ABS light on after cleaning wheel speed sensor

Lately I have been experiencing the low speed ABS problems that are common in the 2001 (and other) Sierras. After reading the techinical bulletin, I removed the front caliper and roter and removed the wheel speed sensor - I clean the end of the sensor (which whas covered in grease) and cleaned the mounting surface. I then replaced every thing and went out to test the truck. When I started it the abs light came on and would not go off. The truck brakes normally, but the ABS probably is not funtioning since the light is on.

Does anyone know what may have happened? Did I possibly damage the sensor, or did I maybe not reinstall it properly?

Thanks,
Nick