ABS braking, how it works and how it might not.

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While I worked for the LVVTA I attended a meeting with Land Transport
staff all about ABS braking. There are a few eye-openers.

The ABS braking system comprises a computer, a hydraulic control box
and a speed sensor on each wheel. The hydraulic brakes function just
as in a non-ABS car until the computer decides it needs to act, having
detected a skidding wheel. It then releases brake pressure on that
wheel and restores it when the wheel is back up to speed. The
hydraulic fluid is dumped back to the reservoir when this occurs and a
pump replenishes the fluid under pressure. A shuddering is felt under
the brake pedal as it drops and lifts again.

Self-test:
When the ignition is turned on, the computer self-tests that the
electronics have power and the pump has power. There is no hydraulic
test. When the vehicle reaches about 10kmh, the computer does a second
check that all wheel sensors are returning a signal. If all is OK, the
warning light stays off.

Hydraulics:
Each wheel has two hydraulic valves, controlled by solenoids. One is
normally open and shuts the pressure to the wheel cylinder whan
activated. The other is normally closed and opens to dump from the
wheel cylinder to the reservoir. These valves are made of steel, in an
aluminium body, with springs the size of ball point pen springs and
machining to fine tolerances. Their correct operation is critical. Any
jamming of any of the eight valves from rust or dirt will cause the
brake pedal to go to the floor, or at least loss of brakes on the
associated wheel.

Activation:
The computer is only activated from sleep when braking occurs (same as
brake light on). The pump starts and the wheel speeds are monitored.
If skidding is found, that wheel has the valves activated and the pump
replenishes hydraulic fluid at full adrenalin pressure. If it does
not, the pedal goes down to the firewall. Note that a sensor cable
break while travelling (after the 10 kmh self test) will make the
system turn off braking pressure on that wheel as it thinks it has
locked up.

Maintenance:
The ABS system requires absolutely clean hydraulic fluid. It is not
tolerant to a bit of water or rust like non-ABS brakes. If the
hydraulic unit has a faulty valve it must be replaced with a whole new
unit. Second hand ones that have had exposure to air are unlikely to
work. New factory units are sealed and must be in service with clean
fluid in a few hours or they are discarded. The man presenting this
talk said that in his cars he flushes the fluid every two years,
bleeding the most distant wheel cylinders first and topping up with
new fluid. Then he drives on a remote country road with seal and dirt
edges (and no traffic). He brakes hard with wheels off the tar seal on
each side to activate the ABS and flush clean fluid through the
hydraulic unit. The car should stop in a straight line. He then goes
home and bleeds the circuits again.

The point he stressed was that the self-test does not show the whole
system is in good order and a failure can cause the brake pedal to go
down to the floor when the ABS operates. Symptoms can go away after
the resulting crash and the crash engineer will find a good hard brake
pedal and conclude excessive speed was the cause. Proper maintenance
is critical.