Auto gearbox problems

[stocky88]

I see many good cars here going cheap because the auto g/box is faulty.
There is a R19 Alize at the moment, in perfect condition for $150 the only problem is the auto will go in reverse, but not forward.
I have noted a lot of renaults suffer from the very same fault, even lagunas safranes etc. my question is, what is wrong causing that problem and how easy/expensive is it to fix.:?:
cheers

 

[stocky88]

Is the fix for this a national secret. a sort of ( if I tell you I would have to kill you ) type of thing, or does nobody really know.
I believe there are 2 soleniods in the g/box that are the usual culprits, can they be got to without removing the auto from the car?:!: