ABS Wheel Speed Sensor TTW1.0
What it does
It measures road-wheel speed and direction. This
information is used for anti-lock brake (ABS), traction
control (TCS) or vehicle stability (ASP) system ECUs
to prevent the vehicle’s wheels from locking
or spinning. This increases the driver’s control
of the vehicle.
How it works:
There are two basic types of wheel speed sensor,
passive and active;
1. Passive sensors;
These are similar in operation to Inductive cam/crank
sensors; they consist of a soft iron, permanent magnetic
pin around which is wound a fine copper wire coil.
The unit fits in close proximity to a rotating trigger
wheel. The trigger wheel is mounted onto a rotating
assembly, such as the disc, drum or hub. When the
trigger-wheel rotates past the pick-up assembly the
alternating teeth generate a small inductive voltage
in the copper windings. This voltage signal is used
by the ECU as wheel speed information. Passive sensors
generally only have two wires, one signal and one
earth/shield.
2. Active sensors;
The operation of the active sensor can be likened
to the Hall type sensor found in distributors etc.
The pick-up assembly has an inbuilt amplifier and
thus relies on a supply voltage which is normally
5v but it can be 12v.
The rotating element consists of a multi-pole (north,
south, north, south …) magnetic ring, which
can be located onto a rotating assembly as with the
passive sensor. There is an increasing trend to incorporate
it into the wheel bearing seal and to use magnetic
powder instead of fixed magnets.
The rotating, alternating, magnetic poles generate
a magnetic flux within the sensor element which then
amplifies and regulates the signal for the ECU to
use as wheel speed information. The output of an active
sensor is digital with a square wave signal, it is
capable of sending wheel speed information down to
0mph, whereas the passive sensor’s accuracy
is usually dubious below, 25mph. Active sensors generally
have three wires; one power supply, one signal and
one earth/shield.
Caution:
Active sensor wheel bearings can be installed backwards
rendering the sensor inoperative. Pay particular attention
to the instructions enclosed with this type of sensor.
Reasons for failure:
The principle cause of failure is the same as most
other automotive sensors, the environment in which
it operates. Wheel speed sensors operate in harsh
conditions; they have to cope with extreme vibration,
water, temperature fluctuations and dirt. The connecting
wires have to cope with the demands brought by suspension
and steering movements. All in all it’s not
good news.
Note:
Some manufacturers use active sensors that have small
aerials built in, to transmit information to the ECU
via radio waves.
Testing:
The principle tool in diagnosing wheel speed sensor
failures is the on-board diagnostic power of the ECU.
Failure of the sensor generally lights “MIL”
lamp on the dashboard. All that we have to do then
is to pin-point the one that failed.
A visual check should be the first check, look for
damaged wires, sensors or a build up of road dirt
etc. The next step is to test the sensors individually.
Use an oscilloscope connected to the sensor and spin
the wheel, check the output pattern for interference
and the correct “shape” (passive = sine
wave, active = square wave). Passive sensors can be
disconnected and tested as a “stand alone”
unit, active sensors, due to their required voltage
input, have to be connected with the ignition on when
testing.
Caution:
Ensure that the correct type of sensor is fitted.
For further technical information
concerning wheel speed sensors please phone our technical
help line 01527 839307
