Reading an ABS wheel speed sensor
Most of the ABS speed sensors that I have seen are CURRENT-MODE devices.  That is: they modulate the current passing though them rather than the voltage.

This has several significant benefits for the automotive manufacturer.  The main benefit is the elimination of ground-induced noise in the sensor.

The easy way to verify this is to install a 100 Ohm resistor in series with one of the sensor leads.  Then measure the voltage across that resistor.

I've made simple adapter boards for local auto enthusiasts.  I had them identify the most positive lead going to each wheel sensor and interrupt that.  I then used some Zetex ZXCT1008 hi-side current sensors to give me a proportional current output that was then converted to a voltage at the add-on board that they were using.  The Zetex part is connected across an 18.2 Ohm resistor in series with the +12V lead feeding the sensor.  

Again, keeping the sampled signal as a current eliminates ground-related problems for the add-on system.