The sensor, itself, is about the size of....
a blasting cap or thin cigarette. Once it is unscrewed, look at the bottom end and you will probably find a build up of grease and dirt. It will take some doing but all of that needs to be cleaned of. Under the mess, you should see two small strips of metal that look like an = sign. Make sure that you gently clean these. I used a wire brush.
Before you put the the sensor back in, I suggest that you use a compressor to blow out the dirt near the mount. Also, clean all of the sprockets / teeth on the speed sensor wheel.