I have an ATX, and my 4th gear stopped working!
Kevin Airth had a problem with his 4th gear not working in his automatic transmission SHO, and found the following solution :
A big thanks to Doug Lewis and Micheal Anderson! My 93 ATX had lost 4th. gear but otherwise worked OK. Micheal posted the same problem and Doug was able to figure it out. The C-clip on the O/D servo had broken, preventing the transmission from shifting into 4th. gear. The O/D servo cover is on the drivers side of the transmission on the firewall side,down low by the steering rack. Remove the drivers side front tire and you can get to the 3-8 mm bolts on the cover. I removed the cover and the servo,found the broken clip laying inside the transmission. Replaced it for less than $1 and now have 4th gear back!