One of the greatest achievements in automotive history was the invention of anti-lock brake (ABS) systems. It provides drivers with the ability to control a vehicle during less than Optimal vehicle braking conditions. The purpose of anti-lock systems is simple, It prevents wheel lockup and loss of vehicle control during braking. There are Two types of ABS systems in use:
* Four wheel ABS, which can control braking at all of the wheels on the vehicle.
* Two wheel, or rear ABS, which controls braking at the rear wheels only.
GETTING SPECIFIC
ABS systems can be divided into three basic categories: four-wheel/four-channel, four-wheel/three-channel, and two-wheel (rear only). Four-wheel/four-channel uses an individual wheel speed sensor at each of the four wheels. This allows the Electronic Control Unit (ECU) to control braking at any one of the four wheels individually.
Four-wheel/three-channel uses individual wheel speed sensors on each front wheel and one sensor in the rear of the vehicle, which is mounted on the differential housing, to monitor rear axle speed. This system allows the ECU to control braking at either one of the front wheels individually, but at the rear wheels the ECU controls braking at both rear wheels simultaneously.
The last type of system, two-wheel, or rear ABS, has only one sensor mounted on the differential housing. In this system, the ECU controls the rear brakes only, and just like in a four-wheel/three-channel system, does it simultaneously.
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While on initial investigation these systems appear to be complex, they are actually nothing more than three fairly basic subsystems. These systems are integrated into a complete vehicle package providing an additional margin of safety to vehicle braking systems. The three subsystems include informational inputs, commanded outputs--the Hydraulic Control Unit (HCU)--and the controller, the ECU.
The first subsystem, informational inputs, is composed of the individual sensor information. This is supplied mainly from the wheel speed sensors and wiring. On the majority of systems, these sensors are the basic magnetic pickup-type, although on a few systems, variable reluctance-type sensors are used. Regardless of which type, they are responsible for relaying wheel speed information to the ECU. There are other informational sensors involved, such as the brake pedal switch, but the wheel speed sensors and wiring are the most critical.
The commanded outputs, which is the next subsystem, is the HCU, which, on four-wheel systems, is nothing more than an assembly of solenoids and valves. When commanded by the ECU, they control hydraulic pressure to the brakes. In the most basic terms, the system provides a combination of isolating and/or relieving brake pressures to avoid an impending wheel lockup. A pump motor is incorporated into the HCU to maintain constant system pressure whenever the HCU is going through its functions as it provides anti-lock control. On rear wheel systems only, the HCU is basically nothing more than a tandem solenoid-actuated valve that either isolates and/or "dumps" hydraulic pressure to the rear brakes. No pump motor is required.
Lastly, the ECU is the "brain" of the system. Based mainly on the wheel speed information it gets from the sensors, it determines the best combination of HCU brake pressure modulation. It accomplishes this by electronically controlling a series of solenoid-actuated valves.
DIAGNOSTIC STRATEGY
The key to successfully diagnosing and repairing ABS system malfunctions is a thorough knowledge and understanding of the system description and operation. This is particularly crucial when it comes to troubleshooting difficult-to-diagnose vehicles that cannot be easily diagnosed "by the book." Never try to memorize any particular aspect, such as fault codes, of any ABS system. As you work on the systems more and more, your experience will dictate what becomes memory for you.
One of the reasons we've broken down the system into three subsystems is that it facilitates diagnosis. By zeroing in on where the problem lies, you can immediately identify which subsystem(s) it is in. This allows you to concentrate on diagnosing that part of the system only, and to eliminate unnecessary diagnostic procedures. The key to making any quality repair is the simplification of diagnosis.
For instance, suppose you have a code for the right front wheel speed sensor. Using the system breakdown we've outlined and the definition of the code, you can immediately dial-in on the subsystem(s) involved. A circuitry code for this sensor tells you that your diagnosis will concentrate on either the wheel speed sensors and wiring or the ECU. Right away you know the HCU is not a suspect. Likewise, any HCU electrical codes will lead you to either the HCU or the ECU. Again, the goal is to simplify your diagnosis.
Always make sure that you understand what the customer's problem, or perceived problem, is. Sometimes a customer will come in with a complaint of brake pulsation, when actually what has occurred is that the anti-lock brakes were activated. Because of the lack of knowledge and/or experience with an ABS system, the customer perceives this as a problem, when in all actuality there is no problem.
