I've got an NB2 6 speed in my 93 coupled to a BP4W and I've started to have an odd problem. Under heavy braking, the shifter will not move left. It will move right just fine and go into 3rd-6th just fine but I physically cannot access the 1st and 2nd gates

Steps to recreate:
Get up to speed
Brake hard
Move shifter to neutral
Try to move shifter left
Problem occurs. Shifter will not move left to be able to access 1st and 2nd gear.

When I let off the brake, even slightly, the problem goes away and the shifter moves left like normal and I can continue my downshift.

Things I've done and checked:
Shifter bushing is only 6 months old and still in very good condition
Shifter itself has no noticeable wear or play
I pulled the shifter out and looked for any debris in the turret. All clean there
I packed the turret with some thick grease to hopefully quiet some rattling (the rattling has reduced)
The transmission only has around 120-150k miles and was allegedly not raced before I owned it
I put XT-M5-QS gear oil in it

Nothing has helped. Any ideas? It almost feels like a selector shaft is moving out of place under the braking force but I don't see how that's possible.

 

If you do this test in 2nd can you get to first under braking? Sounds like a broken detent spring or something, but you'd have to remove the trans to fix that. I'd be interested if the problem is still there with a different 6-speed.

 

Yep. 1st to 2nd and 2nd to first works just fine as long as I don't let the shifter return to center.

Which detent should I take a peek at? Just all of them? What would the actual problem with them be? Broken spring, bad ball, wallered out notch...? I'm pretty familiar with the insides of a 5 speed but not 6 speeds. Seems like they're considerably different between the two.

 

What is the condition of your engine mounts and differential mounts?

 

Diff mounts are about two years old and all poly, motor mounts are AWR and about a year old. I have no noticeable movement in the shifter under acceleration or braking while in gear.

 

Nothing, as far as I'm aware, is serviceable without further tear down of the trans, so it's not a "in car" sorta job. All the detents I've removed have been to remove tail shaft and main gear assembly to reseal the cases since they like to leak.

 

I wasn't expecting it to be, I'm just looking for a direction to look in. I was planning on pulling the gearbox out tomorrow and cracking it open to see if I can find anything. I did a successful rebuild (all synchros and bearings) of a 5 speed a while ago and then raced it for 2 years before moving to the 6 speed for strength. Are the 5 speeds and 6 speeds significantly different internally besides the obvious?

 

As far as I know, yes. Two different manufactures so beyond the bolt pattern, length, and shifter location, there's nothing similar about them. Do some google searches for the 6-speed service manual, it's floating around the internet somewhere. The tail housing is fairly easy to remove.