Difference Between Wheel Speed Sensor and ABS?

when you apply the brake hard enough to lock up the tire the speed sensor tells the computer that the abs needs to kick into action. thats why the brakes are pulsating (it thinks you are skidding) your abs is prob on all the time while braking you can only feel the pulsating at slower speeds. you aren't getting warning lights because the computer sees the abs working properly by what the speed sensor is telling it. sometimes if you hit a bump or turn sharply while abs is working you will get a flash of a warning for a second or two. replace your speed sensors, if you are handy I'm sure you can diy if you research online first prob get parts for 200 or less, and spend a sat afternoon. good luck.