Grinding Second Gear Downshift - 2000 996 6 speed manual

Hi All, I've got a pretty serious grinding sound on my 3 -> 2 downshift, while driving. The higher the RPM, the more it grinds. It does not grind upshifting 1 -> 2. It does not grind in any other shift up or down while driving. The car shifts cleanly in all gears when running, parked, not driving. It does not "pop out" of second gear. I assume it is my second gear synchro going out, but I want to make sure I'm not missing something more obvious. Some background: The car is new to me, and I'm just getting it on the road, so I don't know if this is new behavior or not. When I had the engine out last month, I drained and replaced the fluid with exactly 2.7 liters of Motul 300 75w90. There was no indication of metal bits, flakes, or glitter in the gear oil that I drained out (neither steel nor brass). I upgraded motor mounts (Function First) and transmission mount (Rennline) last month. I put a Numeric shifter in the car, but am using original stock cables, installed at the same position that I marked upon removal. I have a new clutch/flywheel/pressure plate, arm, pivot, inout seal, etc. in the car. I flushed/bled the clutch line, and there isn't any air in the system. To reiterate, other than this one symptom, the car shifts great up and down in all gears, and up into 2nd fine. The kinds of things I think I need to consider based on reading RL and googling: Motor mounts/transmission mount Shifter alignment Fluid level Clutch bleeding Worn synchro Bent shift fork Motor and transmission mounts are brand new, so I don't suspect them at all. The Numeric "seems" aligned right, but I don't have any way to measure it. It is easy enough to adjust if it might help. Moving in the same direction, the 3 -> 4 shift doesn't have any issues, so I am not sure it makes sense to mess with it. I don't know if I should add more gear oil - I put in exactly 2.7 liters, but the way the factory manual is written, it seems like there may be an additional step to ensure it is filled correctly. I've got .3 liters of fluid that I can do this with. Anybody done this and seen a difference in the shifting (especially 1 -> 2)? With it not popping out of gear, I don't suspect the bent shift fork, but only a teardown would really tell me for sure. If it does end up being a synchro, any tips, tricks, advice, links, etc. for that project? I've torn down and rebuilt transmissions in the past on other cars (VWs), so I'm not especially scared of the work (not excited about it, but not dreading it). Any other things to fix while I have it apart? Thanks in advance.

I have heard that synthetic non-porsche 75w90 is too slippery for the synchros. The easy (if not inexpensive) option is to re-fill the case with Porsche 75w90.

I had the same problem. Shifts were all good except downshifting to 2nd. According internet experts my synchro for 2nd was shot. Drained and replaced the trans fluid and I freaked out because it was still grinding. After a few drives the grinding stopped. It's been a year and twenty thousand miles since and no grinding gear changes whatsoever, so I would assume the synchros being shot idea was rather incorrect.

Our 01's synchro was bald when we rebuilt our transmission. Not knowing anything about transmissions I didn't see the problem because it was so uniformly smooth, until he grabbed the new one to show me side by side It did not exhibit what is described here. There wasn't really grinding unless you let the clutch out too early, just hard to get it into 2nd. It was also going up, not down (could have been some going down, but not enough to make an impression). It was the worst when the car was cold and after a mile or 2 it would become much easier though still nothing like the rest of the gears. My suggestions would be to check the cables. There is one that likes to pop out of it's clip and makes shifting a challenge (forget which are the common gears). I'd also suggest going back to the factory shifter or at least have someone verify the Numeric is installed and adjusted correctly. As already mentioned, changing the fluid (and using the Porsche oil) is another good step. Doubt the mounts have anything to do with it. I've never heard of any shifting issues due to bad mounts anyway. Ours were terrible when we changed them and it had no effect on shifting.

Thanks all. I visually verified that the lower end of the cables are in place (I have them safety wired and it was intact). The Numeric is definitely installed correctly and functions well, but I can can easily mess with the adjustment of the cable ends to see if it has any impact. I kept the center console out until I get this all dialed in, so it is all readily accessible. The top ends are firmly in their spots, no popping out. The ends are firmly attached to the shifter (the numeric replaces that little black plastic retention pip on the left side one with a washer and screw). I'll verify the fluid level since that is easy. But it sounds like Porsche gear oil is in my future. That'll teach me to try to save a buck. Thanks again.

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I've got Liqui-Moly 75w-90 (Not Porsche, but I'm pretty sure it is Porsche certified) in mine, and it shifts perfectly... Gear oil too slippery? Hmmm... sounds weird to me.

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I disagree. I think the synthetic is an issue in this case.

Yes, our transmission is famous of being picky on oil. OP, have you checked the magnet inside the transmission drain hole (use your pinky). I suggest you fill it with genuine fluid until the oil drips out of the fill hole (with the trans being level). Should be ~2.7L. If that doesn't solve the problem, you can drain the oil and reuse it after transmission rebuild.

