2. ANTI-LOCK BRAKING SYSTEM (ABS)
An anti lock braking system (ABS) sensor is used for determining wheel rotation speed to
prevent wheel lock up when braking. The Hall effect ABS sensor consists of a permanent
magnet with a Hall effect sensor next to it. The magnetic field strength changes when a
magnetism sensitive object passes through the magnetic field of the magnet. This changing
of the magnetic field causes the output of the Hall effect sensor to change.
In most cases the object used to influence the magnetic field is a disk or ring with evenly
distributed teeth, mounted on the driveshaft or in the bearing. When the wheel is rotating,
the teeth are passing the sensor and the pattern in which they are placed is visible in the
ABS sensor signal. Each period of the signal is a tooth passing the sensor. The frequency
of the signal depends on the rotation speed of the wheel and the amount of teeth on the
disk or ring.
Two different types of Hall effect ABS sensor are
used in cars, with either 2 wires or 3 wires.
3. 2 WIRE
The 2 wire Hall effect ABS sensor has a
12 V power supply wire but no direct
ground. The ground of the sensor is
also the signal wire. The 2 wire Hall
effect ABS sensor is current regulating.
The amount of current (Is) is changed
by the sensor when a tooth is passing
the sensor. Depending on the sensor
design, the presence of a tooth causes
either a high or a low current and a gap
between teeth the opposite. This
current flowing through the resistor
inside the ABS ECU will produce a
voltage (Us) related to the ground,
similar to the square wave signal of the
3 wire Hall effect ABS sensor.
4. 3 WIRE
The 3 wire Hall effect ABS sensor has a
simple power supply and a signal wire with
the signal voltage (Us) going to the ABS
ECU. Depending on the sensor design, the
presence of a tooth causes either a high or a
low signal voltage and a gap between teeth
the opposite. The resulting signal is a
square wave. The voltage levels are
different and much lower than a 3 wire Hall
effect ABS sensor because of the low
currents. Voltage levels can also change
from system to system depending on current
flow and resistor values but a clear square
wave pattern should be visible.
5. DIAGNOSIS
Signal values may differ on different types of engine control units and Hall effect
ABS sensors.
The following signal deviations can indicate a problem:
• No signal:
Cause: back probes have no connection, no power supply, poor or no connection
between sensor and ECU, sensor defective
• Noisy signal:
Cause: poor power supply to Hall effect ABS sensor, poor connection in
connector terminals, wiring of signal wire or power supply damaged
• Signal has faulty pattern:
Cause: tooth ring or disk damaged
