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Feature   February 1, 2008   by

The ABCs of ABS

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The ABCs of ABS


Four good habits to employ when diagnosing tricky brake problems.


By Rick Cogbill


These days, brakes don’t just stop cars. Through the use of computer-controlled anti-lock braking systems (ABS), they assist with traction, control skidding, and help avoid roll-overs.

But, in spite of the fancy add-ons, basic brake work is not a thing of the past. Remember that ABS is built on top of a standard hydraulic braking system, consisting of mechanical pressure (the brake pedal) converting energy to hydraulic pressure (the master cylinder, lines, and calipers/wheel cylinders) to create frictional force (pads or linings pressing on rotors or drums) to stop the vehicle. Any brake-related complaint should start with an inspection of this basic system before moving on to the ABS.

I know of at least one technician who recently confessed to spending hours on a puzzling ABS problem, only to discover the master cylinder was low on fluid. The low brake fluid sensor had disabled the ABS.

Avoiding pitfalls when confronting tricky brake problems involves employing four good habits.

Good Understanding

You can’t know it all, but you should understand the basics. ABS may be packaged differently from car to car, but the principals are the same. Speed sensors at the wheels or differential monitor wheel speed during braking and report any differences to the ABS computer. If one wheel slows down quicker that the others, the controller begins regulating hydraulic pressure to the brakes to avoid wheel lock-up. It’s as simple as that.

So if a customer complains that the ABS is activating when it shouldn’t be – like at slow speeds on a good road – and there are no trouble codes, it’s a safe bet the computer is getting wrong information, most likely from a wheel speed sensor.

In fact, GM has a bulletin out on its trucks for this very problem. Rust build-up underneath the sensor mounts is creating excessive air gap between the sensor tip and the tone ring, resulting in a faulty reading. Remember that all wheels must read the same speed at the same time, so in this case, a good cleaning is all that’s needed.

On the other hand, a hard code that returns every time you clear the system usually means no information or information that’s out of range – a poor or broken wiring connection, or some damaged parts, or a shorted solenoid. Pinpoint and visual tests, plus snapshots during a road test will help you locate the trouble.

Good Parts

It’s a technician’s dream; installing the right part the first time, and see it working as it should. But we live in the real world, where even OEM parts can fail straight off the assembly line. Still, don’t make it harder on yourself than it should be. Rebuilt master cylinders might be cheaper, but will they last? White box pads save the customer money, but do they have the same “feel,” life expectancy, or quiet operation? A used ABS controller might be tempting, but if the first one failed because of rust in the system, how long before the used one does the same?

Many shops have good success with quality aftermarket parts, and if you find a line that works for you, stick with it, because OEM options have their share of problems too. One shop I know of went through four dealer-supplied ABS control modules before a 2001 Chevrolet Silverado was finally fixed. The first three were the new updated part number, and each one caused different problems when installed. Only by going back to the pre-updated part number was the shop able to find success.

Good Tools

ABS service is no place for guesswork. A good digital DVOM, lab scope, or up-to-date scanner is required if you’re going to test the system properly. Also, the ability to take a movie or snapshot during a test drive makes life a whole lot easier. That is, of course, if you know how to read the data.

Too many shops own equipment that no one really knows how to use. Practice with that scanner on known-good vehicles until you really know what you’re looking for. This will pay big dividends in the years ahead as you trim back your diagnostic times and order fewer non-required parts.

System bleeding is a big issue, and often requires a scan tool to cycle the ABS controller in order to remove all the air. If the tool isn’t available, then safely force the system to activate during a test drive (on a gravel side road, for example), pulling the vehicle back into the shop for a re-bleed. You’ll be surprised how much air didn’t come out the first time. Some technicians have also found that pressure bleeding from the bottom up works better than gravity bleeding from the top down.

But the most cost effective tool is a membership to on-line forums like the Canadian Technician Forum ( or iATN, as well as technical help lines offered by your parts supplier.

Bouncing ideas off other technicians will save you hours of fruitless testing, and any membership fees are well worth it. Just ask Rob Ingram of Eldon Ingram Ltd. in Stratford, Ontario. His help request through iATN concerning an intermittent hard brake pedal on a ’91 Pontiac Grand Am brought nine quick replies from other members, seven of which pointed him to a common problem and the solution – moisture in the booster’s vacuum check valve that would freeze it open in cold weather.

Rob says he keeps the iATN webpage open on his shop computer all day and checks their database as a routine part of his diagnostic procedures.

Good Communication

Lastly, spend some time with your customer before you turn a wrench or plug in a scanner. Today’s braking systems are increasingly complex, and the motoring public needs to know there’s not always a simple answer. Some ABS problems can only be solved by testing with known-good parts, and control units are not cheap. If your customer is not prepared to pay, then maybe it’s a job you don’t really need. Remember the old adage: You didn’t build it, buy it, or break it… so don’t think that every vehicle showing up on your doorstep is your responsibility.

On the other hand, ABS has been around long enough now that, just like the standard brakes of the past, there’s a long list of common problems. Once you become familiar with them, you’ll find your diagnostic and repair times coming into line.

Remember, it’s as simple as ABC:
Always check the basics first;
Become skilled with the tools at your disposal; and
Call for help and advice when you need it.

Do this, and you’ll find people lining up at your door for your ABS expertise.


Rick Cogbill is a former shop owner and a frequent contributor to Canadian Technician.


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