Ford transit automatic gearbox problem

Hello friends, I have a question to see if someone can help me. I have a Ford Transit 2.5D from the year 90 (in Autodata the model from 89 to 91 appears).
The problem is that the march behind the gearbox does not enter, accelerate and the van or flinch; And I would like to know what it could be, if it could be something internal (valve box, discs ...) or it is something external (some selonoid or some switch ...). When putting the selector lever on R, the back lights are lit.
All the best