A multitude of problems can trigger erroneous fault codes on antilock brake systems (ABS), service experts said. Many codes are spurred by wheel-speed sensor problems because these sensors are the most vulnerable, highest-wear components in the entire ABS system. However, common problems in the fundamental brake system also trigger many ABS codes. All brake service sources agreed that the ABS system is only as good as the basic brake system to which it's connected. When the ABS control computer sets a fault code, it disables the ABS system and the conventional brake system continues operating. The computer also turns on the antilock warning light on the dashboard.
Speed sensor snafus
Sources said that speed-sensor-related quirks often turn on the antilock warning light. For example, erratic, uneven traction on snowy or icy roads may vary wheel-speed and/or spur extended ABS operation, turning on the warning light.
When technicians can find nothing obviously wrong and suspect snow or ice tricked the ABS computer into disabling the system, they should follow the prescribed procedure for clearing the computer's memory. Then road test the vehicle and see if the trouble code and illuminated warning light return, said Joe Meyer, an instructor at Professional Automotive Career Training (PACT), a division of Conrad's Goodyear, Cleveland.
Service managers TIRE BUSINESS interviewed agreed the severe ice and snow conditions that prevailed in much of the country last winter turned on many antilock warning lights.
At Kieser's Tire & Service Center, Fairless Hills, Pa., service manager Timothy Steigerwalt encouraged technicians to resist the temptation to leap into detailed ABS diagnostics when the initial inspection shows nothing wrong with the system or the vehicle.
Writing down codes, clearing the computer memory and road testing to see if the codes recur is a successful technique for diagnosing engine control systems. But it's equally practical for ABS troubleshooting, he said.
Plus, understanding subtle factors such as the impact of road conditions on ABS operation is part of the practical outlook needed to diagnose the systems accurately, he added.
Although visual inspections reveal many root causes of ABS trouble, technicians may not automatically associate the problems they find with ABS. That's why it's critical to ask the customer if anyone has worked on his vehicle recently. Learning what was done to the vehicle may be the mental bridge that links the tech's findings with the root of the ABS problem, noted Ron Bulla, operations and training manager at Kieser's.
For instance, a car that had been aligned recently at another shop came in with the antilock warning light on and a fault code indicating an erratic front-wheel speed sensor. Visual inspection showed the cam bolt for adjusting camber on that side of the vehicle was extremely loose. The technician torqued the bolt to specification in the interest of safety-and fixed the ABS problem at the same time.
Further investigation confirmed the loose bolt caused the sensor to vibrate violently, causing an erratic wheel-speed signal and triggering a fault code, Mr. Bulla explained.
Likewise, some vehicles come into a shop needing nothing more than routine brake service and leave with an ABS problem. Technicians who are careless with wrenches, hammers, drums and rotors often whack wheel-speed sensors, damaging the sensor or altering its air gap, said Tod Lange, an instructor and field service specialist with Raybestos/Brake Parts Inc.
As explained in earlier service supplements (March 23, 1992 and August 9, 1993), ABS wheel-speed sensors are magnetic-impulse sensors that produce an AC voltage signal. The air gap between the stationary permanent magnet and the rotating reluctor ring is critical to producing a consistent, accurate speed signal.
Mr. Lange said that besides being high-failure components, wheel-speed sensors often become contaminated with fine metal particles. The problem causes erratic wheel-speed signals that confuse the
ABS computer, causing it to disable the system, store a trouble code and turn on the antilock warning light.
Brake service experts at EIS Brake Parts and Brake-Pro Systems both confirmed that metal particle contamination can set false fault codes. It can also cause phantom symptoms such as an antilock warning light flickering on and off for no apparent reason, they said.
The wheel-speed sensor on many popular light trucks is mounted in the side of the transmission or rear differential housing. The inner tip of the permanent-magnet sensor is exposed to transmission or differential oil because it monitors the speed of the transmission output shaft or differential ring gear.
Consequently, metal chips caused by normal drivetrain wear accumulate on the sensor tip over a long period of time.
Mr. Lange urged techs to remove the wheel-speed sensor from the transmission or differential, clean it and retest the vehicle before proceeding with ABS diagnosis. Also, change the transmission or differential oil. It also pays to open up the differential on a high-mileage vehicle and carefully wipe out the bottom of the differential housing.
Brake-Pro Systems technicians said the presence of permanent-magnet speed sensors underscores the importance of washing all drums and rotors after cutting these parts on a lathe. Besides contributing to rough brake application and brake noises, the metal dust may be drawn to the speed sensor.
Plus, grease thrown from a leaking CV boot may soil an ABS speed sensor or reluctor ring. Grease on a reluctor ring may attract metal dust from a rotor or semi-metallic brake lining. A metal-laden grease spot may trigger a speed sensor code because the computer thinks the spot is an extra tooth on the reluctor ring, EIS technicians said.
Some sources said that when in doubt, technicians should wash off a suspect wheel-speed sensor with an appropriate brake or electrical cleaner, then retest the vehicle.
The harmless-looking, fine black brake dust on the sensor could contain enough fine metal particles to disturb the wheel-speed signal.
At PACT, Mr. Meyer said experience confirms that running a space-saver spare tire on an ABS-equipped car may turn on the antilock warning light and set a wheel-speed code.
After reinstalling the standard-size tire, technicians need only clear the ABS computer memory and drive the vehicle.
Basic brake problems
Basic brake trouble creates countless ABS-related complaints. A brake manufacturer's field service specialist told TIRE BUSINESS technicians should forget the ABS system even exists until after they have road tested the car and thoroughly inspected the
conventional brake system.
Mr. Meyer explained that any basic problem that makes the rear brakes grab or apply too quickly may activate the ABS. When this happens, the customer may describe the symptom as oversensitive ABS-the system activates during light and/or moderate braking. It also may set a fault code for extended ABS operation, he said.
Cracked rear brake linings, oil-contaminated linings, weak brake return springs, or a faulty proportioning valve can cause the sensation of oversensitive ABS. (Proportioning valve diagnosis is explained elsewhere in this service section.)
Another possible cause is any basic brake problem that allows the brake on one side of the vehicle to grab sooner than the other. Improper brake adjustment also can create or aggravate overactive ABS operation, Mr. Meyer said.
Once again, vehicle history is vital because simple brake service mistakes can brew unexpected ABS trouble, Mr. Lange warned. For instance, the parking brake link or strut activates the rear brake shoes when the driver applies the parking brake. On one vehicle he diagnosed, this parking brake link fell out because it had been improperly installed. The link then cocked or wedged the brake shoes against the drum, causing grabby brakes and premature ABS operation, he said.
Carefully following a diagnostic flow chart is a good way to isolate the cause of an antilock warning light. At PACT, Mr. Meyer said experience has taught him to prioritize one item that often falls at the bottom of troubleshooting flow charts-check the fuse!
For instance, some ABS systems have a power wire that operates the computer itself and a separate wire that powers the computer's ``key-off'' memory. The antilock warning light comes on if either power feed open-circuits.
These power wires may be fused to the same circuits as the dome light, stop lights, dashboard gauges etc. Consequently, a short circuit-and the accompanying blown fuse-that initially appears unrelated to the ABS may be the real cause of the antilock warning light coming on, Mr. Meyer explained.
Raybestos' Mr. Lange said improperly wired add-on accessories, such as CB radios and cellular telephones, also can trigger a false antilock warning light.