Most factory ABS wheel speed sensors will be a two-wire Hall Effect type. These are powered sensors that produce a square wave pulsed output unlike a non-powered variable reluctance sensor that will produce a sine wave output.

Wiring of a two-wire Hall Effect sensor is not immediately intuitive. They are often directional or polarity sensitive, and require the 12V signal to be hooked up to a specific pin in order to produce an output. You can normally determine this if you have access to a factory service manual, however, in the absence of this information, you may need to make an educated guess by dissecting the factory harness or by trial and error as you can only get it right or have it back to front once.

The second pin will be your signal pin back to the ECU, but instead of simply hooking it up and leaving it to be, you will actually need to wire in a pull down resistor to ground. A 270Ohm 0.5W resistor spliced across the signal wire into the signal ground pin should perform adequately to produce a rising and falling square wave output when the wheel spins. Typically the signal will not rest fully at 0V or rise all the way to 5V, but it does provide a sharp edge for the ECU to pick up on and read as a tooth. Using the Oscilloscope function in your ECU to view this voltage waveform, you can determine the amplitude of this signal and program the arming threshold appropriately. 

Some systems do use a Reluctor sensor so each application should be tested for correct information before proceeding.

Example: Excerpt from a BMW/Bosch ABS Manual

Vehicle Specific Information

Custom should be selected as the sensor type.  The edge does not really matter but Falling is selected by default

ABS Sensor Source	Voltage Supply	Pull Down Resistor (Ohms)	Arming Voltage	Triggering Voltage
VE Commodore	12V	150	2.0	1.5
350Z	12V	220	2.3	1.9