901 Transmission Rebuilt - problems with blocking

Hi,

I had my 901 transmission rebuilt by a very reputable shop that I've been going to for a number of years about 5,000 miles ago.

The problem is that I am getting blocking in 2nd and 3rd gear on a regular basis (both on upshifts and downshifts). Is this normal after 5,000 miles? My only experience with a newly rebuilt transmission is on a 741 for a 356, and that one was smooth as silk after 3,000 miles. For now, the mechanic is recommending replacing the gear oil with normal motor oil, drive it for 10 miles or so, and then replace with the correct gear oil. Seems like its worth a shot before we pull the trans again, but I'm not sure how effective this will be.

Any thoughts?

Thanks for your advice.

Mike

Mike, I assume by blocking you mean that it is difficult to slide into and between 2nd and 3rd gear. I suspect that there might be wear on the slider ring or dog tooth ring mounted to the free-spinning gear. If the front edges are worn or there is any "Mushrooming" of the slider from previous synchro wear, the two parts will be difficult to mesh. However, Usually the wear is asymmetric on the individual side wear on the slider, meaning it will be harder to get in either 2nd or 3rd.

I guess the answer depends on what parts were replaced or reused. I usually use a flat file on the slider ring to take off the mushroom edge if the part is not replaced.

If the problem is equally persistant in both 2nd and 3rd I consider two other options; a misaligned shift forks or improperly adjusted coupler. If you only have the problem in the 2/3 plane I might suspect the shift fork. It should be easy to drrain the oil and pull the selector fork guide and look at the alignment. The 2/3 fork will be the one in the center. All three forks should make a nice even lineup for the selector pin to engage.

I would guess that the shift coupler set screw is loosening up. Maybe the mechanic fogot the safety wire. Remove the access cover on the floor in front of the rear seats and shake that thaing all around!

Email Grady Clay or Matt Monson for advice.

JP is probably correct I would check to see if the slider was replaced as they wear on the teeth end but also where they slide and can get hard to move. They are expensive and sometimes are thought to be good enough...also the new syncro rings seem to be a little bigger and can make shifts seem a little harder it comes down to what was replaced during the rebuild.

All of the above are good possible solutions, another consideration is what oil is in the box now? Some oils are TOO slippery and don't allow for enough friction to let the synchro band work. 4th and 5th may not have been as affected if those parts weren't replaced. If easy cures like oil and shift linkage don't do it then it must be an improper shift fork/ shift rod adjustment.

I'd be interested in what was actually rebuilt. Something isn't lining up correctly. My experience is that with German engineering, if you use the correct pieces and they are in good shape they tend to work correctly the first time. And I'm always leery of a quick fix. Using an engine oil temporarily implies that a lighter viscosity oil will allow parts that are too tight to slide together with a heavier gear oil. It also implies you want to wear down something that fits incorrecly by using an improper oil. That would not be my strategy. If it's not working correctly, it may be smarter to review the work and not ruin an otherwise good (and relatively rare) gear box. Good luck.

Thanks for all of you advice guys - you've been incredibly helpful. I'll let everyone know what the issue turns out to be once I find out.

Mike