98 Windstar ABS Trouble

Alright gents, this is a new one for me. I'm having trouble with the ABS coming on as i come to a complete stop with moderate brake pressure on dry pavement. The brakes seem to work fine except for this. And, i know the ABS is coming on by the sudden drop in the brake pedal followed by vibration and the bzzzzzt of the ABS valves... Like i said, it's only noticeable when i'm coming to complete stop (at say 3 mph or so). Then, i do eventually stop. But it's a bit disconcerting! Also note that it doesn't seem to happen when stopping with light brake pressure; only moderate to hard... Also, i am familiar with the typical buzz of the ABS right after startup; but that only happens once per drive in my experience (i.e. the first time you apply the brakes while pullnig out of the driveway). This issue is independant of the startup buzz.
Tire pressure is good on all 4 wheels so wheel speed differences shouldn't be the problem. There are no warning lights on the dash either. If it were the wheel speed sensor wouldn't it come on all the time? I can't think of any other culprit. If it is a sensor, how do i isolate the faulty one? Your help please!!! :)

Two things can cause this without a code being flagged. The air gap between the sensor and the exciter. Or a broken exciter ring on the drive axle. The only thing you can do is visually inspect the exciter rings on the front drive axles. If one of them is cracked you found your problem. Otherwise a drive with a scan tool with ABS data capabilities is your next step.

What is happening is one of the sensors is sending out a report of one wheel slowing faster than the rest at a slow stop. A broken exciter ring will increase the space between the teeth and will result in a longer time between the signal spikes of the two teeth on either side of the crack. The ABS interprets this as the wheel slowing faster than the rest of the wheels and enables the ABS function to correct the pseudo slowing wheel. Since there is no malfunction in the electrical components of the ABS system there is no ABS light.

Thanks for the insight. I pulled off all 4 wheels and looked at the rings and sensor pickups. No cracks in the rings. The only thing that i did notice was the sensor pickups on the rear wheels were pretty rusted and flaky. Those may be the problem, but i sure would like to verify before dropping two new ones in there. They are about $70 a piece. So, i was gonna pick up an ABS scanner at Autozone, but they were $200 and couldn't be returned. I also wanted to verify with you all what a good make and model of scanner were... While i'm at it, should i buy an engine code scanner too? Also, i need an ABS scanner that will tell me which wheel sensor is bad (note that i have no code or ABS light on). Thanks!

I don't have any experience with affordable scan tools. I can tell you mine initially ran me $3,500 and costs me about $800 a year to keep it up to date. This year I traded it in on a newer model with and that cost me $1,400.

So you have to ask yourself if the cost of a scan tool capable of reading data is worth the expense or would it be cheaper/better to have it scanned and diagnosed?

I would try cleaning the rear wheel exciter rings with a wire brush and brake cleaner.

Thanks for the advice. I brushed off the exciter ring and abs sensor and also blew them out with compressed air. It seems to have taken care of the problem, but time will tell! Thanks!