93 GLX 4dr AT

[joli]

My son got a bargain on a 93 GLX 4 DR- AT & with AT problem. 128k

very clean nice car
but only goes forward - seems to slip partially between N & D or N & R no real Park or Reverse.

If you accelerate when in P/R/N it goes forward some but a slipping feel.
linkage is working under hood area.
fluid is full

so are most of the tranny problems internal ?
Is there any computer or electronic thing to check before pulling the whole thing out?

Is it usually the same problems with most of these trannys?
It was suggested the hydrolic? piston thing inside tranny is malfunctioning.???
any thoughts out there?