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Re: Gearbox
Posted by Joseph Galas (more from Joseph Galas) on Thu, 30 Aug 2001 23:16:25
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

I recently experienced the exact same problem with my '90 900. In order =
to get an idea as to what was causing it I disassembled a spare =
transmission I had laying around from an '88 900. From my examination of =
the parts I could see that it too, was soon headed for the same type of =
problem.

What I saw was a lot of wear on the forks and the grooves (that they =
ride in) on the synchro slider rings. That wear eventually causes the =
engagement of the splines to be reduced dramatically. The reason that =
the transmission pops out of gear only on deceleration  (in 2nd gear) is =
that the ends of the splines for 2nd gear do not have the conventional =
pointed (or triangular) shape that the other gears have. For some reason =
(maybe to help engagement) Saab decided to make the ends of the splines =
wedge shaped, so that there is a angle form one side of the spline to =
the other. This means that under acceleration the splines engage on the =
longer side but under deceleration the splines engage on the shorter =
side. In addition, the wedge shape helps push the slider away toward the =
neutral position.

I have repaired numerous American transmissions (both automatic and =
manual) and for the life of me can't understand Saabs reasoning for the =
design of their synchronizer assemblies. All other manual transmissions =
that I have worked on have detents on the shift fork shafts and in the =
synchronizer assemblies so that when a particular gear is engaged the =
detent for the fork helps keep the fork centered in the slider groove =
and the detent in the synchronizer keeps the slider engaged so that =
there is no wear on the faces of the two. Contact between them only =
occurs during engagement and disengagement.

The Saab synchro assemblies are designed so that the sliders are spring =
loaded against the forks all of the time. The only thing that keeps the =
assembly engaged is the detent on the fork shaft. That means that there =
will always be contact between the face of the fork and the groove in =
the slider ring causing substantial wear to the two over time =
(eventually resulting in the situation we are experiencing). The wedge =
shape on the end of the spline for 2nd gear makes the problem even worse =
as there is even less engagement on the deceleration side.

Unfortunately, the only way to fix the problem is to remove and rebuild =
the transmission or replace it. A used transmission of the same vintage =
may be well on it's way toward the same problem (as I've experienced =
with my spare '88 trans).

I will most certainly rebuild my original transmission because at least =
I know what I am starting with. I just need to find the time!

For all of the good designs that Saab has had... the design of their =
gearbox is certainly not one of them!

Good luck!

-Joe-
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