I can't explain why using AC coupling (filtering out the DC components of the signal) would have helped at all. We really do not want to AC couple unless we specifically desire to filter out the DC. In this case you would not. Perhaps you used a different scaling and were able to see something.
I am not familiar with that specific application but you could be dealing with current based magneto resistive type wheel speed sensors.. You are not likely to get much of anything looking at voltage on these. However, you can check them with a micro amp probe.
Here is a ink to a discussion the first time a micro amp probe was used to sample this type of sensor:http://www.autonerdz.com/cgi/yabb2/YaBB ... 1276215303
Edit; Ha....You beat me to it Alan.