I just put in a new ABS Wheel sensor after replacing a ball joint.

The ABS system activated at very slow speeds, but otherwise seemed normal. The problem was that the ABS wheel sensor was not pushed in all the way. I could see a very small gap between the plastic part of the sensor and the spindle. Some gentle taps with a hammer and re-tightening of the alllen bolt fixed the problem.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside