Won't shift into second above 6000 rpm
 

Title says it all, shifts great to any gear at high rpm UNLESS its from first to second. First to second above 6000 rpms GRINDS like no other. Car is an '85 GS, bone stock.

Sounds like a worn out synchro. The 2nd gear synchro is usually the first to go. You can replace the synchro (lots of work) or you can just double-clutch.

I'm got the tranni filled with a 1/3 lucas synthetic friction modified and the rest with normal fluid. Think if I change it all to Lucas or an equvilent it won't grind?

I don't imagine that would help much.

What the synchro does is it matches the transmission speed with the engine speed while you shift, so that the dog teeth mesh smoothly when you go into gear. If the transmission is old or has been abused by the driver (moving the gear lever too fast and not giving the synchros time to do their job) then the synchros will likely be worn down and won't work properly.

What they did in the olden days before synchros came along (and sometimes in modern day endurance races) was double-clutch when shifting, which means manually matching the transmission speed to the engine speed. When you go to shift, you put in the clutch and move the lever out of gear and into neutral as you normally would, but instead of going into the next gear, you let out on the clutch and match the revs then kick in the clutch again and go into your desired gear. It might sound weird, but it's not too hard if you practice. If you do it properly the transmission won't grind when you change gears.

Sounds like what I'll have to do from now on, sounds a little slow in a drag or autocross event though am I right?

This is from "How to hotrod and race your datsun": Use 50% gear lube and 50% ATF. The detergents in the ATF help to free up sticky synchros, which tend to get clogged with miscellaneous shit after a couple of decades.

I tried this with my 73 Z that was doing the same thing (also 2nd gear). Turned my tranney into a brand new one within a hundred miles or so. I changed out the fluid at about 200 miles and it was black with all the goop it had cleared up. Replaced it with the same mixture and put around 70K on it with no issues.

I highly recommend this for anybody dealing with a twenty year old tranney. And if it doesn't work, you're not out much $$$$ for having tried it.... Good luck man!

Rusty is correct, on both counts, the synchros have taken a beating somehow, and are FUBAR, the only other thing that coulD cause a problem in there is if the shift forks got bent from misshifting, but this useually casues the tranny to totally go bannanas and it falls into a dead spot around 3rd.

Kenn

Sounds like I'll have to try the ATF and fluid trick. If it doesnt work, double-clutch here I come.

dont double clutch
 

youll do more damadge than good and changing the synchros is not that hard just a little time consuming. and for the love of rotaries dont put fluid that is not made for your tranny youll just do more damadge jimmy rigging it than taking the time to do it right, bottom line dude the part is cheep but the job takes some time. if you love your ride do it right. and the gears are called sprag not dog. :withstupi

I believe you are confused. A sprag is a cam or figure eight shaped piece that is used in a sprag clutch, which is a one-way clutch found in an automatic transmission.

In a manual transmission there is a dog clutch that slides on the main shaft with male teeth that engage with female teeth on the dog clutch hub. Hence why they are called dog teeth.

Let me clarify myself. Double-clutching on a synchromesh transmission normally would indeed be a stupid thing to do as it causes extra wear on the synchronizers, but when the synchronizer in question is already worn out, it doesn't make a difference.

Hmmm, from my understanding double clutching has no ill side effects. It is simply a case of doing the synchro's job for them in a manual fashion. Once you match the revs, the synchros have nothing to synchronize. This should mean zero stress on the synchros (unless they worry about job security). I may be wrong, but this is how it was explained to me......If I am way off base, I would welcome an education in this matter for my own personal knowledge. I currently double clutch when downshifting at high rpms in order to reduce the strain on the synchros, so I hope I'm not on the wrong track here...:)

Willkill12a, the ATF trick comes from a reputed source and was used by both of the top Datsun racers at the time. Not sure what kind of differences there might be between datsun and mazda trannys, but I'm pretty confident of my information and my own personal results. If the synchro is shot, it will not help. If the synchro is suffering from buildup, then it will. So, in this instance, I have stated a verifiable fact while you have stated an opinion as if it were a fact. Opinions are great, but should not be confused with (or presented as) fact. Otherwise, its always nice to hear input from people's own experiences...

