The system does a self-test every time you turn on the ignition switch. A defective sensor or broken wire would cause the light to come on right away. Most likely you have a cracked front tone ring. They have 47 teeth, but the crack makes it appear to have 48 teeth. This tells the computer that wheel is turning slightly faster than the other one which would occur when rounding a slight curve, but not for a mile or more! You'd be going in circles!
You can not buy new tone rings; you have to replace the entire half shaft, unless...
I proved to my students you can weld cast iron. When this happened to my mother's '95 Caravan, we took the ring off, ground a depression inside, squeezed it tight in a vise, (which also broke off a ring of built-up rust), heated it with a torch for a few minutes, then filled the depression with the weld. Glued it in place with Mopar RTV sealer which will also prevent water from getting in there, freezing and expanding the ring causing it to crack. I have since acquired three more tone rings for future repairs.
The crack in the tone ring is pretty easy to see, but before you jump in there, verify that all four tires are the same size. Mismatched sizes will affect the wheel speed readings the computer sees. Also, don't overlook low fluid level in the master cylinder although that should also turn on the red brake warning light.
Friday, May 22nd, 2009 AT 3:00 AM