Why does my 1999 Jaguar xj8 l ASC light stay on ?

OK, Carlos, so the ABS light was on and you replaced the wheel speed sensor, but the ABS light remains on? It's a pretty straightforward operation to replace that sensor, so perhaps the incorrect sensor (there are 4) was replaced, or perhaps the sensor itself is faulty. It's really impossible to answer the question without some testing. You can reach us at Cliff's Car Tech, or if you prefer to do your own work, you can access www.alldatadiy.com for answers for the do-it-yourselfer.