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ABS: Variations & Commonalities

Monday, September 1, 2008 - 00:00
Brake Tech Brake Tech Series brakes antilock brakes ABS antilock brake systems antilock braking systems anti-lock brakes hydraulic control unit HCU repair shop repair shops technicians automotive aftermarket

Antilock braking may seem relatively new, but it first was used in aircraft, because the kind of pedal-pumping action once commonly practiced by expert automobile drivers is difficult or impossible in an airplane. The first systems, say some sources, go back to 1929.

Microprocessor control in the 1980s made antilock braking systems (ABS) for cars take off. Gradually. Only about 3 percent of new 1987 models — mostly expensive ones — featured ABS. Costs dropped, though, as installations proliferated, both because higher production lowers per-unit cost and because engineers developed less-expensive systems.

A quick review by looking ahead

In a nutshell, standard ABS senses speed at each wheel. If the system computer determines excessive deceleration at a corner, it commands the hydraulic control unit (HCU) to cut brake-fluid pressure to the affected brake or brakes (though not more than three of them). Then pressure is instantaneously reapplied. If a wheel's deceleration rate still is too much, the cut/reapply cycle continues, several times a second, until either the stop is completed or the lockup threat has passed.

Once engineers figured out how to apply each brake selectively, they developed more ways of using this capability. The system just described includes essentially all components needed to also help launch a vehicle under limited-traction conditions. Brake-assisted traction-control systems are programmed to notice when a powered wheel is gaining speed too quickly. The HCU then partially brakes that wheel to help get wheelspin under control.

The latest enhancement is stability control, and if you haven't serviced one of these yet, you will. The federal government has mandated it to become standard equipment. We're already in the phase-in period, with 55 percent of 2009 models required to include it. By model year 2012, all new light-duty vehicles (with very few exceptions) will have to have it.

These systems help keep cars pointed where the driver wants to go. To oversimplify only a little, stability control systems add a steering angle sensor to determine the driver's intended direction and a yaw sensor to measure which way the car's actually going. (Yaw is rotation about an imaginary vertical axis — imagine a big stick poking vertically through the car's dome light as the axis.) Some systems also sense lateral Gs. These inputs let the controller determine if the vehicle is off course, and which brake(s) to apply to try bringing it back into line.

Changes and evolution

With more than 20 years worth of systems on the road, you can expect to see considerable variation when working on ABS-equipped vehicles. Make a point of keeping in mind what's unique about the system you're servicing, even when you're not working on the antilock system itself.

Not many rules apply to all systems. With that in mind, let's examine some differences found in various systems – and maybe one or two things they have in common, starting on the hydraulic side.

Grace under pressure

With many antilock systems, the accumulator must be depressurized before you crack a bleeder or open the hydraulic system in any other way. (But then, other systems don't even use accumulators.)

Adding to the confusion, some systems that use accumulators can be bled if the component is pressurized or not. On some others, you're required to use accumulator pressure to move the brake fluid when you bleed. But maybe just the rear brakes. It depends. Bottom line: Check the manual for the system you're servicing or you could make a mess and/or get hurt performing an operation that used to be both universal and routine.

Don't use the wrong brake fluid

One of the few rules that's universal with antilock brakes is to avoid silicone (DOT 5) brake fluid. Under high pressure, agitated-pulsation cycles through the small valves and passages in an ABS hydraulic control unit (HCU), this fluid is likely to aerate, leading to erratic operation. That's why no manufacturer recommends DOT 5 with ABS, and most service manuals specifically caution against its use.

Fix foundation first

OK, there's more than one rule that applies across-the-board. Like this one: Antilock braking systems can't function properly unless the foundation brakes are working normally. So, when both the red "brake" and amber "antilock" lights are lit, fix the foundation brakes first.

Depending on the system, the "foundation" brakes may be conventional, with a standard master cylinder and booster and the antilock HCU downstream. But some systems integrate the HCU with the master cylinder and booster, and it can be difficult to differentiate which components in these systems are part of the basic brake system and which are there for ABS functions. Any such confusion is understandable, since some items, in some systems, perform both foundation and antilock braking duties.

How much is too much?

Start your assessment of a malfunctioning ABS with a visual inspection; one thing to notice is the brake fluid level — especially if the problem developed after someone "topped off" vehicle fluids. But even if the fluid level is below maximum, you can't assume the reservoir is not overfilled.

Not without first checking the appropriate manual. That's because some (not all) accumulator-based systems' brake-fluid levels are to be checked only when the accumulator has been depressurized, releasing its stored brake fluid to the reservoir. This can noticeably raise the brake fluid level, sometimes enough to trigger a diagnostic trouble code (DTC.)

This one's definitely worth keeping in mind if an ABS problem develops shortly after the customer visits a quick-lube shop advertising "free top-off service."

Say 'No!' to fluid backup — safely

It used to be OK to drive brake fluid back toward the reservoir when retracting caliper pistons to make room for new pads, as long as you avoided overfilling the reservoir. Try that with ABS and expect to find a DTC alerting you to HCU problems, often in the valve block. Be sure to not pump brake fluid back up the line to avoid ABS complications. And while once you could line-clamp brake hoses to prevent fluid backflow, that practice may damage linings in many modern hoses.

So how do you prevent fluid backflow today? Open bleeders and retract deliberately. And, in systems with conventional master cylinders, use a pedal depressor to hold the brake pedal down about 1/4 inch.

You'll block the ports in the master cylinder, preventing fluid from coming back up the lines and into the HCU, where it may unseat valves, or sediment/debris can interfere with ABS operation.

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