92 Bonneville ABS not working

[Alpha One]

My 92 Bonneville's ABS is not working. Some time ago I went over a pot hole and the ABS light turned on. I tested the brakes over dirt and ABS didn't work. Otherwise brakes fine. For the next week or two sometimes the light would go off and the system would work. Now it never works. I tested the four weel sensors and all four send a signal. Any ideas on what may be wrong? Where is the computeter for the ABS system located?

Thank you