915 won't go into first gear while moving
 

On my 86' Carrera I have a 915 gear box with the factory short shift option, and I can't get it into first gear when the car is in motion. If I start from second gear and bring it strait back up to first, with a double clutch, sometimes I can get it in but otherwise it grinds.

To avoid grinding the gears I don't go into first unless I'm stopped or barely moving. I have also read up on the issue and spoken to some knowledgeable friends and it seems to be systemic to the 915's.

The reason I pose the question is because I recently drove a friends 73' Carrera also with the 915 and you can get into first gear anytime, no grinding. This leads me to believe i have a problem with mine or it needs a rebuild.

Is this a problem with my 915 or do I need to learn to work with it?

Don't know how many miles on my 915 (if/what/when work was done to it) but I have 234k on the car.
I can get into first moving very smoothly when downshifting. I do match rpms to the tranny but no issue.

My 915 has just under 100k on it...

synchro's probably worn...normal wear/tear. You may have done this but check all of the linkage/clutch adjustments first...if that is all proper then time for some work ....and depending how the box has been used they do not last the same way the engines do.

My 86 915 does
 

My 86 915 goes in rolling at a reasonable speed for low RPM & first gear(traffic & stop sign lines). Shifts better warmed up. With my out old 915 was an exercise to get into 1st even sitting still. Make sure everything else is in good order(shifter bushings, clutch adjust, etc..) because 915 parts have gotten expen$ive. Lots of good 915 threads worth checking out. An inexpensive rebuilt unit will not be a good deal unless it is out of a wreck that you can verify the history. You are also limited to the 84-86 915 unless you want to cut your own sensor slot, change drive flanges, etc.... Best of luck.

Oh, hell yes. I did not even know my 81 could go into first gear when moving until I had it rebuilt.

You don't have to rebuild. Work around it. You can do this for a long time.

Bob's right "work around it" because I know now how much bearings and syncros cost for a refresh $1800.00+.

Blame the PO for how well the gearbox currently shifts; it's either good or not.

Does anyone still believe synchros aren't a wear item?

The double-clutch technique to overcome a balky synchro also does wonders for delaying gearbox rebuilding, but then everyone knows that, right?

This conversation would be less likely if this were a Chevy or an RS GT2 forum.

Sherwood

I do not want to try shifting in to first while the car is rolling. Sometimes i get lucky and it goes in, but I usually do not.
Mine rebuilt last year by THE man.

if you must go into first while rolling, try bumping the RPM's up to match the speed of 1st

As a rule you dont want to go into first anyway unless you are barely rolling. These things have more then enough torque in 2nd. The only time i ever even want to go into first is stop and go traffic and when I am rolling up to a red light, and it works fine at that speed

How fast are you going when you want to go into first ? anything faster than a crawl and you are looking at bigger issues than synchros I would think.

If you want to apply maximum torque toward forward motion, you will want to use the best gear to do that. If that's not the objective, then use the next up-gear and gradually apply throttle to reduce excessive load on the engine.

However, realize that attempting max torque at low rpm with a higher ratio gear will lug the engine (not good for engine life). To demonstrate, with a fully warmed up engine in 3rd gear, slow down to approx. 15 mph, then accelerate with full throttle.

As a rule, you want to downshift to a gear that brings the engine rpm into an ideal torque range. That's why Porsche added synchromesh to all the forward gears, and that includes first gear. It just so happened the PO wore it out by not downshifting properly and you're now the recipient of that shifting strategy.

Sherwood

Is it really possible to shift smoothly into first while rolling, without rev matching, in ANY older trans.? With both my old BMW ('76 2002) and my G-50 911, I rev. match and double-clutch to roll into first. If I don't do that, there's resistance in the gear lever. And both of those tranny's are in good condition (no problems going 1-2, etc.).
Never having driven a 915 car, I wonder if they are really all that different from manual gearboxes of that era, as if only in 915s downshifting into first requires finesse.

personally i dont see the need other than something may be wrong with the tranny. but then i had a very bad 3rd gear syncro in my 77. drove it like that for over 10yrs.

as for the 930? i take care of 1st. its way to quick in first so going down or up form 1st i am very careful. nail it in second then a slow shift to 3rd and hope the cops arent around.

i remember some guy on here complaining that he could not down shift into 1st below 20 mph. ???? or was it above. ????

I have a 915/70 trans in my 3.6 transplant with about 30k on it. It shifts into first
like butter whenever I want to. (within reason). However, I rarely downshift into first except at a stop, as I have enough torque in 2nd to accelerate from a very low speed.
Sounds like the OPs 915 1st gear syncro system is toast!

As a precaution I almost always double clutch and never try to engage first unless at a near stop. Can a fresh transmission be expected to shift down into first at 20mph?

From wikipedia
Manual transmission - Wikipedia, the free encyclopedia
<<
Synchromesh

Synchronizer rings
If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or synchromesh, which consists of a cone clutch and blocking ring. Before the teeth can engage, the cone clutch engages first, which brings the selector and gear to the same speed using friction. Moreover, until synchronization occurs, the teeth are prevented from making contact, because further motion of the selector is prevented by a blocker (or baulk) ring. When synchronization occurs, friction on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves and notches that allow further passage of the selector which brings the teeth together. Of course, the exact design of the synchronizer varies from manufacturer to manufacturer.

The synchronizer[3] has to overcome the momentum of the entire input shaft and clutch disk when it is changing shaft rpm to match the new gear ratio. It can be abused by exposure to the momentum and power of the engine itself, which is what happens when attempts are made to select a gear without fully disengaging the clutch. This causes extra wear on the rings and sleeves, reducing their service life. When an experimenting driver tries to "match the revs" on a synchronized transmission and force it into gear without using the clutch, the synchronizer will make up for any discrepancy in RPM. The success in engaging the gear without clutching can deceive the driver into thinking that the RPM of the layshaft and transmission were actually exactly matched. Nevertheless, approximate rev. matching with clutching can decrease the general change between layshaft and transmission and decrease synchro wear.
>>

2002 Bimmers used Porsche style synchronizers until about 1971, thus, I imagine they have the same user complaints. Thereafter, BMW switched over to the more robust Borg-Warner synchro. Porsche eventually switched over with the G-50 in '87.
BMW 2002 Transmission FAQ

FWIW, thumbs up or down self surveys numbering one sample are useless as data to diagnose another person's issue. When someone offers that POV, I wonder if they're offering their unit for sale or trade - being in great shape as it is, or is it just a public announcement they personally have no issues with theirs. Either way, it's questionable information for the orig. poster or for subsequent discussion. It's like saying, "Oh, you have a flat tire? My tire is fine - going strong for 30,000 miles.

Sherwood

I would think so but I have never done it on mine (I don't think going that fast). There is no reason. I just tried it a few times after the rebuild and it worked when slowing down at a stop sign. I said "Sweet" and then never did it again. Brake pads are way cheaper.

Very common, same in my '86. I've gotten used to only going into first while stopped, and always go down into 2nd before going up into first (while in neutral with the clutch out at a stoplight, the trans will be turning, so going down into 2nd aligns the gears before going up into 1st... or something like that!).

^^ +1 I thought I was the only one doing this work around. Same on my 86, glad I am not alone.