ABS in 95 XLT

[warzzero]

First time posting, be gentle

I had my rear brakes completely redone, new rotars, new brakes, ball joints, all that stuff. Also had a leak in the rear diff which is now fixed, but it seems that since doing this stuff, a bigger problem has creeped up. I'm trying to narrow down exactly what the cause of the problem is, and hopefully with your help I'll be able to figure it out.

When I get the car up to 45mph I can feel a small vibration in the brake pedal, as soon as I go under 45mph this turns off. I'm pretty sure this is when ABS kicks in. When my rear dif was looked at, the mechanic told me that there was shrapnel found in there but they couldnt see any of the gears missing teeth. the ABS sensor was also crushed, mechanic said the shrapnel might have caused this. So i had the ABS sensor replaced thinking this might have been the cause of ABS problems, on the way back from the mechanic, the brakes were acting up again, ABS sensor didnt help at all. What happens is occasionally (very often, but mostly when going freeway type speeds) when you hit the brakes the pedal feels like it is fighting your foot a little bit, it also makes a creaking noise (like an old door opening). The brakes actually respond (it has no trouble braking). Once you push on the brakes a little, after it passes a certain point, it acts normal. I *think* this is something to do with the ABS not knowing what to do ? Anyway, also on long drives, the ABS light will come on, when this happens, the brakes are absolutely fine. The pedal doesnt act up at all.

Upon researching this problem, the mechanic mentioned that there is something called an Exciter Ring or a Tone Ring that the ABS uses to determine wheel speed.. ? I have a feeling this might be the source of the problem, since the shrapnel was found in the rear diff, and basically ive already replaced everything else. What I havent been able to find is anything in detail about this tone ring, dont know where it is, dont know how hard it is to change out, etc. Any help?

 

[Brandon1993Sport]

Well I did some searching on it and it looks like there is a tone ring mounted inside the differential like you were talking about. I guess the ring in yours might have some messed up teeth on it which is causing a bad reading at a certain speed. Btw when the ABS light comes on, that means it has shut the ABS system off which is why the brakes work fine then. One of the sites mentioned that the ring is welded to one of the gears in there so I don't think it's something that can be done yourself. Also couldn't find a part number anywhere on it. Here's a pic of what they look like:

Sorry I couldn't help more, good luck

 

[warzzero]

Curious where you found all this stuff, i've been looking all over! So Let me see if I understand this a little better... the ABS Sensor has a positive and negative 'sensor' or whatever.. and it counts how many times the 'teeth' on the Tone Ring are going around? Those teeth arent spinning another gear, right? just being counted by the ABS Sensor? That would make sense then.. if there are some teeth missing from the Tone Ring (which im *guessing* thats what the shrapnel found in the rear dif was from) .. then its getting some misreads and not knowing what to do. There could also be something floating around in the rear dif that is screwing up the magnetic field between the ABS sensor and the Tone Ring ..... ?

 

[mrboyle]

The ABS sensor is basically an elecrto-magnet that makes a square wave signal an the teeth of the tone ring pass. When the peak (high point on the ring) passes the sensor the magnetic force is inceased. Then as the valley (low point on ring) passes the magnetic foce drops. This happens over and over as the vehicle is driven and ceates the square wave patteren. If the ABS controller senses a break in this pattern, when the brakes are applied, it assumes the wheels have locked up and it will pulse the ABS (its more complicated than that, but you get the idea). If the ABS module senses a break in the pattern, and the brakes are NOT applied, it assumes there is a problem with the system and will shut off and turn on the amber ABS light.
From your description it sounds like you have a problem with the tone ring in the rear diff, but normally if there is a broken tooth on the tone ring the ABS will sence this as soon as the vehicle reaches 5mph, so its hard to say if this is the reason.

 

[warzzero]

Thank you for the explanation... that helps me understand it a bit better. I do have a question though, why is the ABS only seeming to have problems at higher speeds? and what is with the vibration in the brakes at exactly 45mph? Some people have said this is when the ABS 'kicks in' , but as you said, abs should always be on and notify me if theres a problem within 5mph