Antilock braking systems (ABS) have been with us since the 1980s. But as of 2016, ABS has never been a federally mandated passenger

vehicle safety feature. In spite of that fact, the vast majority of late-model pas-senger vehicles have been equipped with this technology since near the end of the last decade. By contrast, Fed-eral Motor Vehicle Safety Standard (FMVSS) No. 126 requires electronic stability control (ESC) systems to be installed on all passenger cars, multipur-pose passenger vehicles, trucks and bus-es with a gross vehicle weight rating of 4536Kg (10,000 lbs.) or less, beginning with the 2012 model year.

ESC is built upon the capabilities of the ABS system. While ABS deals with the loss of traction only during braking, ESC uses computer-controlled braking of individual wheels to assist the driver in maintaining vehicle control in critical driving situations.

As with any technology, there are bound to be a few glitches. These sys-tems must analyze data from a vari-ety of sensors, then make split-second decisions on when and how much to apply or release the brakes to individual wheels. A number of factors can cause incorrect decisions, which can lead to what has been variously referred to as unintended, unwanted, incorrect or false activation of the ABS and/or ESC systems. This edition of Brake Shop will delve into several causes and cures for false activation of these systems.

Abnormal ABS activation occurs when the ABS control unit makes a wrong decision about when to activate the ABS hydraulic unit. The driver may sense that something is wrong if the ABS activates when the weather and

road conditions are normal. Abnormal ABS activation typically occurs at low-er vehicle speeds (3 to 18 mph) and is normally related to just one possible cause—the wheel speed sensors and their related components.

In an ABS system, the wheel speed sensors are the only inputs the ABS control unit can rely upon to determine

the rotational speed of the vehicle’s four wheels. If one or more of the wheel speed sensors sends an input indicating an abnormal wheel speed or, worse yet, sends no input signal at all, the ABS control unit’s default response is to ap-ply the ABS.

Early ABS systems utilize self-pow-ered wheel speed sensors that gener-ate an alternating current as the wheel rotates. The amplitude as well as the frequency of the output from one of these sensors increases in direct rela-tion to the wheel speed it’s assigned to monitor. Problems can usually be traced to a wheel speed sensor signal that’s either too weak or too erratic to be properly evaluated by the ABS con-

trol unit. So what causes these weak or erratic signals?

As mentioned, the amplitude of a self-powered wheel speed sensor de-creases as wheel speed decreases. The ABS control unit can’t handle a wheel speed sensor input that falls below a certain amplitude. In addition to rota-tional speed, the proximity of the wheel speed sensor to the signal-generating pulse wheel can also diminish the am-plitude of the sensor signal. Typically, rust in the wheel speed sensor mount-ing area will cause the sensor to move away from the pulse wheel. The process is called rust jacking, because the force of the rust moves the sensor upwards.

Similarly, rust or metal particles that accumulate on the tip of a self-powered wheel speed sensor may diminish the sensor’s output amplitude. The cure for both of these problems is to clean the area to remove any traces of rust or debris and to restore sensor air gap to its intended dimension. In some cases, wheel speed sensor replacement may be necessary, as removing a sensor that has already become rusted in place may damage it in the process. Likewise, an otherwise good wheel speed sensor may become damaged during routine brake service if reasonable care isn’t exercised. An errant hammer or wrench blow may deliver damage to a wheel sensor that isn’t immediately apparent, but can cause intermittent problems later.

A rusted or damaged ABS pulse wheel may also cause false ABS activa-tion. The pulse wheel’s evenly spaced notches allow the wheel speed sensor to generate an output signal that’s uni-form. Each peak in the alternating cur-rent output signal is followed by a cor-responding valley. A pulse wheel with irregularly shaped notches, or one with one or more missing teeth, will cause

FALSE ABS & ESC ACTIVATION

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Active wheel speed sensors are commonly used on ESC-equipped vehicles, as well as many vehicles equipped with more ad-vanced ABS systems. Active sensors can produce an output all the way down to 0 mph and are also able to communicate the vehicle’s direction of travel.

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the wheel speed sensor to generate a pattern that’s less than uniform. The ABS control unit may be able to deal with this flawed signal when vehicle speed and sensor output frequency are higher, but may reject or misinterpret it at lower vehicle speeds, resulting in false ABS activation.

