97 Voyager abs

My abs light has been on for several months now; all the brakes and calipers have been done.. .what makes this light go on?

The ABS is separate from the Hydraulic brake system but it cannot work without it. If it comes on as soon as you start the car it is detecting a fault in its self test. Typically these are wheel speed sensors but it can be a number of other things. The best thing to do is have the abs system scanned for codes. This will give a better direction of what the actual problem is.