Fimbultyr

Helping my friend with his boat with a sbc with trans2 and trs.
the problem is the reverse wont work.
forward and neutral works fine.
The gearchanger works fine so something is wrong inside.
we will get the tranny out of the boat this weekend.
where do a start looking?
serched the whole internet
but no one seems to have the same issue?

please help
best regards from the cold north /Sweden