On my last camino in february/march, the detours started after granje de moreruela (n-630). Between the n-630 and n-631 up to n-525, there are extensive AVE construction going on. I am not at home now, but as i recall, the detours start when you are walking from granje de moreruela on your way to tabara. I believe you will pass through a pueblo, fontanillas de castro, on your way. The waymarkings at this point are quite patchy. I got lost here. At another intersection, there are yellow arrows pointing in 2 directions. If you are walking during working hours ask anyone on the construction site. As you approach tabara, about 2-3 kilometers, look for the temporary yellow arrow. At this point it does not matter as you can see tabara ahead of you.
As you leave tabara for santa marta de croya, there will be detuors again. I believe you can walk to a pueblo called litos. Actually litos is not on the route. I got lost again. The guidebook recommends to go through either berciano or another pueblo. You will see many temporary yellow arrow signs. Unfortunately there are many other areas that are not sign posted, or the posters blew off the post.
I will be backed home in london on monday, give me a shout if i can be of assistance.
Que tengas un buen camino y que dios los bendiga.
Ps - credential pelegrino: by this stage you may run out of space for your sellos, you can get one at the municipal albergue (the cafe-bar) in granje de moreruela. I was told the albergue is very cold by some peregrinos just for your info. I believe it is the only lodging in town.