How Can you identify Active or magneto-resistive Wheel Speed Sensors?
A common mistake is assuming that all two wire wheel speed sensors are analogue AC (passive) sensors. Many are but not all; more and more vehicles are being fitted with two wire Active sensors. This is due to the greater low speed accuracy and ability to detect the direction of rotation. (Think about hill start assist systems.) Technical data sources often do not identify the sensor type, and the fault codes provide no clues either. So the only way to be sure is to measure the Voltage at the sensor with the ignition on. If there is approximately 12 Volts on one of the wires you are dealing with an Active sensor.
Do you need specialist equipment to test active wheel speed sensors?
A scan tool is a good starting point, check for codes and live data. If this suggests a fault with the wheel speed sensor(s) then you will need to test the circuit. An oscilloscope is ideal, but you can use a good multi-meter.
If you wish to measure the very small current flow a micro Amps clamp can be used.
If you have a LED test lamp this can be used to test the circuit with the sensor disconnected as it should illuminate with the small current supplied. However the circuit protection will switch off the supply on many vehicles as soon as an open circuit is detected so it cannot be relied upon & therefore is not recommended.
What is the correct output?
The sensor produces a square wave output when the wheel is rotating. However it is not switched to ground or to battery voltage as you would expect. What you will see using a scope or multi-meter is the voltage drop across a single resistor or a pair of resistors in parallel depending on the position of the reluctor/tone ring, just by moving the wheel a small amount you should be able to detect this change. Try that with a passive sensor.
The output should be a square wave form the difference between the two voltages (Delta Voltage) should be approximately 0.6V or if you are using a micro amps clamp in the region of 14mA and 7mA.
Only the frequency should change as the wheel accelerates and decelerates. Unlike analogue AC sensors where the amplitude and frequency of the signal changes proportional to wheel speed.
It is also possible to detect physical faults with the reluctor/tone wheel using an oscilloscope. Cracked, blocked and damaged teeth produce signals that should identify the fault quickly.
If the teeth have come into contact with a foreign object they can distort or have a wear pattern that produces a waveform that has more tooth signal than gap signal in the area of the damage.
Can you diagnose them by measuring their resistance?
The sensor requires a small current in order to operate this is supplied by the ECU through a resistor. Resistance measurements are made with the circuit disconnected; therefore the results would be misleading, wrong or possibly damage the sensor.