I know that I am chiming in late in this discussion, but I recently had some REALLY wierd problems with my ABS on my 97 Blazer that I bought new and has about 250K miles on it.
The brake pedal was randomly going to the floor, with the ABS light coming on and off, and sometimes the brake light coming on too. The ABS motor was turning on every time I stopped. Scary to drive because the brakes would work really well, and then not at all. I read a lot of forums and the problem seemed to be the ABS unit itself. So I found one on eBay and replaced it. No help. OK, now what?
Maybe a bad replacement unit? But the ABS light was out now, so maybe not. I talked to an ABS control unit rebuilder and explained the symptoms. He thought the problem was hydraulic because the pedal was going to the floor. Well, I had never replaced the calipers or wheel cylinders, so I did that, along with the pads, rotors, and shoes, which were pretty much worn out as well. And the master cylinder too. Good quality NAPA parts.
Now I had really good braking but still had the wierd symptoms with the pedal randomly going to the floor. Air in the system? OK, we flushed and bled the system 3 times. STILL no help. What did I miss??
Did some more reading, including this forum. OH, maybe a bad ABS speed sensor or a broken wire? OK, I pulled the rotors to clean and check the sensors, like some of you guys said. Yeah, good idea.
When I pulled the rotors, you will NOT believe what I found.
Yeah, this sounds crazy. Almost could not believe it myself. There are three large bolts that hold the bearing assembly to the brackets on each of the front wheels. These bolts also hold the backing plate with the ABS sensor on it in place.
All three bolts on the right front wheel were LOOSE and the backing plate was at an angle instead of being parallel with the wheel. The ABS sensor had been rubbng on the speed ring and was generating all kinds of wierd signals which were driving the ABS controller crazy. Yeah, and me too.
How I missed that when I replaced the caliper and pads on that wheel, I will never know. As soon as I tightened the bolts (and torqued them to spec) all the ABS problems disappeared instantly. No red lights, high pedal, and the brakes really work well now. Well, after replacing almost the entire brake system, they ought to. Have not replaced the sensor and ring on that wheel yet, but parts are on order.
Lesson learned: If your ABS is going nuts, check the sensors before you disconnect or replace the ABS unit. It's just a computer and it only does what the sensors tell it to do. The interesting part is that you can get the sensors at any auto parts store but not the ABS unit. Wny? Because stores stock what they know they can sell. The sensors go bad and they sell. I'm guessing that the ABS units don't sell much, so maybe they don't go bad a lot.
Anyway, thanks to all you guys for pointing me in the right direction.
Hugs.
Skypilot. Yes, I am am pilot - and (surprise)
female. I bet you were not expecting that. LOL