T56 - Shifting to 3rd issue
Any ideas? I took it to a trans shop for a free diagnosis and obviously that only covers them shifting it and nothing else because then they wanted $750 just to drop the trans and nothing else. So I figured I'd hit the boards first maybe get any ideas.
The teeth on the 3rd gear
The blocker ring
The 3/4 synchro
It sounds to me that you are able to shift into 3rd when you are getting the speed synchronized manually. So it might be your blocker ring.