I had the exact same issue recently. All shifts were fine except for the 3 to 2 downshift at higher revs. Even in normal around town driving it was fine, but I don't do much of that. I chalked it up to abuse - lots of downshifting at higher revs really makes the synchro work hard. I had my indie open it up - not only was the synchro worn, but so were most of the gears. So, after a big repair bill, I now have a gearbox with mostly new gears and it works flawlessly. I also got him to put in a limited slip diff at the same time - you know, while you're in there

And that is precisely what I’m concerned about.

Until the O.P. is sure the correct Porsche gearbox oil (made by Shell) is used and to level, other options are moot. We've discussed the gearbox lube issue and no, sadly there is no alternative to the correct Porsche dealer supplied lube. Lots in Search "Markus"

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Thank you. I don't believe have a magnet in my early transmission (in my research, my understanding is that the magnet came in later evolutions of the transmission). However, there were no materials in the old fluid that I drained. I put in a later model magnetic plug, and there are no materials on the magnet. Exactly 2.7 liters didn't bring me to the point where the oil dripped out, so perhaps I'll just try to top it off to see if that matters. I'm hopeful but not optimistic. I'm running out of driving season, so this may end up being done in the spring. I need to drive it it recreate the behavior. Thanks!

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Yup, all options point to this as a likely culprit. I'm going to try topping it off with the Motul, and if I get more driving weather, try that before draining and refilling with the good Porsche labelled stuff. You say "to level" - what is your technique - fill until it starts to drip out the fill hole?

Personally, I wouldn't obsess over the oil level. Unlike an engine that relies on oil level to keep the pump pickup submerged, the transmission just churns the stuff around. Close to full is fine IMO. I suspect others here will strongly disagree but think about it.

As follow up, I had Motul 300, tried Redline MT90, and recently went to Porsche factory fluid. The redline resulted in the worst grinding. I don�t think the Porsche stuff is better than the Motul, about the same give or take (no way to quantify). I�m not looking to spend a ton more on it. Some good advice came my way about dropping the transmission, pulling the cover and replacing the 2nd gear synchro parts. I�m not eager to do this, although it is within my capacity - the parts are pricey (and I am driving my 2002 now, this one is for sale). On a post in another thread just now, some reference was made to the 2nd gear pop-out fix and that it might help with grinding. Anyone seen this? What is the relationship? How does that upgraded part work? As a reminder, the transmission upshifts great in all gears and downshifts great into every gear except 2nd (3rd to 2nd downshift). Edit: I put the stock shifter back in (moved the Numeric to my 2002) and it didn�t change anything.

http://www.gboxweb.com/detent.html

Thanks for the link. But I don�t have a pop out issue. Is this likely to help with the grinding?

Sorry, not sure about that. You may want to check with GBox or hopefully somebody else can answer it.

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I agree !! But for information purposes, the correct fill level is 2.7, that brings you to 10mm below the fill plug level, fashion an "L" shaped wire (read coathanger) to measure this distance. Obviously many people fill to the fill plug, and there seems to be no adverse effects in doing this overfill. I have to idea why the spec is 10mm below the fill plug. As for 2nd gear grinding it can occure for two reasons, one being the syncro not letting the slide engage the dog teeth on the loose 2nd gear, or the slide engaged, but then pops out slightly causing the grind.Sometimes it is hard to tell which is happening. 2nd gear gets the most abuse, and the factory admittently got the positioning of the 2nd gear ideal position off by 0.015. The factory fix for pop -out is to adjust the shim stack to move the second gear 0.015 further forward allowing the slide to engage further. The g-box detent allows further engagement of ALL the gears , but this isn't a problem. The CMS (California Motorsports) has a modified shift arrester that does the same thing but only to 2nd, 4th, and 6th gear, and requires the rear case to be removed to install. The loose 2nd gear "dog teeth" are "V" shaped to allow fast and easy engagement. every time a grind occures, the tip of the V is ground or rounded. If too much of the Dog teeth are ground off the slide will no longer stay engaged and/or be very difficult to engage. If grinding has occured, you can be sure that "some" damage will be done to the "v" of the dog teeth. The extent of damage must be determined and the moderate rounding of the tip will result in less effective shift. If the grinding is stopped in time, the shift quality may still be acceptable for normal driving.

I had similar issue in my 2003 996 Turbo, it was grinding only on the downshift into second gear above 30 mph, other wise it shifted fine with no pop-outs. I had the oil changed with genuine porsche oil according to spec for $355, and it shifted perfectly.

Dredging up my own thread. I'm trying to sell the car and the grinding 3-2 downshift doesn't help. I think I'm going to pull the transmission and replace the synchro as Porschetech3 suggested. Before I buy parts - can I get someone knowledgable to tell me if the specific synchros involved in a 3-2 downshift are called the 2nd gear syncros or the 3rd gear synchros? I think it is that middle stack: Which of those do I need? Are there other parts I'd be looking for in this synchro change? I'm not looking to spend a ton of money, I just want to fix this one specific problem. Otherwise the car is terrific.

Part# 3-4-5 is the set that makes up the 3-piece syncro for 2nd gear (also used on 1st gear). Part #2 is 2nd gear, inspect closely the small teeth for damage to the leading edge "V", also the guide sleeve part #6 for damage to the leading edge "V"..Compare the "V" to that of other known good gears in the trans.

