[T3] Transmisison fun

[Soren Jacobsen]

Here's a new failure mode for me!  Thought you all might enjoy a bit of
schadenfreude and information about a strange design quirk of the manual
transmissions we're cruising around with.

While on a lengthy out of state trip, I hear a strange clicking/grinding
type noise from the rear of the car.  It only happens once, and I'm
still happily cruising along the highway.  No cause for immediate
concern, just something to note.

An hour or so later, I stop at a riverside park to use the bathroom.
It's the boonies, so I've left the car running, and I hop back in
quickly, throw it in reverse, release the clutch, but...nothing happens.
This isn't an "oh, you probably have a mis-adjusted shifter and hit 2nd
instead of reverse" type thing -- this has been my main car for 10+
years and I am very much in tune with the feel of the shifter.  It's
right where it should be.  No, this is a "you engaged the reverse gear
selector fork, there just aren't any gears engaging" thing.  I'm on
level ground, thankfully, so I push the car a short distance and drive
the remaining 7 hours home.  Every once in a while, I hear that same
noise.  It troubles me, but I'm tired and the car is rolling smoothly,
so I keep going.

Back at home, I start tearing stuff apart to see what's up.  It doesn't
take long for something...interesting...to pop up, or should I say out.
When I pull the engine, the input shaft to the transmission comes with
it.  Shoving it forward with some pliers, I manage to get the engine out
and then start freaking out.  Seems like a big deal, right?

Well, it turns out that the culprit here is something very simple: a
stud that sometimes, if you're unlucky like me, can work its way out.

The input shaft / main shaft is two pieces held together by a short
stud.  On top of them goes a reverse gear, which, if things work loose,
can slide out of its proper location.  This was what I was observing
when driving.  The reverse gear had slid out far enough that it couldn't
engage, but everything else was still together enough to keep the
overall transmission operational.

The problem, on a deeper level, is this: the stud holding these two
shaft pieces together is too short.  On top of that, the shafts have
chamfered edges leading up to the threads (if you've got an alignment
tool made out of an input shaft, take a look at it and you'll see what I
mean).  This means that you've got a very short section of threads
actually engaging, and enough room for the stud to rotate to one end or
the other.  Why does this only rarely happen?  I'm not smart enough to
answer that.  What I do know is that VW later fixed this issue not by
lengthening the thread, but by introducing a stud with a shoulder in the
middle.  I'm not exactly sure when this fancy stud was introduced, but
my impression is that it happened during the water cooled era.

Anyway, I guess the moral of the story is that cars are difficult to
design, and if your input shaft falls out it's probably not as big of a
deal as you think.

Soren