Braking System (ABS) relies upon information coming in from the sensors fitted
to the hub assemblies.
heavy braking the ABS Electronic Control Module (ECM) looses a signal from one
of the road wheels, it assumes that the wheel has locked and releases that
brake momentarily until it sees the signal return. It is therefore imperative
that the sensors are capable of providing a signal to the ABS ECM.
operation of an ABS sensor is not unlike that of a crank angle sensor, using a
small pick-up that is affected by the movement of a phonic wheel, moving in
close proximity. The relationship between the phonic wheel and the sensor
result in the production of a continuous Alternating Current (AC) sine wave.
ABS has been
a common safety related fitment to vehicles since the early 1980's with
systems from ATE, Bosch and Bendix. The systems all have a similar operational
strategy and are all electro-mechanical based.
The ABS ECM
(Electronic Control Module) is wanting to see a continuous stream of sine
waves for all four of the vehicle's wheels. If however the brakes are applied
and a wheel 'locks', the ECM will loose the signal from the skidding wheel. If
the ECM sees this happen, it will immediately release the hydraulic pressure
to the wheel in question and very quickly 'pulse' the pressure within the
hydraulic system allowing the braking efficiency to be maximised.
fitted with an ABS system will have improved braking abilities in adverse wet
or slippery conditions. Some vehicles may only have ABS fitted to the front
wheels of the vehicle.
sensors can also perform another role when the vehicle is fitted with traction
control, instead of looking for a loss of signal from a 'locked' wheel the
traction control ECM will be looking at the frequencies of the signals to see
if any of the wheels are spinning. If a spinning wheel is detected, the
engine's power output is reduced until such time as all frequencies from the
ABS sensors are the same and traction is regained. Some systems will
momentarily apply the brake to the spinning wheel to aid traction on the other
is a typical front hub ABS set up, whereby the driveshaft has a series of
teeth on it and the speed sensor is mounted in close proximity to them.
alternative set up is shown here utilising a castellated drive ring mounted
onto the wheel bearing. The inductive sensor is the built into a hub cap
located on the inside of the vehicle's hub assembly.