915 won't go into reverse

[cowtown]

Hi Everyone,
I had a hard blast around the farm roads a couple weeks ago, and then the car sat until today. I went out to the coffeeshop this morning, and everything was great...except when I tried to back out of my parking space, I could not get the shifter into reverse. Pushed the car back, then got home fine, with gears 1-5 working normally.

Here are the specifics:

-It's an aluminum case 915/61 ('81, I think) installed in a '75 911S with a '77 930 engine. I purchased the 915 as a rebuilt unit about a year ago (new synchros, dogteeth, sliders, etc.) from a person in Socal. It's been problem-free until now.

-All shift bushings are basically new and check out OK

-Factory short-shift kit has been installed and in use for about 2 years. Shift tower is bolted down tight.

-I spent about an hour trying to adjust the linkage. I did get it to go into the reverse position once, but then when I selected any forward gear, it was stuck in two gears (I think) and wouldn't move. After shifting it around some more, I was able to get it into first and move the car again.

Here's what I've done so far:
-Drained the fluid. There were no metal pieces in the oil, just a normal amount of shavings on the drain plug's magnet.

-Pulled off the bottom inspection plate. The 1-2 and 2-3 rails (is that what they're called?) seem lined up OK (see pic). The static fork was loose, so I reinstalled the three studs with loctite and used new nuts and CV-style lockwashers. However, this made no change in getting into reverse.

-Unbolted the backup light switch, and the reverse pin moves freely in and out when I push it.

I've parked the car (no oil in the trans) and now need to decide what to do. Before I resign myself to pulling the transmission and taking off the nose cap, is there anything else I can try to diagnose this problem?

Though this "feels" like a linkage adjustment issue, I just can't seem to fix it by adjusting the linkage. And gears 1-5 all work just fine.

I'm stumped.

Thanks for your advice,
Colin

[Jim Sims]

With the shift coupler disconnected from the horizontal shifter rod (in the tunnel) can you shift the transmission through the gears without it locking up? If not, I would suspect that a detent was not properly re-installed during the rebuild. Are all the detent hole plugs in place on the the exterior of the transmission? Jim

[cowtown]

Thanks for the reply, Jim. I cannot "bench shift" the transmission into R by pushing it without the shifter attached. However, I have a hard time pushing it into any of the forward gears in this manner as well, so I'm not sure if it is conclusive.

I don't know what you mean by "detent hole plugs." Can you tell me where to look, and I will go out and check?

Thanks Jim,
Colin

[Jim Sims]

With the horizontal shift rod (the one going forward to the shifter) disconnected and the shift coupler still attached to the shift rod going into the front of the transmission one should be able to go into all the gears (if all is well with the transmission) with the clutch pedal depressed. Use the shift coupler as a handle.

Detent plugs:

On passenger side: 2 with threaded plugs on side and lower down and one higher up (vertical hole with tap in force fit type plug - right under black wire in below image).

[ChrisBennet]

Reverse is overrated anyway...

[SpeedracerIndy]

That is exactly how my transmission acted when the static shift fork (as you called it) was loose. It happened after a few really hard 1-2 shifts. Mine would also get stuck into two gears at once though. After rebuilding the tranny myself, I don't do hard 1-2 shifts anymore.

[Tim K]

I had the same trouble once. I the little metal "gate" on the side of the shifter rod was bent in such that there was not enough room for the tab on the shifter housing to fit between the "gate" and the rod. It would just hit on the end of it and would not slide between. Just lift up the shifter boot and take a look.
I suppose that the shift knob may have been bumped while parked causing the bent gate.
But as Jim suggested, if the transmission is the problem, you can eliminate the shifter from the equation by disconnecting the coupler and shifting it from there.

Tim K

[cowtown]

Originally posted by Jim Sims
With the horizontal shift rod (the one going forward to the shifter) disconnected and the shift coupler still attached to the shift rod going into the front of the transmission one should be able to go into all the gears (if all is well with the transmission) with the clutch pedal depressed. Use the shift coupler as a handle.

Detent plugs:

On passenger side: 2 with threaded plugs on side and lower down and one higher up (vertical hole with tap in force fit type plug - right under black wire in below image).

I crawled under there again tonight, and verified that the 3 plugs you pointed out are in place. They look just like the ones in your picture.

Shifting with the linkage disconnected, I can get into gears 1-5, but not Reverse (the position closest to the driver when moving the shift coupler by hand). When trying for Reverse, the selector goes about 1/4 of the way in, then runs up against a very solid feeling stop.

So...what's next?

I'm guessing transmission removal is coming up, but I'm more than happy to try anything else you can think of!

Thanks for the help,
Colin

[Jim Sims]

There are a couple of more detents (3-4 and 5-R) under an additional "drive in plug" located on the passenger side near the joint between the gear housing and the front nose cover. One can see part of the plug just above the black cable in the below image.



I suspect that the detents are not the problem but instead a bent or loose 5th/reverse shift fork or a worn or damaged feature on one of the shift fork shafts. Hitting the hard stop going into reverse is what makes me suspect the bent/loose shift fork. perhaps John Walker will render an opinion. Jim

[john walker's workshop]

from the famous 915 rebuild thread by rcecale, here's a pic of a 915 w/reverse engaged. too bad it's not from the other side so you could see the tang on the main shift shaft engaged in the notch in the 5th/rev shift rod. as you can see, there's not much that could get in the way of engaging reverse. even if the teeth broke off, the shifting operation wouldn't necessarily be impared. something bent, or distorted, like the tang on the main shift rod, would be my guess. you've eliminated the outside issues by disconnecting the coupler and shifting it by hand. remember, the teeth on the reverse idler gear and on the shift sleeve need to meet tooth to groove, or it won't go into gear.

