First gear selection problem - help!

[billjam]

I have just cross-posted this thread here from the dark side as there might be more Pelicanites here who are aware of the solution. This isn't strictly a turbo forum issue, although we are dealing with a 930 4 speed tranny ...

I am about to embark on a project to change 2nd, 3rd and 4th ratios in my 4 speed tranny, BUT, I have another problem I’d like to solve while I’m in there.

First gear is really hard to engage sometimes.
It’s a regular PITA at the lights or stop signs as I frantically try to grab first … clutch in / try first gear, no go / clutch out / clutch in again / look at pi$$ed off driver in mirror / try again / try second, no problem / try first again, no go / back to second and off we go, v slowly!
It might work fine for days and then catch me out right when I am next to some redneck in his monster truck or a little ol’ lady in her Toyota Yaris. As they head off down the road, I’m left behind playing with my knob

I initially thought it was just a synchro ring problem so last time I had the tranny out, I replaced the synchro ring (41) only. It looked a little worn but serviceable, so I changed it anyway. I couldn’t see any other obvious wear problems on dog teeth on 1st gear (48) or the brake band and blocks (38, 47, 47a) so I reused them all. End result was no change.

I’ll try to explain how the gear-change feels so that hopefully someone here will recognise the symptoms and point me to the problem.
The problem occurs about 30% of times when trying to select first, usually with car stopped.
Most times, it slides straight into gear, but sometimes the gear lever just seems to come up against something solid. If I push really hard it sometimes goes in, but I can feel the shift mechanism flexing under the load. Its almost like the slider (45) is blocked from moving and I can visualise the fork and selector shaft bending – not nice!
With the clutch in and pushing the lever into 1st, if I hold pressure against the stick and ever so gently let out the clutch, just as the clutch starts to bite and things start turning in the tranny, the gear will engage.
Changing down from second into first on the move usually works OK. However, for a while I’ve been double de-clutching for each change into first and it has no beneficial effect. Sometimes works, sometimes not.

So, over to you – what’s the problem?
Which bits do I need to order?

By the way, which parts should I replace when I fit my new 2nd, 3rd and 4th gears?
I am installing 2nd 17/26 (1.53:1), 3rd 26/27 (1.04:1), 4th 30/23 (0.77:1).

[docrodg]

Normally that would be a synchro issue, but you replaced that. I would take a look at the stop, guide sleeve, needle bearing, and clearances on all parts as well as the shift fork. Sounds like it may be the gear is moving around on the shaft or the synchro having too much play on the fork.

[matt demaria]

Typically for the Porsche gearbox especially when a lot of miles are on the internals, the synchro sliders and the dog teeth wear. Instead of having a sharp triangular edge on each tooth, usage will wear off the edge. At first it will round off the edge of the tooth. In advanced wear state a flat edge (sometimes with a burr) will be produced on both the slider and the dog tooth. When the gear is engaged two flat edges will butt up against each other and stop the gear engagement.
The sharp edges are meant to force the dog teeth and the synchro slider teeth into alignment (and engagement). Once they are worn away, the design feature is lost. In Porsche transmissions these are considered consumable items. They are meant to be thrown away at every rebuild like old spark plugs. That is why rebuilding these transmissions are so expensive.

[billjam]

I know exactly what you mean Matt.
I have rebuilt a few boxes before and I am aware of the potential problem with wear on dog teeth and slider.
Last time I had this tranny out (when I replaced the synchro ring six months ago), I carefully checked these parts and they looked pretty good so I left them.
I may just replace them this time around regardless.

BTW, in this thread's twin on the turbo forum, there have been a large number of 930 drivers reporting exactly the same problem.

I have also found the following service bulletin which may be relevant. My car is an '86 model. Porsche introduced this fix in '87.