What is that noise??, tranny? Options

[jr91472]

Ok, I have been listening to this sound for a couple a hundred miles and am stumped.

Best described as a roaring sound coming from the back, but does not occur at all speeds or driving conditions. Specifically, only when in either 4th and 5th and only at lower rpms (say between 2800 and 3200).

Additionally, I only hear it when holding at a constant speed. I.E. if I accelerate through that rpm range - no roar. Only occurs if I let off power, then re-apply power but only to maintain a constant speed. Consequently it only seems to occur while "cruising" in 4th and 5th (at the stated rpm range).

My first fear was tranny bearings, but wouldn't I hear it at all speeds?

CV joints?

please help, so far my only solution is to either downsift or turn on the radio (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[lapuwali]

Backlash is out much clearance there is between the teeth on meshing gears. When one gear is driving another, one side of the driving teeth are pressed against one side of the driven teeth. When the loading changes (such as letting off after accelerating), this reverses, so briefly the "driven" gear may drive the "driving" gear. The teeth will tend to bounce off each other, taking up clearance on one side, then bouncing to the other side, back and forth. The more clearance, the greater the forces involved. This creates noise, and can damage the gears over time. There is always SOME backlash, and always some noise, but in a "good" gearbox, the backlash is either small, or the noise frequencies of the various bits all end up mostly cancelling out.

The forward speeds in the 914 gearbox are all "helical-cut", with the teeth cut at an angle. This gives more tooth surface area for a given width of gear, which makes the gear a bit more robust in wear, and cuts noise as the backlash is naturally less. The reverse gear, however, is straight-cut. It has to be, as this gear, unlike the other five, is engaged by meshing and unmeshing, which can't be easily done with helical-cut gears (a diagram would help here). Straight-cut gears are much noisier, as the tooth lash is much less constrained. The whine is basically the gears ringing from the teeth bouncing back and forth.

If your forward speeds are making a similar whine, then it's either caused by a chipped or worn gear or two, or by a failing bearing. Rolling element bearings make all kinds of noises as they fail: whine, rumble, roar, squeaking. It depends on exactly what's causing the failure. The fact that it's only happening in the upper gears and only at certain speeds makes me think that what you're hearing is some resonance that could be caused by several things almost anywhere in the gearbox. The sound is always there, probably changing in frequency, but only under the circumstances you describe are the frequencies "matched" sufficiently for you to hear it. In other gears and speeds, the frequences cancel each other, or when then add up are still so low that they can't be heard.

What to do about it is another matter. I had a '99 Miata for five years and 70K miles that made a similar noise mostly in 4th from new. Several cars did it, and there were enough complaints that Mazda investigated and determined it was an unfortunate resonance, and they decided not to do anything about it. I have yet to hear of an actual failure caused by this. I simply didn't use 4th all that much, so it didn't really concern me. You may pull the gearbox apart and find nothing. Simply replacing one gear pair or a bearing may alter things enough that the noise changes (perhaps better, perhaps worse).

The last bit is backlash in the final drive. Again, due to resonance effects, it's still possible that it could be this even though it only shows up in certain gears. This is the spacing of the pinion to the ring gear, and is adjustable via paper shim gaskets between the intermediate plate and the gearbox housing. Setting this is a royal pain, but well worth the effort. These parts are sufficiently stressed that too much backlash will kill the final drive eventually.