To deal with the shortcomings of self-powered wheel speed sensors, more recent ABS and ESC systems are typically equipped with powered or ac-tive wheel speed sensors. Active wheel speed sensors use a magnetoresistive design to output a digital, square wave signal with an amplitude that isn’t af-fected by wheel speed. The best sensors of this type can allow the ABS control unit to measure wheel speed all the way down to 0 mph and can also help the control unit determine whether the wheel is currently rotating forwards or backwards. Active wheel speed sensors aren’t as easily affected by metal parti-cle contamination, either.

An example of active wheel speed sensor operation is shown in the illustra-tion on page 20. Rather than relying on a toothed pulse wheel to generate an al-ternating signal output, a multipole ring is used as a pulse wheel. It consists of alternating magnetized plastic elements arranged on a nonmagnetic metal car-rier. The north and south poles adopt the function formerly performed by the teeth of the pulse wheel. The sensor’s integrated circuit (IC) is positioned in the continuously changing fields gener-ated by the magnets. The magnetic flux through the IC changes continuously as the multipole ring turns. The sensor converts this into a digital output that can be more easily understood by the ABS control unit, regardless of vehicle speed or rotational direction. But prob-lems can still ensue, as we’ll soon see.

As with many other vehicle systems, rust is the enemy of ABS and ESC sys-tems. The photo on this page is a dam-aged combination hub and wheel bear-ing assembly removed from a Chevy Equinox by Jim Henderson of Jim’s Automotive in Barberton, OH. Jim re-ported to Motor contributing editor Dan Marinucci that the control unit had stored a “plausibility” trouble code for the wheel speed sensor adjacent

22 October 2016

to the damaged wheel bearing assem-bly. When he road-tested the vehicle while monitoring the vehicle speed with his scanner, the vehicle speed reading would drop out completely and errati-cally at about 7 to 8 mph, he said.

ESC systems also experience false ac-

tivation problems, which may be traced to one or more defective sensors, or improper wheel alignment angles. ESC problems typically manifest themselves when the vehicle’s wheels are turned. The customer may remain unaware of what’s going on until a mechanical problem develops with the foundation braking system. Clues may include one wheel that’s covered with brake dust or brake pads that have worn out prema-turely at just one wheel. In some cases, a customer may complain that the ESC activates on slower corners or on free-way off-ramps.

There are many forces that act on a typical vehicle that have a direct effect on ESC operation. Consequently, ESC systems require several more input sen-sors, on top of the wheel speed sensors employed in an ABS system. Look for two accelerometer sensors—one for measuring braking and acceleration, and the other to measure cornering forces. A yaw sensor measures the vehi-cle’s rotation on its vertical axis.

These sensors are typically located in a group under the center console, close to the vehicle’s center of gravity. You’re unlikely to encounter problems with these sensors unless they’ve been un-

bolted from the vehicle body or the ve-hicle has been involved in an accident. In either case, these sensors will need to be either recalibrated or replaced.

The sensor that’s most likely to give you problems is the steering angle sen-sor (SAS), which measures steering an-gle, steering wheel speed and torque applied by the driver. This is the only sensor that monitors the driver’s ac-tions. The other sensors tell the ESC control unit how the vehicle is respond-ing to those actions, as well as the ac-tions of the control unit.

It’s probably incorrect to blame the steering angle sensor for problems, as it’s actually an incorrectly adjusted steering angle sensor that will cause problems—for example, if a vehicle has had its wheels aligned, but the steering angle sensor was not recali-brated to compensate.

Consequently, the steering angle sensor may inform the ESC control unit that the steering wheel is turned 35° to the right when the vehicle is actually traveling in a straight line. On a freeway off-ramp, the extra 35° of steering angle may cause the ESC to activate because the control unit in-terprets the situation as an understeer condition. It will apply the brakes or limit the engine’s output in an attempt to compensate.

False ESC activation may also be caused by a vehicle that’s incorrectly aligned and has an excessive rear axle thrust angle. The ESC control unit can’t measure the thrust angle directly, but it can measure its effects. The yaw sensor may show that the vehicle isn’t traveling in a straight line. The accelerometers and steering angle sensor may show that the driver is attempting a correction. If the accelerometers report that nothing is happening as a result, the ESC con-trol unit might interpret this as vehicle oversteer. In an attempt to correct the oversteer condition, it may respond by pulsing brake pressure to the inside rear caliper. Correcting the thrust angle mis-adjustment should eliminate the false ESC activation.—Karl Seyfert

This article can be found online at www.motormagazine.com.

A ribbon of magnetic material was orig-inally embedded into the circumference of this Chevy Equinox hub and bearing assembly. Rust has done its work and you can see that part of the magnetic material is missing. This resulted in a “plausibility” trouble code.

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