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Re: Thanks Pete! CMyles... any more thoughts on the ma
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Posted by CMyles [Email] (#1126) [Profile/Gallery] (more from CMyles) on Mon, 28 Aug 2006 16:33:57 Share Post by Email
In Reply to: Thanks Pete! CMyles... any more thoughts on the matter, backman, Mon, 28 Aug 2006 12:19:02
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backman,
I'm reluctant to get in the middle of a major disagreement with a Saab tech here but here is the logic he's suggesting; Your tranny bearings wore and as a result the final drive gears wore to such an extent that when they were returned to their "original" confirmation (offset and backlash) they no longer mesh ideally and hence the noise. The logic continues; the gears, now rolling in fresh, properly preloaded bearings, will wear until they arrive at a point which approximates their original mesh charicteristics. (whew!) Those gears will definitely wear and they may become quieter. They may also break. An important question is: how bad were those gears? When these trannys are run low on oil the gear teeth can have horribly rough and worn contact faces, but may for some guys be reusable. Here is a critical point: if the gear teeth are fine (they have typical, limited wear) and the bearings are replaced then the final drive should be quiet IF the pinion offset and ring gear backlash are set correctly. Given the tight production tolerences of modern bearings many guys, when installing new bearings, simply replace all of the shims just as they were and have quiet transmissions. There is generally no need to do a complete re-shimming of these C900 trannys. Re-shimming BADLY WORN gears to their original spec's. will make a noisy final drive. The best approach with really bad gears is to set the offset and backlash using a transfer medium to gauge the contact profiles, forget the book spec's. (other than the preloads, which, if done right, are not related to gear noise). Now you said that you localized the noise near the chain case and I assume that the car was in neutral and not rolling when you determined that. That's why I suggested that this is not a final drive issue but a chainwheel or input bearing or tensioner problem. Those would all cause noise which would be heard while driving and would change with RPMs and load. Redline may quiet the problem but why is there a problem? So to sum up: Either your gears were really, really badly worn or there is a problem with the workmanship. It's critical to locate the noise (be real sure that it isn't a tire or brake problem or something silly like that) then identify all the conditions when the noise is heard and not heard and then look inside the unit. Remember, if the noise occurs when the car is not rolling, then it's not final drive (and it won't go away with time). Drive the car so that you hear the noise, leave it in that gear and step on the clutch while maintaing the throttle. If it's a tranny problem you'll still hear it although the pitch may change. Then slip it into neutral (keep your foot on the clutch). If it goes away then it's one of those problems in the chaincase and not the final drive. Also final drive noises change very noticably as you go from load to coast. The ring and pinion pressure moves to opposite sides of the teeth as the torque flow is reversed. If it really seems to be a final drive problem then the rear cover can be removed and the backlash checked or better yet the contact patterns could be established using a transfer medium without pulling the unit. Saab master techs aren't taught by Saab to check contact patterns but it's the definitive way to adjust final drives. Ring gear backlash can be reset in the car but if the pinion offset is way off you're screwed. The backlash would be off as a result of negative side bearing preload (less than none) but if he made that mistake you need to find someone else to go to. Hope this makes sense and helps. Good luck. Let us know how it goes.

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