[cowtown]

Jim and John,
Thank you very much. I will check the remaining plugs just to be sure. The 915 is still a big black box to me. I need go back and re-read Randy's thread tonight.

If I understand your replies, you're saying:
-the big straight-cut gear in Randy's picture is Reverse
-it's likely that the bronze-colored shift fork toward the bottom of Randy's picture is either bent, or something in that area is not sliding correctly

To diagnose this, can I just pull the engine/transmission, leave them mated together, and pull off the "nose" cover of the transmission?

[Jim Sims]

Yes and you can buy the replacement paper gasket needed (between the front cover and the gear housing) individually for about a $1.

PN info: dumb part number question - trans + pelican catalog

Jim

[john walker's workshop]

the large straight cut gear is the 5th /reverse shift sleeve. it engages the reverse idler, which is constantly in mesh with the small bevel cut gear on the mainshaft, thus reversing the direction of the pinion shaft. just the end cover needs to come off to see this. i'm still trying to imagine what would cause this problem. maybe i'll get a brain flash later on. it doesn't seem like a detent problem, because the two inner shafts are in the neutral podition, and you have 1 thru 5. to get 5th, the tang on the main shift rod needs to enter the notch on the 5th/rev shift rod, and pull the rod inward, (outward for reverse), so all that seems to be in order. wierd.

[john walker's workshop]

like i mentioned, reverse won't engage until the teeth on the slider and the teeth on the idler are lined up tooth to groove. tooth to tooth will not engage. that's why in normal operation, you can't get reverse on the first try sometimes, until you turn the mainshaft a bit by letting out the clutch for a moment, then trying again to get into reverse. so when you tried to shift into reverse after disconnecting the coupler, and failed, maybe it was hitting tooth to tooth. a long shot maybe. try rolling the car a bit while you try it again with the coupler off.

[wevoid]

Colin,

Like John, no exact reason jumps to mind for this issue.

When you mentioned that you "bench shifted" it - and the forwards gears were hard to select, this is normal - synchros are quite stiff to overcome.

Try again to get reverse, it will be with the main shift rod (of the transmission) rotated all the way counter-clockwise - viewed from rear of car, then pulled forwards.

As there is no synchro ring on reverse, at this point you are only overcoming the friction of the shift rail assembly.

You can even try this through the bottom cover hole, as the 5th /rev selection can be achieved without the bottom cover installed. The main shift rod will travel about 11 - 13mm to fully select reverse. Again - full counter clockwise rotation and push forwards on the main shift rod.

Seeing that you have a 930 motor with a 915 trans, my suspicion is that you have a drive tooth broken off one gear and it has found it's way through the oil bleed port for the front end of the 5th reverse rail and is now blocking the travel of the 5th/rev shift rail. With the shift rail in the reverse position, there is virtually no additional free travel. Any blockage in there will be a loss of travel into reverse.

If that's the case - and you become really lucky, a small telescopic magnet might lure it out. The oil port faces downwards, try with the transmission in 5th gear to give you best chance.

At very least, I would use a magnet and go fishing through the bottom cover towards the front end of the case, as there are a couple of places where a larger piece of debris will block the travel of the 5th/rev shift rail assembly.

I have seen a front cover with a whole pinion gear tooth sitting in there, it was blocking the shift action.

Regards

Hayden

[cowtown]

Tried rolling the car around, and it didn't make a difference in getting into 5th. The additional detent plugs all look OK.

Hayden, I'll definitely try going fishing with a magnet, and try shifting it from underneath as you suggested. It doesn't look like I have a lot of other options before pulling the unit out.

It's probably going to be this weekend or the following weekend before I have time to do any more. Thanks a lot for all the help, and I will post back with what I find.

[cowtown]

Hi Everyone.
I finally got a few hours free and pulled the engine/trans this morning. The good news is that nothing appears obviously broken.

The bad news is that something just looks - and pardon my use of technical language here - out of whack. The gears don't seem lined up the way they should be.

With the nose cover off, I can move the selector into four (?) positions, as shown below.

Pictures (after the diagram)
1) Fifth gear engaged, but with the selector travelling too far and part #16 "Sliding Gear, 5th and reverse" touching the slant-cut gear of part #19,
2) Reverse "sort of" engaged
3) Reverse "sort of" engaged, but in a different way
4) Neutral. I can rotate the rod and select other gears from this position.

I hope this post makes some sense. Is this possibly a slipped/out of adjustment 5/R selector fork (#15)? What should I do next?

Thanks!
Colin

[john walker's workshop]

top pic is 5th engaged, next one is out farther than it should be in reverse, 3rd is normal reverse, 4th neutral. is the shift rod guide fork plate back on for this test? is the tang on the main shift rod engaged into the notch in the 5th/reverse rod for all the pics? you can pull the 5th/rev rod out farther if the tang isn't limiting it's travel. that's how they are removed.

[cowtown]

John,
The shift rod fork plate is back on in this test. But, I did remove and replace it a few days ago, because the fork was very loose. Does it need to be set up in a certain way?

The tang on the shift rod was engaged for all 4 pictures. I pulled the rod out as far as it would go (with the tang engaged in the notch) - shown in 2nd pic. You're saying it should come out even further than that?

Thanks very much for the reply.
Colin

[john walker's workshop]

no, the 3rd pic is normal engagement. they don't mate up 100%. so now you're getting it into reverse. what's different now? put the end cover on with a couple of nuts and see if it goes in.