Here are some ohm and voltage tests you can run.

Front ABS Sensor Ohm reading across the Sensor End connection

4x2 F250/350: 2,000-4,000 ohms

4x2 F450/550: 650-800 ohms

4x4 F250/550: 650-800 ohms

Rear ABS Sensor (VSS) Ohm Readings

All: 800-3,500 ohms

A volt meter, set to the low scale AC volts, should show a fluctuating reading while the differential, or front hub, sensor ring is turned at a quick rate.
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