gear whine

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dave v. in nc

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Another post made me wonder...What causes transmission gear whine?..and what is the prognosis (how much time do I have, doc?..) I know that if you open the box to repair something like a bad synchro (2002 history...), you might as well rebuild the whole thing...or so I have been told? In old English cars, I just assumed that they all sounded that way...Thanks.
Dave
 
When I put in my 265 there was a lot of whine at first. Over a few months it became much quieter (or I simply stopped noticing).

If you're using a trans that hasn't been run in a while as I was, it is likely a good idea to run some new fluid for a month or so, then change it out for a fresh batch. Whine I believe is play between gears and shaft, needing new lube to get in and smooth things out, but that's just my speculation.
 
Gear whine is caused by error in the teeth pattern between the gears. This stacked mesh error causes different noises depending on which parts of the tooth profile are in error. Different gear sets have different errors, which is why a given transmission can whine in 3rd but not in 2nd. The best transmissions have "matched" errors such that the total error is close to zero for each gear path. (Ex: input shaft to main shaft: +0.02, main shaft to first gear:-0.01, first gear to output shaft:-0.01, total error:0.00)

Using a thicker oil helps mask the noises by buffering the gear contact patches, which helps "zero" the error.

This is, of course, assuming nothing is actually wrong with the transmission, like a bad bearing or burned bushing. This type of noise as I described will be present during the entire life of the transmission, although it will get worse with age as gear faces wear and the gaps increase.
 
Typically gear whine in a BMW transmission is caused by bad countershaft bearings from my experience. If an overdrive trans gets quieter in 4th gear or a dogleg gets quieter in 5th, my bets would all go to replacing countershaft bearings and of course all the rest too as long as you're in there, not all that expensive if you already have the pullers and a press. Make sure to get the correct shimming on the preload of the countershaft bearings or you'll be doing it again sooner than you want to. Good luck, Leroy
 
How about whine downstream of the transmission? Auto tranny just rebuilt, driveshaft also, must it be the differential? Also some click clack on acceleration deceleration like when motor mounts are bad. If so how do you quiet a differential?
 
diff noises

Diff noises are often caused by too much backlash (amount you can turn ring gear with pinion held solid) which will cause a click or in the case of more wear,a clunk. Typically this is caused by wear of the bearings, in particular, the pinion bearings. Another cause could be insufficient or over tightness of the nut that holds the u-joint yoke onto the pinion gear. This can be caused by someone who just changed a leaky pinion seal and did not follow approved repair practices. A diff is not all that difficult to re-bearing and reseal but approved clearances, torques, and turning torques must be adhered to for success. Regards, Leroy
 
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