A combination of inadequate maintenance and improper service procedures causes wheel-speed sensor problems on ABS systems, sources said. Accurate wheel-speed signals are vitally important because they alert the ABS computer when wheel lockup is imminent. Then the computer can react and prevent wheel lockup by modulating brake application pressure via the electrical-hydraulic (EH) actuator.
These sensors are usually the highest-failure components in an ABS system because they are under the vehicle, exposed to road splash and other debris, vibration and temperature extremes. When the root cause of an ABS problem is not obvious, brake experts recommend double checking wheel-speed sensor operation before moving into more-complicated diagnostic tests.
ABS wheel-speed sensors may be called magnetic pickup sensors, permanent magnet (PM) sensors or variable reluctance sensors (VRS). But all are speed-sensitive AC voltage signal generators. Each sensor has a pickup coil, a stationary permanent magnet wrapped with a coil of fine wire.
A reluctor or interrupter is a set of metal teeth or vanes mounted on the axle or brake rotor. This makes the reluctor or interrupter turn at the same speed as the wheel. The reluctor is designed to spin at a specified clearance from the pickup coil. Remember that increasing this clearance or air gap weakens the AC signal the speed sensor produces.
Every time a reluctor tooth or interrupter vane passes the pickup coil, the coil produces an AC voltage signal that goes to the ABS computer. The faster the wheel turns, the more powerful the AC voltage produced.
What's more, the faster the wheel turns, the more frequently the signal pulses between a positive and negative voltage. Most voltage signals technicians encounter occur only on the positive voltage scale. However, an AC signal constantly cycles back and forth between positive and negative voltage scales.
The ABS computer determines wheel speed by monitoring how often this positive-to-negative cycle occurs. The faster the wheel turns, the more often the voltage signal cycles. Frequency-which technicians measure on a meter or oscilloscope-is the number of cycles per second the voltage signal makes.
When a wheel-speed sensor doesn't produce any AC voltage or frequency reading, it suggests a failed sensor, bad sensor wiring, or an extremely wide air gap between the pickup coil and reluctor/interrupter. Some speed sensors have an adjustable air gap, but most of them do not.
Most technicians instinctively check for loose or broken sensor wires when there's no speed sensor signal being generated.
According to Ned Lineback, a brake service instructor at Wagner Brake Products, rain water sometimes penetrates a wheel-speed sensor harness connection, shorting out the signal. Correcting the problem calls for a new sensor harness on some cars. On others, you can fix it by cleaning the connection with electrical contact spray cleaner and coating it with a waterproof silicone grease such as that used on spark plug boots.
Sometimes, water in the speed sensor connections mimics other ABS malfunctions by causing the ABS to operate at very slow speeds right before the car comes to a complete stop. Mr. Lineback stressed that this water-induced condition frustrates techs because it may or may not set a trouble code.
If a speed sensor produces an erratic voltage or frequency at a steady road speed, it suggests loose wiring connections, a loosely mounted sensor, or a contaminated sensor. It also could mean a broken reluctor tooth or bent interrupter vane.
Vehicles equipped with rear-wheel ABS have a speed sensor located in the rear differential housing where the ring gear teeth act as the reluctor. On other rear-wheel ABS systems, the speed sensor is in the transmission tailhousing and it senses the speed of the output shaft.
A four-wheel ABS system has a speed sensor at each
wheel or one in the rear differential and one at each front wheel. According to Jeff Masterman, a technical trainer for Standard Motor Products and its EIS Brake Parts division, comparing signals helps technicians isolate a faulty sensor or sensor circuit on four-wheel ABS.
To do this, locate the ABS computer and identify the wiring terminals for each wheel-speed sensor. Then check the AC voltage or frequency reading at each sensor at the same road speed-for example, at a steady 30 mph. The voltage or frequency readings at each sensor should be the same or extremely close to the same. Typically, the reading at the faulty speed sensor or sensor circuit differs greatly from the others, he said.
Manuals usually list resistance specs for the wire coil inside the pickup coil. However, the resistance test is a static sensor check that usually only identifies the two electrical extremes of a shorted or open coil winding.
But diagnosticians prefer live sensor testing because it
shows how the sensor actually performs-the quality of
signal it actually produces-when the vehicle is running. When a wheel-speed sensor or sensor circuit fails, the ABS computer disables the antilock capability and the vehicle stops on its conventional braking system. Plus, it illuminates the instrument panel's ABS warning light.
But experience shows that contaminated sensors cause erratic wheel-speed signals that may force the ABS computer to ``fail safe'' and turn on the ABS light. Because the sensor's pickup coil is a magnet, it attracts debris such as fine metal powder from the rotors and semi-metallic brake pads. Sensors mounted in the differential or transmission collect the fine metal slivers resulting from normal drivetrain wear and tear.
In other cases, a contaminated sensor may trigger erroneous ABS trouble codes and cause ghost-like problems such as a flickering ABS warning light.
Tom Amundson, a brake service trainer at Raybestos, warned that contaminated speed sensors may cause peculiar ABS behavior. His favorite example is a Dodge minivan on which the ABS would activate during a normal braking situation. When the minivan had nearly rolled to a stop, the ABS would suddenly activate, pulsing the brake pedal!
All of the service experts interviewed strongly urged technicians to visually inspect wheel-speed sensors and reluctors during every undercar inspection, routine brake job or ABS diagnosis. Always clean off any contamination and retest the ABS system before proceeding to more-involved diagnostic steps.
At Stan's Tire and Auto Service, Lafayette, Colo., service manager Dave Hayes noted that consumers often ignore recommended service intervals for transmission and differential fluid changes. Mr. Hayes said the presence of an ABS speed sensor gives service personnel extra incentive to sell these fluid changes.
He also encouraged technicians to treat an ABS speed sensor as another periodic maintenance item-remove it and clean it whenever they change transmission or differential fluids.
Mr. Masterman said techs should carefully clean all metal debris from differential housings after replacing a bad bearing or damaged differential unit. Otherwise, the debris will collect on the ABS speed sensor.
Furthermore, they should treat ABS speed sensors with plastic-safe electrical or brake cleaners. Some speed sensors have plastic housings that melt or distort when exposed to certain cleaning chemicals. Plastic-housing speed sensors are also vulnerable to damage from overtightening, he added.
Technicians should not pierce or crack the jacket around ABS speed sensor wiring. Besides helping to protect the wiring from the elements, the jacket shields it from electrical interference that could disturb the wheel-speed signal and mistrigger the ABS computer.
At Wagner Brake Products, Mr. Lineback warned techs to watch for ABS speed sensors during routine brake and alignment work. One slip with a hammer could destroy a speed sensor or change its air gap.
Some techs create an intermittent electrical connection in an ABS speed harness by stressing its wiring. They do this by allowing a heavy steering knuckle to hang by the sensor wires. Or, they jerk the sensor harness harshly because they're trying to remove an axle hub without disconnecting the harness first.
Mr. Lineback also advised exercising care with Ford Motor Co. rotors. A ring of interrupter vanes for the ABS speed sensor is pressed onto the back of some popular Ford brake rotors. Take care not to bend the ring because a distorted ring causes an erratic wheel-speed signal.
Finally, be sure a replacement rotor has the correct interrupter ring for the application. A ring with the incorrect number of vanes would cause a wheel-speed variation that would force the computer to ``fail safe'' and set a trouble code, he said.