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In a straight line with water or oil only on one side of the car, ABS offers a major benefit. If the conventional brakes
are applied hard, one side of the car will slide and the car will turn violently. Alternatively, if the brakes are not applied
that hard, it takes a long distance to stop. ABS allows the brakes on the dry side to be applied hard while the brakes on
the wet side are kept from sliding, and the car can be brought to a rapid halt in controlled fashion.
In a curve, ABS provides perhaps its greatest benefit. If conventional brakes are locked in a curve, the car will slide in
a straight line, which runs it off the side of the curve and into a tree or over a cliff. Skilled drivers know this and never
apply the brakes hard in a curve; they will aim the car straight, even if it’s aimed off the road, before applying the
brakes hard. With ABS, however, it is perfectly workable to apply the brakes hard in a curve, since the wheels won’t
lock and control will be maintained. Since the curve causes a weight shift to the outside, the outside brakes can be
applied forcefully without locking the lightly-loaded inside wheels. However, this is yet another possible reason that
ABS hasn’t lived up to its promise of increased safety; drivers may still refuse to apply the brakes hard in a curve.
Loose-packed snow, gravel, or the like, may be one of ABS’s most notorious shortcomings. On such surfaces, the
quickest way to stop a car is often to lock up the wheels and let them dig in. However, ABS prevents this from
happening, so the car seemed to roll along forever, pulsing all the way. If the car ahead doesn’t have ABS and has dug
its wheels in and stopped in short order, this can be a real problem.
Of course, stunt drivers don’t care for ABS. When you see those guys in the movies spin the car around backwards and
take off in the other direction, rest assured they do not have ABS; that 180º spin requires that the steering wheel be
turned first one way, then the other way hard and the brakes locked. With ABS, they’d probably just drive off the side
of the road.
ABS brakes were introduced on the XJ-S in mid-1989.
JAGUAR XJ-S ABS BRAKES (TEVES): Here’s a brief synopsis of how the ABS brakes work on this car: There is
an electric pump and a pressure accumulator that provide pressurized fluid. When the brake pedal is depressed, the
pedal linkage moves a piston within a chamber in the ABS master cylinder. This piston directly applies pressure to the
fluid leading to the front brakes, which means the front brakes will work even if the pump stops working.
This leads some to conclude that the front brakes are not assisted, but they are mistaken. The force applied to this
piston also actuates a valve that allows pressurized fluid to enter the chamber behind this piston, helping push it forward
and applying the front brakes. The result is a 4:1 power assist on the front brakes when the system is working properly.
The pressurized fluid applied behind that piston to assist the front brakes is also ported directly to the rear brakes. The
rear brakes are therefore actuated entirely by the power assist to the front brakes. If the pump fails, you will have no
rear brakes whatsoever, save for the handbrake. Combined with the loss of power assist on the front brakes, the fact
that something is very wrong with your brakes will be apparent immediately -- even without all those warning lights lit
up on the dashboard.
Now to the ABS operation. There are three systems here: the LF, the RF, and the rear. Each of these has two solenoid
valves within the valve block. If a wheel sensor detects that a wheel has stopped rotating, the first solenoid valve closes
off the line from the master cylinder so no further pressure can be applied to that caliper. Then the next solenoid valve
opens, relieving some of the pressure within that caliper back to the brake fluid reservoir. As soon as the wheel sensor
detects that the wheel has begun rotating again, the second valve closes and then the first valve opens, applying more
pressure to the caliper. In practice this all happens over and over very quickly, leading to the characteristic pulsing of
the pedal.
With the rear brakes that are fed entirely by the pump, that’s all there is to it. With the pedal-operated fronts, however,
this alternate relieving and repressurizing would quickly allow the brake pedal to fall to the floor. So there is one more
valve involved that applies pressurized fluid directly to the front brake circuits during ABS operation to keep the pedal
up.