Guiding Lights

| December 12, 2008

A little awareness of anti-lock brake warning systems goes a long way toward ensuring that your ABS is ready for duty.

One of the most complex systems in newer trucks is the anti-lock brake system. Diagnosing or fixing typical ABS problems requires more tools and technical expertise than most owner-operators have. The basic diagnostic unit or software can cost over $1,000.

Far easier is understanding the ABS warning lights. Because the government mandates built-in electrical diagnostics, you can know at a glance when you have a tractor or trailer ABS problem. Every time the truck powers up, the system checks itself, and the ABS dash lights will do one of the following:

LIGHT UP FOR A SECOND OR TWO AND THEN GO OUT. Assuming the system is correctly installed, the flash-on/flash-off is a reliable signal that there is no problem. Manufacturers “perform extensive tests to ensure the cables are connected correctly,” says Tim Frashure, ABS engineering manager of Bendix Brakes. However, should the sysem not perform correctly, have it inspected.

NOT LIGHT UP AT ALL. Check to see whether the bulb is burned out, and replace it if necessary. If the problem is not the bulb, consult a technician.

STAY LIGHTED. “If the light comes on and stays on, or comes on and goes off again intermittently, there’s a problem,” says Rick Romer, director of electronic products at WABCO Meritor.

One possibility is the system is not getting power. If possible, make sure the electronic control unit’s power cable is plugged in securely at both ends. Check the ends for corrosion or dirt that a quick cleaning or a shot of WD-40 might cure. Also check the fuse panel for a blown ABS fuse. If the ECU is inaccessible, visit the shop.

The other possibility when the light stays on is that at least one of the five main ABS components – electronic control unit, connecting cables, wheel speed sensor, tooth or exciter ring, or modulator valve – is not working properly. To check further, activate the ABS dash lights’ flashing blink code, which most ABS systems have. This is a Morse-like malfunction code that locates and identifies the problem, Romer says.

Using your service manual, you or a technician can interpret the meaning of the blink codes. Say the light blinks twice, then pauses, then blinks four times and then gives a long pause. Meritor WABCO’s maintenance manual says this means a fault in the left rear drive axle’s modulator valve, though code translations often vary by manufacturer. You’ll also see illustrated, step-by-step troubleshooting procedures in the manuals.

Even though the blink code may show that part or all of the ABS isn’t working, the truck still can be safely driven. “You have your basic brake system, so if something goes wrong with the ABS, you will get home,” says Mark Flick of LINK-Radlinski, an engineering firm that services brake component manufacturers. “But I’d get the problem taken care of quickly.”

Two common ABS problems involve:

WHEEL-SPEED SENSORS AND THEIR CONNECTING CABLES. Wheel-speed sensors and tooth rings are tucked tightly inside wheels. Cables lead from sensors to the ECU and modulator valves, which might be anywhere. A typical problem is “that cable gets cut or broken, or the sensor comes loose from the wheel,” says Larry Walls, Northern Alabama Peterbilt service chief.

Sometimes you can fix this yourself. These sensors fit into a bushing, which keeps them pushed against the tooth ring. “That can get moved out of place during maintenance on wheels and brakes,” Frashure says. He recommends simply pushing the sensor all the way back into the bushing. This is easily done on steer wheels, but ABS sensors on drives and trailer tandems are usually out of reach.

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