Wondering why this suddenly decided to snap? I'm a fairly new C4 Corvette owner I have a 1985 C4 with the super t10 or doug nash 4+3 transmission and when I got the car I replaced the clutch and everything was fine it never drove weird and was easy to get in and out of gear. I didn't hear a noise while driving but suddenly my 1st and 2nd gear seemingly disappeared so I managed to get home with only 3rd and 4th since I was close and didn't wanna tow it home got it up in the air and took out the plate covering the transmission forks and then I saw the issue right away. Any ideas what may have caused this?

 

Just age and usage is what caused it.

 

Only has 119k original miles that seems a bit ridiculous for them to break with such low mileage and usage but age makes more sense.

 

Everything is subject to breakage and does not always require a "reason". Mileage has nothing to do with breakage. Wear, yes, but not breakage. To use your logic, there wouldn't be a car in a workshop for breakage repair until they had 200,000 miles.

 

So long as they're treated right and regularly had fluid changes yeah, unless its a chrysler product.

 

Your issue could've very easily be 'self inflicted' if the linkage wasn't correctly aligned. There's also some small parts used in the cover w/shafts that are important some are actually selective I believe also. The shaft failures I didn't think were thought to be weak links but yours is 'busted'. It ain't a 'busted' fork.

 

Yeah I wasn't quite sure the right terminology to use as the only name I found was "shift lever" but it's possible whoever owned it before had something done to it as I only messed with the clutch and nothing else.

 

This exact same thing happened to me. I was visiting my daughter in Northern Ontario, 600 miles from home, and I was out impressing my grandson with the exact same Corvette as yours. I did a nice little power shift for him and thought nothing of it. I was on my way home the next day, and as I was passing through Timmins, the shifter arm let go, luckily in neutral. I had to go about 500 miles using 3rd, 4th and OD only. It wasn't fun. I found the exact same condition that you had, and I blame it on the POS shifter. Unlike a good Hurst shifter, it has no stop bolts to prevent overtravel of the linkage on a hard shift, so you pays your money and takes your chances.
The tough part was actually fixing it. In Canada, I couldn't get the shifter arm from any GM dealer, so I ended up getting one from Ebay, from a 70's muscle car with a Super T-10. Two problems. First, the flats on the shaft do not match the existing flats. The angle is all wrong. So, I had to make a new shifter arm from a piece of 1/4" thick, 1" wide steel stock, carefully cutting, drilling and doing a lot of filing by hand. It took me 4 tries before I got it right. There cannot be ANY play whatsoever, or else the transmission will simply lock itself into first gear and that's the end of that. You have to lift up the car and remove the linkage and shift it with a wrench. So, if you go this way, be very accurate with what you make. The other problem is that there is no flat spot for the overdrive enabling switch. I got around that with a 12 volt proximity switch. (I'm a retired industrial electrician.) I spliced the contacts into the existing switch contact in parallel, and set it so that it detects the shifter in 4th gear. I took the 12 volt power supply from the radio feed. There's very little current draw. I've always thought that the 4+3 feature is a bit of a gimmick, and I'm good with a simple 5th gear overdrive only. Hopefully, you'll have better luck than I had at the time, and you'll be able to find the proper shaft.
Don't forget to adjust the linkage when you're all done. I made a nice adjustment tool shown in the picture.


So, no more power shifts for me. I'm being very gentle with this transmission, due to the crappy shifter. Don't get me wrong... There's lots of ability to have a good spirited drive, but you need to recognize that it's a crappy shifter, and breaking a shaft is all too easy with a bit of misuse. And it may not even be you, but a previous owner that weakened that 1-2 shaft.
BTW, I refilled the transmission with Lucas synthetic 75W90 oil. It's never shifted better! And put it in the differential while your under the car. It's got "friction modifiers" that make the posi work perfectly.
Good luck!