Active and magneto-resistive sensors

A discussion about sensors results in a recap of essential diagnostic principles

Published:  20 December, 2013

Andy Crook

How can you identify active or magneto-resistive sensors?

A common mistake is suggesting that all two-wire 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. For an example, consider hill start assist systems.

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 multimeter 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 around 1.2V and 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 as the pattern will look as if the vehicle had been rapidly decelerating and accelerating many times in the space of a few seconds.

If you want to know more about Andy's technical and training events, call Brian Thomas at Autoparts South Wales on 01443 405726.

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