Make Sure You Understand ABS Fundamentals Before You Attempt Diagnosis or Repair

If you live in a heavy-traffic area, perhaps you’ve noticed something interesting: Where you used to frequently hear the wailing squeal of skidding tires during some unfortunate motorist’s panic stop, now it’s an unusual sound indeed. That demonstrates both the proliferation and performance of ABS.

Speaking of that proliferation, the trend toward anti-lock braking systems is a change in vehicular stopping technology just as profound as the switch from mechanical to hydraulic brakes, or the adoption of discs over drums. And we’ll all be safer for it.

Subaru got on this safety bandwagon early on. You could buy a Legacy with ABS back in 1990 — that’s 15 years ago.

Demo

We were once given an impressive demonstration on maybe a half-acre of Mylar film sloshed down with soap and plenty of water from fire hoses. If it hadn’t been a perfectly level parking lot, you could barely have walked on this terrifically slippery surface. Two rows of cones were set up, and we were asked to try to stop a car between them. But first a cobbled-up switch on the dash was turned to “off” — it was the juice feed to the ABS controller. Well, whether we pumped furiously or just tried the gentlest decel possible, we got seriously sideways and wiped out cone after cone. Then we tried it with the switch on, and it was almost as if we were driving on dry pavement. We stopped short and straight.

Like most other defensive drivers, we don’t have to stand on the brakes very often, and we’re wary of limited traction conditions, so it’s highly unusual when we get into trouble trying to stop. But, of course, it only takes one bad instance, which recalls a motto often used by Robert Bosch Corp. (a former supplier of ABS components to Fuji Heavy Industries): “If you need it just once, it’s paid for itself.” And that’s what the carmakers are concerned about, that one moment of inattentiveness, or when dangerous conditions stack up all at once.

Basics

The idea, of course, is to prevent lockup during hard deceleration, especially in slippery conditions. This greatly increases control (a locked wheel can’t be steered) and reduces stopping distances. It can also save an expensive set of tires from flat-spotting in a panic stop. 

The basic operating principles are easy to understand. The electronic control module watches the speed of each wheel by means of gear-like reluctors or exciters mounted on the disc or axle and hermetically-sealed pickup coil sensors that count the teeth as they pass. The computer continually monitors and compares this input, which means it processes thousands of signals per second. 

If one wheel slows down more than the others during brake application, or deceleration doesn’t match a profile in the microprocessor’s memory, the control module responds instantly by sending commands to electro-magnetic fluid valves housed in the main control unit, which freeze or release hydraulic pressure to that wheel or circuit, then reapply it when the tire speeds up again. This cycle is repeated up to 15 times per second — the best driver in the world could never pump the pedal that rapidly, and obviously he can only pulse the system as a whole. With ABS, he can apply the brakes as hard as necessary and give them no further thought while he concentrates on steering.

During normal braking, ABS watches and waits. Hydraulic pressure simply passes through the valves as if they weren’t there.

Solenoid Valves

With the dual-diagonal hydraulic system split used on typical FWD and AWD vehicles, there’s one inlet and one outlet valve for each front caliper, and another pair for each of the rear brakes, making a total of eight in the valve body.

As we said, the computer activates these solenoids appropriately to tailor fluid pressure to conditions. When a wheel begins to skid, the electrical circuit to the normally-open inlet valve is completed, making it close and blocking the movement of fluid into the caliper. If that corrective action isn’t sufficient to stop lockup, the normally-closed outlet valve is opened, which allows pressure to be relieved through a bypass to an accumulator and a return pump so the pads or shoes can retract. No sooner has this been accomplished than the signals are reversed and fluid pressure is again applied to the caliper. This continues on-off at a rate determined by traction.

Semantics

ABS terminology can be confusing. First, there are channels. This refers to the number of fluid passages from the actuator. Early systems have three channels, one for each front wheel and a third for both rears. It is, however, still called a four-wheel system because all four are modulated, even though the rears get one signal and are thus treated alike. So, with the basic FWD dual-diagonal hydraulic arrangement, there are four solenoid sets, but since the rears receive identical control, it still falls into the three-channel category. Later models have four-channel ABS, wherein each rear wheel gets individual control.

As far as componentry is concerned, there are two types of systems, add-on or non-integral and integrated. As the name implies, the add-on variety is used in conjunction with a regular master cylinder and vacuum booster, and this has been the type Subaru has used all along. The integrated approach uses a single unit that combines the master, a hydraulic booster, and the ABS modulator. It’s compact, but generally more expensive to manufacture and more complicated to service.

The microprocessors involved typically provide a nearly fail-safe redundancy. If the logic doesn’t agree on something, the system reverts to non-ABS hydraulics, and a dash light comes on to warn the driver that his car no longer has that extra margin of safety. 

Fortunately, Subaru anti-lock brake systems have shown themselves to be extremely dependable. As one independent specialist tells us, “In 11 years of working on Subaru vehicles, I’ve only replaced a few wheel speed sensors, which had rust problems, and one control head, which was leaking through an O-ring.” As time marches on, and given the public’s cavalier attitude toward maintenance, however, we’re bound to start getting hydraulic problems — water, corrosion, and dirt in the lines and valve body. See our sidebar on regular fluid changes.

Finally, the latest Subaru models are equipped with EBD (Electronic Brake-force Distribution), the sophistication of which goes beyond that of previous ABS. But that’s a topic for a future issue of The End Wrench.

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