Many pigtails aren’t properly secured, or loose connections result from the constant pigtail flexing when the driver makes sharp turns or simply hits normal potholes and bumps. When the pigtail has a bad connection, the trailer ABS has a difficult time sending electrical messages to the tractor. If the tractor can’t properly communicate with the trailer ABS computer, the ABS light will come on. 

Another area that causes many ABS trailer malfunctions is the electrical harness under and just in front of the trailer’s undercarriage. One of the circuits in this harness is the ABS circuit. When the belly cord or harness is damaged by road debris, it can cause an ABS malfunction. I have seen standard butt connectors used to repair the damaged wires, which then resulted in poor electrical connections, causing the ABS light to illuminate.
A proper repair requires a connector, solder and heat-shrink tube. The communication is simply voltage sent in a pattern from one computer to another. When the circuit has resistance due to corrosion or bad connections, the ABS warning light can illuminate.

There are times when the ABS light turns on that are good for the driver to note. If the trailer has a loose wheel bearing, the ABS computer will see a fault at that wheel position, therefore, it is always advisable to check your wheel bearings very carefully when you see that ABS light turn on. This may prevent a wheel hub from separating from the vehicle while the trailer is in motion.
The ABS system goes through a self-inspection every time you turn on the key or power up the truck. The ABS light should cycle on and then back off once the test is complete. If the light stays on, the ABS system is not functioning properly and must be diagnosed.

Sensing the Problem

Another very common problem we see when diagnosing an ABS fault is a bad sensor. These are very simple sensors that act like small electrical generators. They have a magnet and a coil of wire. The wheel hub has a metal ring with precise gaps cut in it. As the gaps pass the sensor, a small electrical signal is generated and sends a message to the ABS computer. If a technician suspects a bad sensor, he or she can jack up and spin the wheel to measure the electricity generated by the sensor. It’s just that simple. Many times the sensor is not generating because of damage to the sensor’s harness caused by road debris. Keep a sharp eye on those ABS electrical harnesses running to the inside of your trailer hubs when you perform your walk-around inspections.

Most root causes of ABS malfunctions are simple. Sometimes it’s just a blown fuse or a bad pigtail between the tractor and trailer. The simplicity of the system must not be overlooked. Usually, when I’m involved in diagnosing an ABS problem, I find that it’s something very simple that was overlooked. Finally, don’t forget the ABS light on the side of the trailer. Sometimes it’s not working because the bulb is burned out. Keep it simple, and you’ll keep it rolling. 

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