TroubleshootingAnti-lock
Diagnostics Help Keep ABS Working
ED THOMAS
CONTRIBUTING EDITOR
Publicity over anti-lock braking systems has pretty much died down since ABS gradually became required in the mid-to-late '90s. Unlike the controversial systems 20 years earlier, modern electronic ABS seems pretty reliable and safety records show it reduces accidents for combination vehicles, which benefit the most from its stability enhancing function.
However, ABS is something else to buy and maintain, and it's neither trouble-free nor maintenance-free. Federal law requires it to be kept working, so both drivers and technicians should know something about it.
For Drivers:
• ABS "intervenes" only in hard braking or braking on slick surfaces. Most of the time it just lurks in the background, electronically monitoring your wheels to watch for skids. It begins operating when it senses one or more wheels slowing down faster than the others.
• When ABS is functional (that is, if you see no warning lights), drive like you always do, except do not pump or "fan" the brake pedal when in a sudden or "panic" stop, as this interrupts the circuitry and prevents ABS from doing its job.
• If a warning light begins shining (on the dash or at the rear of a trailer), don't get nervous. You still have good brakes because the "fail-safe" mode of all ABS products is to let the service brakes revert to "normal" operation. This means you'll not have ABS to help you in a sudden stop on a slick road, so proceed accordingly.
• If you see a trailer warning light, it may well indicate a power malfunction. Stop at a safe place and double-check the connector cable between tractor and trailer to be sure it's tight. Trailer ABS gets its steady power through one of the connector's circuits, and if it's loose the power may be interrupted. Otherwise, report the malfunction to your company, or if the truck is yours, to a technician.
Anybody can do a basic check of ABS. For instance, if ABS is working properly, the warning light will flash on briefly, then go off, when you turn on the ignition. You can further test it by cranking up the engine, releasing the parking brakes and accelerating to a speed above 6 mph (below that, ABS automatically cuts out). Then apply the brakes and stop the truck. If the light stays off, the system is working. If a light comes on, the system's brain has detected a malfunction.
Most technicians know that tractor and trailer ABS operate separately. On late-model equipment, where a trailer ABS malfunction must "light the light" in the tractor, all brands work together for the signaling. So a brand X module on the trailer will send a malfunction signal through a brand Y module on the tractor, and thence to the dash light.
This industry standard is called PLC, for power line carrier. It includes standardized blinking codes so no matter what brand is on the tractor or trailer, techs (and even drivers, if they care enough to look up the codes) can figure out what's wrong. If you're using an electronic service tool, the codes may be displayed on a screen, so get a copy of the troubleshooting guide published by the maker of the system installed on your company's or customers' vehicles. Read it carefully before proceeding.
Disconnect power from the ABS, plug in and activate the service tool, and you should quickly pinpoint of any problem. For example, Haldex' troubleshooting guide lists two- and four-character letter and number codes that indicate everything from a defective control module to a miswired warning light. One symptom of a miswired light is that it stays on steadily, even after fault codes have been purged from the module's memory.
If the warning light never illuminates, even for the initial check when you turn on the ignition, then the bulb may be burned out. Check that first. If the bulb's OK, power may not be getting to the control module. Check for power continuity between the vehicle's main harness and the multi-pin connector on the control module, following specific procedures as outlined in the manufacturer's troubleshooting guide.
Fleet managers say that most problems with ABS relate to its mechanical parts: the toothed tone wheel installed near the hub, and the electronic sensor itself, mounted against the tone wheel to measure the speed of the wheel and therefore the vehicle. Sometimes these things get bumped as wheels are pulled off or put on the hub. Any warning of a malfunction may well involve something as minor as that.
