Addressing ABS issues

Dec. 16, 2016
It's still important to also evaluate mechanical aspects of the antilock braking systems of a vehicle for proper diagnosis.

Vehicles affected: any vehicle with an ABS system

Tools used:

  • Vehicle information system
  • Scan tool with bi-directional capabilities
  • DMM
  • Lab scope
  • Brake pad measuring tools
  • Brake rotor measuring tools
  • Dial indicator
  • Brake pressure tester
  • On-car brake lathe

Over the years, brake systems on vehicles have gone through numerous innovations. Most changes were made to improve braking performance such as hydraulic brakes, power assist, and two and four wheel disc systems. Every type of braking system works well for its era and has both benefits and drawbacks, including maintenance and repair.

In this issue we will be discussing the latest variation of braking systems, Antilock Braking Systems, commonly referred to as ABS. We will discuss some common issues that affect wear or cause other ABS concerns, along with the proper techniques and tools to diagnose and repair these problems.

Issue 1: ABS activation at slow speeds

One of the more common problems with ABS is that it engages at a slow speed without necessarily needing to. In order to recognize what causes this, it is necessary to have some understanding of how ABS operates.

ABS systems are split into two major types: a set of wheel speed sensors on the front of the vehicle and a single sensor on the rear; or a system that utilizes a sensor on each wheel. Wheel speed sensors are used to determine if a wheel, or wheels, are rotating at a different rate than others during a braking event. If a slower rotation rate is detected, the ABS computer controls brake pressure to each individual wheel preventing the brakes from locking and allowing the tires to maintain traction on the road surface.

There are two basic types of wheel speed sensors: passive and active. Passive sensors work very much the same as a crank or cam sensor, where there is a variable reluctance sensor that creates an AC signal that increases frequency and amplitude as the reluctor wheel attached to the axle increases speed. Active sensors utilize a hall-effect type of sensor that produces a digital signal. These sensors have a power circuit, a ground, and a signal return to the ABS module. Depending on the vehicle, the ground circuit can be either through the body of the sensor or a third wire in the sensor.

Step 1: Diagnose the vehicle

Due to the variations of sensors and strategies vehicle manufacturers employ, it is imperative to research your information database to verify the type of system as well as check for TSBs and diagnostic test procedures.

Since there is an ABS control module that manages the system, and in most cases it is part of the CAN network, it is a very good idea to use your scan tool to check all control modules for communication errors or “U” codes. If “U” codes are present, repair those prior to testing other systems.

Wheel speed sensors that are not functioning, or have opens or shorts, typically result in a DTC for that particular sensor. If no codes are present, it will be necessary to observe sensor data while test driving the vehicle. To do this safely you will either need to use the graphing function of your scan tool, or have a helper drive the vehicle while you are monitoring the signal from each wheel. Keep in mind the ABS module and the scan tool both need to have fast enough update rates to provide reliable data, especially with intermittent problems.

If there is an issue with an individual sensor you will most likely see a slower reported speed for that wheel compared to the others (See Fig. 1).

If wheel speed data is not available, the sensors can be tested using a DMM or lab scope. I prefer a lab scope because it has the ability to observe minor variations or signal losses across a wide range of speeds. The image in Fig. 2 shows an example of a signal dropping out intermittently. The image in Fig. 3 represents a common passive sensor, note at slow speeds the frequency (speed) and the amplitude (peak voltage) are significantly less than at higher speeds.

Step 2: Repair the sensor

One common issue that occurs with ABS sensors is when a sensor gets pushed away from the reluctor wheel, due to rust buildup in the sensor housing (See Fig. 4). Because the sensor is further away from the reluctor wheel, the amplitude and frequency will be misreported to the ABS module, causing the system to assume the wheel is moving slower, thereby activating the ABS system and limiting pressure to the wheel.

The proper repair for this condition is to remove the sensor, clean the base and housing, then re-insert the sensor (See Fig. 5). It is a good idea to clean every sensor at that time and in the same manner, to ensure the sensors are all able to provide an accurate signal.

