Download TECHNICAL TRAINING TIP 017 Method for the diagnosis of ABS

TECHNICAL TRAINING TIP 017
Method for the diagnosis of ABS sensors
With the advent of the magneto resistive sensors toward the end of the 1990s,
diagnosis of ABS sensors has become more complex.
Magneto resistive sensors are characterized by the fact that they can detect very low
magnetic fields. Their principle of operation is based on the variation of the electric
resistance of a ferromagnetic material as function of the applied magnetic field. When
subjected to a magnetic field, the resistance decreases by 10% to 15%, which in turn
decreases current flow by 7 to 14 milliamps. Unlike the pulse generator sensor, the
voltage variation of a magneto resistive sensor is minimal; it is the variation of its
current which acts on the PCM’s Schmitt trigger.
Magneto resistive Sensor
It is difficult to diagnose this type of sensor using an oscilloscope since the change in the
sensor’s voltage, depending on the application, is only a few hundred millivolts. The
magneto resistive sensor is also sensitive to variations in external magnetic fields, which
implies that the object to detect must not have varying magnetic fields. Any
accumulation of corrosion on the surface of the reluctor will also adversely affect the
signal.
The most common symptom is the activation of the isolation valve during low speed
braking with no ABS intervention. The most effective method to detect the faulty wheel
is by monitoring, with a scanner in graphical mode, the isolation valves. The valve which
is activated during braking will identify the wheel with the faulty sensor.
© OBD3 Solutions 2009
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Diagnosis using a scanner in graphical mode
The above graph shows the activation of the isolation valve of the RH rear wheel.
The above graph shows a cut in the rear RH ABS sensor.
Caution: Before replacing the sensor, check that the reluctor is not damaged and is free
of any corrosion. Also check the condition of wheel bearings.
© OBD3 Solutions 2009
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