The next step in any diagnostic sequence is retrieval of codes, both on-demand and memory codes, whether the amber ABS warning light is on or not. Record them for later use. At this point the memory codes should be cleared.
Then test-drive the vehicle. This will give you an idea of how the basic brake system is performing. It will also alert you to possible suspension and/or steering problems that may contribute to or exaggerate any type of ABS problem. Don't be fooled by a problem with the mechanical parts of the brake system or hydraulics that may manifest itself as a perceived ABS problem. Take the time to make a thorough road test, and then determine if you need to inspect the basic brake system prior to proceeding to ABS system troubleshooting.
Upon returning to the shop, recheck for codes. Diagnose any "on-demand" codes first. Ignore any memory codes that were initially recorded but did not repeat after the road test. Note them on the repair order for the customer so that if he or she returns on another visit to the shop, they can be addressed accordingly.
Address any memory codes which repeated after first rectifying any "on-demand" codes. But beware; some memory codes are really "on-demand" codes, such as a wheel speed sensor code. The ECU doesn't distinguish between a sensor with no output and 0 mph. Therefore, the ECU doesn't recognize a fault until the vehicle is moving and it "sees" information coming in from all the "good" sensors. The code is then stored in memory.
DIAGNOSTIC PROCEDURES
When it comes to diagnostic procedures, don't complicate things. You need to have a wide array of tools for diagnosis, but make sure you use the simplest tool available for the job. For example, have a lab scope and use it when there is a need to do so. But when checking a sensor for an open coil, an ohmmeter is all that is really needed. Don't make your diagnosis more cumbersome than it needs to be. Keep in mind the basics of electricity and electronics. When using a scope to check sensor signals, remember what each sensor is in its most basic electrical form and what kind of signal you should expect to see. For instance, a magnetic pickup should show a signal that is an analog sine wave. Avoid the tendency to read more into your test results than you are actually seeing. Remember, the ultimate goal is to fix the vehicle.
As far as using pinpoint tests, or "trouble trees," try to avoid them. Develop your own personal diagnostic strategy that works best for you. At every test step in the process when using a pinpoint test, ask yourself what you are trying to test at that step and if the procedure is valid. Mistakes in a pinpoint test procedure are not uncommon.
Never underestimate the importance of a thorough visual inspection. Many a problem has been quickly diagnosed and repaired by just "eyeballing" the suspected area of concern thoroughly, prior to breaking out all your test gear and tools.
First determine the definition of the code that has been set, and then perform a visual inspection. If no problems are uncovered at this point, consult the wiring diagram. For instance, suppose a code is retrieved for "left front wheel speed sensor low input." You immediately know that the voltage coming in on the signal line to the ECU is low. Consult the wiring diagram to find the affected circuits, and using the principles of basic electricity, determine where the problem ties. One caution: in the rare instance where you feel you have a faulty ECU, always double-check all power feeds and grounds to the ECU. Doing this with an ohmmeter is not acceptable. The only way to do this is to voltage-drop test every power and ground circuit to the ECU. The measured voltage drop should not exceed 50 milli-volts (mv), in most instances. This is not the industry standard, but there have been occurrences where it made a difference.
On occasion, you will encounter a vehicle that is exhibiting problems, but there is no fault code present. This is where your thorough knowledge of the ABS system will save you. By knowing how the subsystems work together, and after you have verified the symptom, you will be able to once again determine where the problem ties. Using a scan tool, lab scope or whatever your tool of choice is, you can dynamically test the individual components of that subsystem to determine the cause of the problem.
After you have been at this for a while, you will have--without realizing it--developed your own personal diagnostic strategy and "pinpoint tests" in your head. In the long run, they will allow you to diagnose and repair ABS systems in a more timely and profitable fashion.
1. Here is a typical Ford Hydraulic Control Unit (HCU).
2. Never overlook the importance of a visual inspection. Corrosion such as this in a connector area may be an important clue in your diagnostic process.
3. This is typical of a differential speed sensor used on a three channel system.
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COPYRIGHT 2004 Advanstar Communications, Inc.
COPYRIGHT 2004 Gale Group
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