Volvo Repair: Volvo ABS light, transient surge protection, abs light

Question
My 1994 Volvo 850 ABS light is on.  Is that a way to find out if it is the ABS controller is defective, or the wheel speed sensor has problem.  

BTW, when I first start the car, there is no ABS light on.  It comes on after driving for 100 ft or so.  

Thanks for your advice.


Answer
it could be many things from a bad relay to bad control unit.it will need to be looked ata for a proper diagnsosi.it will need to be fault traced according to the fault tracing manual.since it comes on when driving and not start up this eliminates the transient surge protection relay and fuse.