BinaryRotary; if you try this, please post back on the results. Good luck!

So double clutching its not Just some The Fast and the furious BUllCrap!?
Remember?**not double clutching like you should**
sorry But i had to ask this B/C i'm having problems w/ my second gear also :(

As for the "ATF trick", I fully support that it will help clean everything inside a tranny. Yes, it is thinner in viscosity than the reccomended fluid, but remember, newer manual shift tranny's use ATF alone as lube. Thinner fluid will circulate into the small areas easier, and flow faster. The extra solvents in ATF are designed to keep a transmission clean.
Using a 50/50 mix of 90W and ATF will not harm any components inside the tranny under most conditions. Especially when used as Kentetsu suggested. A long enduance race, in extreme heat, could result in possible damage with this mixture, but would likely occur even with a full 90W lube. Hence, the reason for fluid circulators used for transmissions and gearboxs for these purposes.
Double clutching will not result in excessive wear, when done correctly. Afterall, those large trucks running all over the country have not a single syncro in the tranny, and must be double clutched to shift. (although most old school truckers shift without the clutch)
Second gear is usually the one that gets the worst punishment from hard shifts. The slight delay from moving the shifter lever over to the next shift arm usually helps prevent 3rd from getting quite the same wear, and since 3rd will suffice for most any hiway onramp, 4th doesn't get hit hard nearly as often. At least, that's my theory.

Rogue, when I said that it causes unnecessary wear I was referring to a synchromesh gearbox, not a sliding-mesh gearbox. I realize that big-rigs and whatnot still double clutch since they don't have synchros (just one more thing on a commercial vehicle to wear down, right?)

Kentetsu, I am not entirely sure why, I've just seen that it's the general consensus among techs that it is not recommended that you double-de-clutch with a synchromesh tranny as this can cause undue wear on the synchro rings. I'll look and see if I can find more info on that.

TurboSE, yeah, the TFTF reference to double clutching made no sense at all, though it is a legitimate technique (if you can call it that). If your synchros are good you don't need to double clutch or double-de-clutch. Of course, standard heel + toe is necessary on downshifts, but you don't have to rev-match the transmission.

One other thing that should be mentioned, if your synchros are tired but not necessarily gone, you can usually avoid grinding by simply hesitating a bit longer in between gears. Though that will only work for a while until your synchro finally gives up altogether.

Double clutching will not hurt ANY tranny. All it does is take the work off the synchros, and lets the tranny and engine synchronise. It takes a lil longer, but it does work. I believe it also extends the life of the clutch. I have driven ALOT of vehicles....and dont usually double clutch....tow truck, fire truck, 2.5 ton army truck, semi. Heck, dont even use the clutch half the time...even on my motorcycle. BUT....back to topic. If you double clutch....you can really extend the life of your clutch and tranny synchro's

Hmmmm Interesting!my dad use to have this diesel BUS and i remember .....Back in 95(i was 15) i only use 1st gear and then all the other shifts w/ no clutch(except downshift) But U have to do it @ certain RPM's......U think i can do this w/ my 7? I've tryed doing it on other cars But it doesnt feel that easy

Double clutching on a syncromesh tranny just forces you to shift slower, which will usually helps to eliminate grinding on a tired unit.
I really don't reccomend "floating" the gears (no clutch shifting). Really hard on a transmission when not done perfectly.

Thanks !
I guess i have to look 4 another transmission..............It grinds all the time in second(if i dont let it rev down to like 1500 rpms or wait a couple seconds=(

thats what i was just gonna say. If your having transmission trouble...and dont know how to float a gear....your gonna do alot more damage than you have now. Just replace the synchro and youll be better