Rear Axle Sensor.

Connect DVOM across terminals No. 7 and 9 of ECU connector. Resistance should fluctuate between 600-1600 ohms as rear wheels rotate.
If resistance is not within specification, check resistance of sensor at sensor connector located at fuel filler pipe in trunk. If resistance is now within specification, repair wiring harness to ECU. If resistance is still incorrect, replace sensor. See Fig 1

Fig 1: Volvo ABS Rear Speed Sensor

Courtesy of VOLVO CARS OF NORTH AMERICA.

NOTE: If distance between rear speed sensor and pulse wheel is incorrect, system disengages and ABS warning light will illuminate.