Hello everybody, first time poster here. I recently purchased a 1985 vette with the infamous 4+3 manual tranny. The OD appears to be working fine, and it shifts up through the gears fine, but I have a problem downshifting. Specifically, I have a problem downshifting from 3rd to 2nd. I have been shifting from 4th to 3rd to 1st to come to a stop. It almost seems like I am locked out of 2nd gear. The car shifts up fine through 2nd, with no grinding or other drama, but I am somewhat puzzled as to why it wont downshift.

Any ideas? Thanks in advance.

 

On the surface, it sounds like a worn synchro or a mal-adjusted clutch, But, It is also possible to have a worn trans side cover bushings or worn fork in the trans allowing the shifting fork to be slightly off center when shifting "backwards" 4-3-2-1.
I have a 4+3 in my 57 chevy truck, and it has never given me that problem.
I changed the side cover when I changed out the overdrive a few years back, and noticed the cover bushings were a little loose for the internal detent plate and the arms. Check that the detent ball springs are good.
you might just pull the cover off and look at the detent plates and see if they are worn, causing an excessive ramp in the 4-3-2-1 sequence.
Bear in mind that the fork arms are 1-2 and the other one is 3-4.
The only other thing that comes to mind is the shifter could be worn and not quite adjusted perfectly.
Jack it up, and take the shift rods off the trans with the nuts where they go into the trans and see if the shafts move so ever slightly with the shifter in neutral.
If when you put the shift arm back on each stub shaft and you have to "tension" the shifter to get the link arms back on, your shifter is not in neutral, but the trans is.
adjust one shift rod so the shift arms move on and off freely when you put the arms back on with a 1/4" drill bit or bolt in the shifter in neutral.
Don't forget to check reverse the same way.

 

My first feeling is the synchro's are worn which would not surprise me, and like above you can check for wear of forks. But before opening a can of worms trying to adjust shifter arms, get a good gear box oil and drain and fill the trans, and I am sure this will open a large discussion of who makes best oil. in the past before things like Royal Purple & mobil 1 were popular I had used standard gear oil with a tube of rear Posi additive.

While you are under the car working on the T-10 half of the trans. do a trans. service on the O-D unit, the ATF in the O-D unit tends to get Burnt and very dirty in a much shorter time compared to other Auto transmissions.