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Brake Maintenance: ABS
The modern ABS system has advanced from a crude, rear-wheel antilock system to a very sophisticated four-wheel, computer-managed control system. It is more effective at braking safely in slippery conditions than most drivers under most road conditions, and more effective than all drivers in true panic situations. In the pre-ABS days, pumping the brakes in slippery driving conditions was raised to an art form. The trouble is that many drivers never did get the hang of it. Or if they did, promptly forgot how to do it. Or didn’t think to do it in a crisis.
The automakers came to the rescue with antilock brakes. Now all one has to do is do what comes naturally: panic and slam on the brakes as hard as you can. The computer does the rest. Pumping the brakes on an ABS system can actually be detrimental.
The advantages of ABS are many: fewer crashes outright, fewer major and minor crashes, less liability exposure, longer tire life and the resultant lower costs of maintenance, repair and downtime. Disadvantages exist, however. Improved technology means greater complexity and greater training requirements for the technician. Improved technology means more expensive repairs in the event of component failure. All in all though, few of us would prefer to go back to the old days.
The most common ABS system incorporates four speed sensors, one for each wheel. The control unit samples the rotational speeds from each sensor and compares them several times per second. If, during braking, one sensor is showing a significant speed difference from the rest, the control first isolates the brake fluid supply to that caliper and, if necessary, opens a bleed port to decrease hydraulic pressure to the affected wheel. Because this is done several times per second and is wheel specific, the response is much quicker and more effective than any driver trying to pump the brakes.
The ABS system can be divided into two sections, the hydraulic and electronic. It is the cooperation of these two subsystems that allow the antilock brakes to work. Maintenance of the ABS hydraulic portion of the modern brake system is more of a list of what NOT to do rather than a list of what to do. Brake fluid is hygroscopic, meaning it attracts and absorbs moisture. This moisture travels throughout the interior of the system with the fluid and can lead to corrosion if levels exceed a certain point.
The tendency of the moisture is to migrate to the lower reaches of the system—the calipers and wheel cylinders. As the pads and shoes on the vehicle wear, the pistons in the calipers and (in the case of drum brakes) to a lesser extent, the wheel cylinders move outward to compensate for this wear. As more fluid accumulates into these components, moisture accumulates as well. The resultant corrosion also accumulates. If unchecked, the corrosion can lead to a number of problems.
First, if the bleeders are not opened at the time of pad replacement, when the calipers or wheel cylinder are retracted, the fluid and any contamination present can be forced backwards through the system to the electric hydraulic control unit (EHCU), also referred to as an electronic brake control module (EBCM). The tolerances in this unit are such that it does not take kindly to such treatment. If you get an ABS light on the dash immediately following a brake job, this is something to question the technician about.
Also, if corrosion is present in the caliper and the piston is retracted past it, the piston can bind in the bore of the caliper, leading to the new brakes not releasing as they should. The result is overheating and failure of the new brakes. These are all good reasons to flush the brake system periodically, especially in humid climates or when the maintenance history of the vehicle is not known.
Additionally, the seals in the calipers are square cut O-rings. When the caliper is actuated, the square rings deflect into parallelograms, and when the pedal is released, they return to their square shape. In doing so, they retract the pistons enough to allow the brakes to release.
When the seals become hard from age and heat, they have a tendency to maintain their square shape, and the piston simply slides past. Then when the pedal is released, the piston does not retract that small amount and you have the brakes still slightly applied. This can lead to overheating during normal driving without brake application. This condition is more difficult to detect if you don’t know what to look for when retracting the pistons during a brake job. You can actually see the slight return of the piston when pressure is released in either direction. If you don’t, replace the affected caliper.
The location of the EHCU on the modern vehicle favors two possibilities. Either it is installed in a place impossible to access and therefore bleed. Or it is installed below the vehicle where it is prone to damage from snow removal chemicals or road debris. Because proper bleeding procedures are important in the event of fluid loss while in operation, access to the bleeders and valves (if necessary) is important. When bleeding the system, manufacturer procedures should be followed precisely.
Since the ABS system is electronic, the signals from the wheel speed sensors are vital to braking performance. The harnesses to these sensors are often in harm’s way, and it is critical to maintain all clips and holders to prevent them from contact with the chassis or tires. Proper spacing between the sensor and the toner ring is similarly important because too small of a space will allow contact and possible sensor damage while too large of a space may cause a faulty signal and either false system activation or system shutdown. Because the purpose of the PM is to find problems before they happen, inspecting the harnesses should be part of the basic PM checklist.
If you live in an area where chemicals are used for snow removal, wire insulation integrity is essential. A pin hole (from testing or from road debris) can allow corrosive chemicals to wick up into the harness, leading to false signals, harness failure or component failure.
Do your drivers apply the brake pedal with one foot and the accelerator with the other? If they do, break them of that habit. On some vehicles, the ABS computer can become confused and turn itself off and the dash ABS light on. The technician ends up chasing a nonexistent problem because of the ABS warning light. ABS is a good system that needs only the most cursory checks and a few maintenance procedures to remain trouble free.
Kevin Roberts is the president of Roberts Repair in Rhinelander, WI. The company has specialized in emergency vehicle maintenance since 1989. Roberts is an ASE Certified Master Automobile and Master Truck Technician. He can be reached at firstname.lastname@example.org.
Published in Police Fleet Manager, Mar/Apr 2008
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