Auto Service World
Feature   September 1, 2008   by Mike Cleary

ABS Diagnostics Simplified

One of the greatest achievements in automotive history was the invention of anti-lock brake systems (ABS), providing drivers with the ability to control a vehicle during less than optimal vehicle brak...

One of the greatest achievements in automotive history was the invention of anti-lock brake systems (ABS), providing drivers with the ability to control a vehicle during less than optimal vehicle braking conditions.

The purpose of anti-lock systems is simple: prevent wheel lock-up and loss of vehicle control during braking. There are two types in use: four-wheel ABS, which can control braking at all of the wheels on the vehicle, and two wheel, or rear ABS, which controls braking at the rear wheels only.

Anti-lock brake systems (for this article we will focus solely on “traditional” ABS systems) can be divided into three basic categories: four wheel/ four channel, four-wheel/three-channel, and two-wheel (rear only). The first system, four-wheel/four-channel, utilizes an individual wheel speed sensor at each of the four wheels, allowing the Electronic Control Unit (ECU) to control braking at any one of the four wheels individually. The second, four-wheel/three-channel, utilizes individual wheel speed sensors on each front wheel, with one sensor in the rear of the vehicle (mounted on the differential housing), monitoring 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, and in this system the ECU controls the rear brakes only, and just like in a four-wheel, three-channel system, does it simultaneously.

While on initial investigation these systems appear to be complex, they are actually nothing more than three fairly basic subsystems integrated into a complete vehicle package providing an additional margin of safety to vehicle braking systems.

The three subsystems are informational inputs, commanded outputs (the Hydraulic Control Unit, or HCU), and the Electronic Control Unit.

The first subsystem is composed of the individual sensor information, mainly the wheel speed sensors and wiring. Three types of sensors are generally used: magnetic pickup, variable reluctance, or magneto resistive. Regardless of which type is used, they are responsible for relaying wheel speed information to the ECU. There are other informational sensors involved, such as the brake position switch, but the wheel speed sensors and wiring are the most critical.

The next subsystem (the commanded outputs) is the Hydraulic Control Unit, which, on four-wheel systems, is nothing more that an assembly of solenoids and valves which, when commanded by the ECU, controls hydraulic pressure to the brakes. In the most basic terms, it provides a combination of isolating and /or relieving brake pressures, to avoid an impending wheel lock-up. 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 Electronic Control Unit 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 by electronically controlling a series of solenoid actuated valves to accomplish this function.

Diagnostic Strategy

The key to being successful at diagnosing and repairing ABS system malfunctions is to have 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 for breaking down the system into three subsystems is it facilitates diagnosis. By zeroing in on where the problem lies, you can immediately identify which subsystem(s) it is in, which will allow you to concentrate on diagnosing that part of the entire system only, and eliminating unnecessary diagnostic procedures. Simplification of diagnostics is the key to making any quality diagnosis and repair.

For instance, suppose you have a code for the right front wheel speed sensor. Using the system breakdown noted previously, and the definition of the code, you can immediately focus on the subsystem(s) involved. For example, a circuitry code for the above sensor tells you that your diagnosis will concentrate on either the wheel speed sensor/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. Remember, simplify your diagnosis.

Always make sure that you completely understand what exactly the customer’s problem, or perceived problem, appears to be. Sometimes a customer will come in with a complaint of brake pulsation, when actually what occurred is that the anti-lock brakes activated. Due to lack of customer knowledge and/or experience with an anti-lock system, the customer perceives this as a problem, when in actuality there is no problem.

The first 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. These are recorded for later use. At this point the memory codes should be cleared.

The next step is to road test the vehicle. This will give you an idea of how the “basic brake” system is performing in addition to any 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 themselves 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 return to the shop, re-check for codes. Obviously, any “on-demand” codes are diagnosed first. Any memory codes that were initially recorded but did not repeat after the road test are ignored at this point. They are noted on the repair order for the customer so that if they return on another customer visit to the shop, they can be addressed accordingly. Any memory codes which did repeat are addressed after first rectifying any “on-demand” codes, as previously noted. 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, so the ECU doesn’t recognize a fault until the vehicle is moving and it “sees” info 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 in your arsenal for diagnosis, but make sure you utilize the simplest tool available for the job at hand. 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 ohm meter 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: fix the vehicle.

As far as using pinpoint tests — or “trouble trees” as they are sometimes referred to — try to avoid them. Try to develop your own personal diagnostic strategy that works best for you. When using a pinpoint test, at
every test step in the process, 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 when it comes to diagnosis. Many a problem has been quickly diagnosed and repaired by just “eyeballing” the suspected area of concern, prior to breaking out all your test gear and tools. You want to make this as thorough as possible in the areas that are easily accessed.

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 lies. 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 typically should not exceed 50 mv.

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 lies, and 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 which will allow you to diagnose and repair ABS systems in a more timely and profitable fashion.

Now you’re dangerous.

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