Since I am not a diagnostician, nor do I play one on television, but with my limited knowledge and experience I realize that there are at least three schools of thought for this repair. The first ( unlimited resource method) would be to take it to the dealership, or a Volvo specialty shop, have it properly diagnosed and repaired. On the opposite side of the spectrum dwells perhaps the most common approach, that is to replace the components in question in an ascending order of cost---least to most expensive. Following this method you would order a new speed sensor at a cost of between $30.00 to $125.00, install said component and hope for the best. I have experienced recent symptoms recently in two of my Volvo fleet (both 2000 v-70's) and found that in each case it was the ABS module. In both cases, I was able to secure a good used one from a u-picket yard for about $35.00. You have to make sure that you are getting an exact numbers match to the one you are replacing.