I have the same issue, I have replaced all of the ABS sensors, the rear one and both the front ones. Not sure what to do next..does the Transmission speed senor also cause the ABS light to stay on? If so how difficult is that to replace?

 

I have a '99 f150 extended cab, auto, 5.4, and have been having trouble with the ABS for a couple years now. I've replaced the two front wheel sensors and the rear axle sensor. Sometimes my light is on all the time. Sometimes it isn't on. When it isn't on, I can feel the ABS working at inappropriate times (light braking on clean dry pavement).

My regular mechanic has read the codes but he says they're not giving him any useful information.

Is there a troubleshooting guide for the ABS system that I can refer to? Can I buy a book?

I just did a little googling and found this: http://www.youfixcars.com/ford-abs-light.html

And this is what I've been wondering all along: What does the wheel sensor read? I'll be looking for damage to my reluctor rings.