This is a long one, folks ... thanks in advance for your time in reading this.

For reference, my previous thread on my ABS problem was titled "Replacing the EBCM (Electronic Brake Control Module) in '89 Coupe" and can be found at:

http://forums.corvetteforum.com/zerothread?id=615500

The ABS problem is not yet resolved. However, I've noticed a distinct pattern of the symptoms, so I want to bounce it off other forum members for input.

Background of the Problem
My ABS typically goes "off line" (shows ANTI-LOCK) within a few seconds of starting the car. However, as I mentioned in my other thread, on occasion (about 5% of the time) the ABS system will NOT go off-line right after the car starts. In nearly all these cases, it seems to work fine for a few minutes ... until the vehicle comes to a complete stop. Within a couple seconds of coming to a complete stop, the "ABS Active" light will come on, the brake pedal will chatter a bit, and then the system will take itself off line (ANTI LOCK lights up and stays on for the remainder of the drive).

Evidence of a Consistent Pattern
I recently began thinking about the fact that nearly every time the system DID start out working properly, it worked fine up until the point I came to a complete stop. Tonight, on my drive home from work, I got to prove this pattern to be correct. My drive from work to home, which is about 15 miles, is usually full of stops. But, this time luck was on my side and I made it home without bringing the car to a complete stop anywhere along the way :cool: . And, to make things even better, this was one of those times when the ABS system came on-line after start-up, and functioned without error.

Just as the pattern has demonstrated previously, the ABS system stayed on-line during the entire drive home (presumably because the car never came to a complete stop during the drive). As soon as I drove into my driveway, stopped the car for about 3 seconds, the brake pedal chattered, the ABS ACTIVE light came on, and a couple seconds later the system went off line and there was my ANTI-LOCK light again.

Aha! A (fairly) solid hypothesis!
My hypothesis: Whatever is causing the ABS system to malfunction is directly tied to motion (or lack thereof). If this is true, then this suggests the problem lies within the sensors themselves, or at the hub/bearing assemblies (where the sensor teeth are). I believe this would rule out an issue with the computer, ABS pump or associated wiring harness -- because they are not directly affected by a complete stopping of the car (other than, perhaps, the lack of motion :confused: )

As I've stated previously, I hate throwing parts at a problem. But, I'm tempted to replace the sensors, one at a time, to see if this fixes the problem. If that doesn't work, then I can throw some hub/bearing assemblies at it, one at a time, to see if it fixes it. But, I am curious to hear what others have to say regarding this issue.

Whattayathink?


[Modified by SkunkCar, 10:29 PM 8/14/2003]

 

:lurk:

 

Hey, I resemble that remark :lol:

TTT


[Modified by SkunkCar, 10:31 PM 8/14/2003]

 

Well you could remove each wheel sensor, and shine a light down the hole to inspect the teeth, as you rotate the wheel. You could also measure each sensor for continuity and resistance, they should be around 900 Ohms.

 

When testing the sensors for continuity, from what two points do I take the measurement?

 

Hey fellow Arizonian....I live in chandler. Have you removed and looked at the sensors? my rear ones were covered with iron filings.

 

Hey Jeff - it's me, Steve (I was over about 3 months ago when you were doing the rear/left bearings on your '88). Yes, I've pulled all 4 sensors and inspected. I did find iron filings too, and cleaned them off. I looked best I could at the teeth inside the hub/bearing unit, and from what little I could see, they looked ok too.

[I must add that I checked the bearings, and they seem to be ok as there is little or no play in the wheel when attempting to move at the 12 and 6 positions.]


[Modified by SkunkCar, 8:24 AM 8/15/2003]

 

Hey! my guess from your symptoms is that one sensor is not reading the fact that it's tire has stopped spinning. I have not seen exactly how ours are programmed but I know how they are supposed to work. If you come to a stop and the computer sees a wheel that has not stopped then it senses that something is wrong. If (like you said) all of the parts but the sensors have been changed out. I would go to a junkyard and aquire a set of sensors to swap out. I had read a while back about somebody's sensor loosing the iron bar across the end. This would definately reduce its sensitivity. Just some thoughts......

 

Good idea ... have you heard of 20th St. Auto Salvage? It's like around 20th St. and Washington. They have several '87-'89 Vettes, so it's a good source for us. I've found them to be much more reasonably priced than other places too. I'll have to grab a set of sensors from them...

 

They are right down the street from my work. I actually got some parts for adifferent car a while back - very helpful.

 

ttt

 

There is a multi pin connector in the box behind the seat. it has ten wires, 5 pairs of different colours. One pair is black, dunno what they do, but each other pair is connected to a wheel sensor. So measure the resistance between the two reds, the two yellows and so on. (Can't remember the exact colours, but you get the idea)

Each measurement should be around 900 Ohms. (somebody else may have more accurate info). If one is wrong, it could be the sensor itself, or the wiring to it from this plug.

 

One other obscure idea.....have you tried removing the sensors and try to lightly move the sensor ring with a flat blade screwdriver.? It is feasable that a weld broke and allow the ring to continue spinning when you stop?????