This system uses a variable reluctance or analog sensor. All the big 3 light duty trucks can have an issue just like this. As the air gap increases slightly it decreases the signal output of the sensor. Before it goes too far it can create an issue of pre-mature ABS applications. The fix for this is to remove the sensor and clean the film of rust off of the knuckle then reinstall the sensor. Don't grind it or the air gap could be effected. The sensors should be less than 10K ohms according to Ford specs. If the sensor begins to get too much resistance (or too little) the controller will set a code for a failed Wheel Speed Sensor. The resistance values should be close, and there is a test for checking the output of both front sensors.
Also, it can be a hub/axle bearing beginning to develop play that can effect the ABS operation also. Anything that allows the air gap to change can create an ABS false activation if the signal doesn't go out of preset parameters.
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