Thanks! Parts 3-4-5 ordered. I'll inspect the others when I have it apart.

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And then...

I changed my gear oil with some generic 75w90 synthetic I had at my shop after I installed a new trans mount. I noticed it would grind a lot more going from 1st to 2nd when cold. I ordered some from porsche just now, hopefully it helps.

I have the transmission apart, and the only damage appears to be a worn synchro. I have several threads opened recently about the project. I am in the reassembly stage, but distracted by other projects and my day job.

Subscribed, we need pics! please

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Nice work

That there is some machinery. Where did you end up sourcing your parts? (asks the guy who is going to try new tranny fluid first.)

Good idea trying the gear oil first. I tried three different kinds including the genuine $$$ Porsche gear oil. None of them fixed it. Some made it worse. The genuine fluid was definitely the best. I also tried four different shifters (996, 997, Numeric, B&M short shift). I kept the 997 shifter in. None of them improved anything. The B&M made it worse. Once I got it apart and got to the second gear synchros, I could see why. The brass synchro that should have a rough grippy surface was completely smooth, and I could easily turn the assembly by hand, whereas the replacement pieces locked tight when assembled. I got genuine Porsche parts (cone ring, synchro ring and ring - part numbers above, note different parts by model year for the brass synchro) from an ebay seller. The price was significantly less than the dealer prices I found. The teeth on the second gear dog ring (part of the loose second gear) were fine, so were the teeth on the slider (outer part of the sliding gear selector assembly). If they required replacement, the repair parts could have made this job more expensive than a used gearbox. Since I�m not paying for labor, have the tools and great help from this community (porschetech3 has been my guru!), it was worth the efffort to try the repair. Worst case, if I fail, I buy a used gearbox.

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Rereading my own thread, I wanted to make a correction to note that I was wrong - there is a round magnet inside my transmission.

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Hi, and sorry to resurrect this thread, but I am about to try and rebuild the 2nd gear synchros, and you can imagine I am quite nervous and trying to get as much information as possible. Could you please confirm that the extra shim is in inches, so 0,015 inches = 0,38mm? Also, the shimming is added? � 2nd gear will be further away from 4th gear after shimming? Thank you!

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The shim stack adjustment is the "factory fix" and requires that ALL the gears be pressed off the shaft. There are other fixes from G-box and CMS that also fixes the second gear pop-out and don't require all the gears to be pressed off and total disassembly.. The G-box detent doesn"t even require the transmission removal, and the CMS shift arrested just requires that the rear case housing be removed.. Are you also replacing the two big pinion bearings? Or just fixing the second gear pop-out..

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Well that is unfortunate, my hope was to shim while removing 2nd gear only. Thing is, it never pops out, but its hard to get into gear. When warmed up, 3-2 is pretty good, I would say I hit it 90% of the time. But 1-2 causes issues even warm. I've refreshed the tranny + engine mounts, and it helped a bit but its still rough. What is most annoying is that I can not tell if it goes into gear or not on the gearstick — the only sign is the grinding sound Since 2nd gear can be pulled using only a puller, I was going to refresh the synchros and install the CMS arrester for extra help. I am also thinking about refreshing the cables to take out the slack from the assembly in hopes of making the gearstick more precise.

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Yes, from your description the new syncro's and the CMS arrester should fix your problems as long as the "dog teeth" on the gears are not damaged...

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Yep, hopefully that will be the case once I take it apart. I presume I will be able to visually check that by removing the front cover only, right? I would love to do this properly (remove all gears, change bearings etc.), but its hard to find a shop I would trust with this where I live. Just for my curiosity, the shims are placed between the bearing and the retainer plate, as drawn here: https://www.rosepassion.com/en/diagr...ary-shaft-2561 ?

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Yes that is correct but I definitely would not go through all that just for a second gear issue...you can replace 1st and 2nd gear and their syncro's pretty easy, but the other gears or pinion bearings are a PITA...better to stay away from that if you can...

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Well, I am in deep, currently removing the gears but ran into an issue. I lost the circlip for the secondary shaft. I've found the part number, but it comes in 5 different thicknesses. Is there a default, stock thickness, or does it differ for each car? https://www.rosepassion.com/en/diagr...-117935-z69425

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It differs with each car but you can use careful and precise measurements to see what C-clip you need. Measure from the shim/inner bearing race to the end of the shaft, then measure back from the end of the shaft to the first edge of the C-clip groove then subtract that number from the first number, that will give you the maximum thickness of the C-clip. Then select the thickest C-clip that will fit within this distance..

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Thank you so much, measured 2,30mm, so will order 2,30mm and 2,35mm just in case. I also lost one of the four rollers in the middle shift fork. Unfortunately the PET has no mention of it, but I did find a roller in the 996 Turbo gearbox diagram. Hopefully it is the same part. https://www.rosepassion.com/en/diagr...1-roller-57951 This job is relatively straightforward if you don't count the hours lost looking for things on the garage floor