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A few minutes of investigating can lead you to the cause of an ABS alert

By on May 4, 2015

Any warning light warrants attention, but the ABS light on the dash of your truck or the side indicator light on your trailer should never be shrugged off. It indicates a malfunction with your ABS system, which leads the vehicle’s brake system to default to normal braking with no ABS functionality.

Obviously, this is a Federal Motor Carrier Safety violation that must be corrected. But that’s not the only reason to take action. The lack of antilock braking capability can result in a jackknife or dangerous brake skid situation.

So it’s extremely frustrating as a driver to see those alerts flash on while driving down the highway or light up as soon as the truck starts. It does indicate a malfunction, but it may not be as challenging as you think to identify and correct the problem.

First look

If you just hooked to a trailer, be certain to check the connections at the pigtail. The only way the trailer ABS system can communicate to the tractor is through the pigtail. The center pin in the pigtail, called the auxiliary circuit, is typically used to power trailer options such as interior cargo lights. Most trailer manufacturers use this existing circuit to send a unique voltage signal that is superimposed over the existing 12 volt signal. Because this auxiliary circuit is used for multiple functions at the same time, it is critical to establish a good connection. If you get a voltage drop or loss across the circuit, you will experience ABS problems — including the alert light coming on and off.

Conduct a visual inspection of the pigtail and the plug pins in the nose of the trailer, as well as the plug on the rear of the tractor when this happens. Be certain to take a good look at the overall condition of the pigtail. You want to look for frayed and/or kinked wires, wires trapped between the catwalk and the frame, and just the general condition of this circuit.

Homer-2013Watch the wheel hubs

Once you feel satisfied that the connection between the tractor and the trailer is adequate, take a walk around the vehicle. Look at the belly wires under the center of the trailer. If these wires are ripped or torn or are dragging on the ground, you have likely located the problem. Continue to look for loose or damaged connections on the tractor and trailer frames and undercarriage. One key area to focus on is the wheel hubs.

I recommend you keep an infrared thermometer in the truck at all times. It can be purchased at most home improvement or automotive parts stores. These thermometers allow you to quickly take a temperature reading of your wheel hubs, which can give you a good indication that a wheel bearing may be failing.

If a wheel bearing gets loose it will increase the gap between the sensor and the ring that is attached to the wheel hub. When this gap increases beyond the allowable range, the ABS light will come on. So, a quick walk around the vehicle can be very productive if you look in the right places.

The chuff test

Listen carefully and this test can quickly tell you which wheel or axle position has the problem. When you turn on your ignition switch, the ABS computers initiate a self-test of the ABS modulators. Some vehicles have an ABS modulator valve at each wheel position and others have them at each axle. There are additional configurations but these two are the most dominant in the market. You should hear a “chuff” noise — indicating escaping air — from each modulator. If one of the modulators does not make this sound, it is not functioning properly and that may tell you where to focus the search for the problem.

With just a few minutes and a good walk around your vehicle, you may just save some critical downtime and enjoy some productive uptime!

Homer Hogg, Technical Development Manager for TA and Petro, has worked as a truck technician for more than 30 years. He is ASE Master-certified, a Daimler Certified Trainer and a member of the Nashville Auto Diesel College Hall of Fame.

Homer Hogg’s “Maintenance Matters” airs on the Dave Nemo Show (Road Dog Trucking, SiriusXM 146), 8 a.m. ET, the first and third Thursday of each month

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