Issue 2: Uneven or premature brake wear

When diagnosing any issue on a late-model vehicle it is easy to assume it will boil down to one of the many technological advances that have improved vehicles over the past two decades. When we presume it is a technology problem and limit our focus to the computer operated systems we can overlook some mechanical issues that may be causing the concern. When you look at basic operation, a late-model vehicle with an ABS system still uses brake rotors, calipers, pads and a hydraulic system.

Step 1: Obtain system information

Check for TSBs using your information database. Many problems are consistent with other vehicles using the same system so this is a great place to start. It is also necessary to check how the system operates, specifications and proper service procedures, including the system bleeding sequence.

Step 2: Perform a complete inspection

In order to perform an inspection it will be necessary to remove each wheel and caliper. It will be necessary to measure each brake pad at the leading and trailing edges as well as the top and bottom. By doing this you will be able to determine if each set of pads is wearing at the same rate and see if they are seating correctly in the caliper.

Brake pads that are wearing the same from one side to another, but are wearing prematurely, are usually due to a hydraulic system issue such as a proportioning valve, master cylinder, or an ABS issue. Pads that wear on one side but not the other can be caused by a caliper, a caliper mount, a brake rotor, or a brake hose.

In addition to measuring the pads, you will need to measure the brake rotor thickness and the run-out of the rotor. The thickness of the rotor may be an indicator of brakes that were not serviced properly or an overheating issue that is causing excess wear. Use a dial indicator to measure if the rotor stays perpendicular to the caliper when rotating.  

Rotor run-out can not only cause a vibration but, in many cases, may cause excess drag on the brakes causing uneven wear. Excess rotor run-out can be either a cause of additional brake issues, or an effect of having an issue.

Step 3: Diagnose the issue

You will then need to perform tests based on the results of your inspection.

For the most part, if the system uses an ABS design that has sensors at each wheel, the brake fluid pressure is delivered individually to each wheel. If there is a single rear wheel speed sensor and two front sensors, the fluid is delivered to the front wheels individually and the rear wheels together.

This is important to understand when you are diagnosing the system. On a single rear sensor system, if brake pads are worn out on an individual rear wheel the fault will be at that individual wheel, as opposed to an ABS problem where  both rear wheels have excessive wear.

Using a brake pressure tester allows you to determine if the individual wheels are receiving equal applied pressure and let you know if residual pressure is alike or excessive.

Step 4:Repair the vehicle

If you have determined there is a hydraulic issue, repair as necessary, then bleed the system per the vehicle manufacturer’s recommendations. In most cases this involves using your scan tool to activate the ABS module, then bleeding each wheel in a specific order.

If there is a rotor run-out issue, the best practice procedures require the use of an on-car brake lathe to ensure the rotor variation remains within tolerance.

Vehicles produced after 2012 are required to have stability control systems installed. Additionally, many vehicles manufactured prior to that date utilize the technology. Stability control systems function by using steering angle and wheel speed sensors, along with accelerometers, to determine if the vehicle is moving in a straight line, turning excessively, or leaning (tipping) while braking. Based on input from the various sensors, the ABS control module will apply brake pressure in order to regain control and straighten the vehicle out. A fairly common issue that has arisen from using stability control is excessive brake wear on a single wheel when the system assumes the vehicle is not travelling in a straight line.

In many cases a DTC will be present if there is a faulty steering angle sensor. There are many different types of sensors, on some vehicles you will be able to look at a PID for the sensor to determine if there are faults in the system, others will require the use of a lab scope to monitor the signal for any glitches.

The most common problem arises when a technician fails to reset or calibrate the sensor when necessary, causing the ABS system to perceive the vehicle is turning while holding the wheel straight. Think of this as centering of the steering wheel will read 0 degrees and a slight turn will read 10 degrees. If the ABS system thinks you are having to correct by 10 degrees to the right, it will apply the left front brake to correct the steering, causing the left brakes to wear.

Steering angle resets will be required any time an alignment is performed or a steering angle sensor, an ABS module, or the vehicle battery are replaced. Follow the manufacturers recommended procedures to perform the reset.

As with many vehicle diagnoses and repairs, it’s important to try and not get lost with technology. Start with testing the primary brake system fully, then move on to the electronics only after you have corrected any issues with the basic system.

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