Volvo: ABS Problems 2000 S80, wheel sensors, volvo dealership

Question
Symptoms: dashboard check engine, abs, tracs lights on.  obd code P1550(ECM-510F)- fault in vehicle speed signal detected in abs control module; can turn this off but it comes back on at engine restart.  Removed my abs control unit and had it installed in another s80 - worked fine; upgraded the unit anyways and reintalled it in my s80 - same problems.  Had volvo dealership read trouble code - 0021.  Problem with right front wheel sensor.  Dealership says abs control unit has internal fault - they tested wheel sensors and they are all okay.  Had a second abs control unit swapped with my upgraded unit, but still had the same problem.  What can the problem be??

Thanks

Answer
Ron,
Poor diagnosis. If all the sensors test good and the c/unit is new and double checked then what else could it be? Go back and talk to the service manager and tell him the story. Maybe he can have a more experienced tech look at the car. The control units do have a high failure rate.