Gear box noise in 2nd and 3rd only

[MartinH]

I have just finished replacing the diff output shafts on my S3 DHC , bolted everything back together new diff mountings and bushes and torque rod bushes. I also noticed the gearbox mounting bolts were loose so tightened these up (spider chassis) Took it for a test run and it now makes a grating noise in 2nd and 3rd only. Once in top it runs along perfectly with no noise. It also makes no noise in first( the gearbox was rebuilt about 8 years ago and has done max 12k miles). I then slackened off the gearbox mounting bolts to see if this was the cause maybe pulling the box against the chassis perhaps, but it makes no difference the noise remains. It did not make any noise prior to an output shaft breaking about a month ago.

Anyone got any ideas, suggestions?

Martin

[MartinH]

another piece of possibly relevant information re this gearbox noise. The car has been sitting for a month with the back end way up in the air while I took the diff out and did the repair. So the gearbox oil will all have been sitting at the front of the box.......... or am i talking rubbish?
Martin

[Craven]

Hi,
Can?t think of a gearbox specific fault to produce noise in 2nd & 3rd only, but perhaps a different prop shaft alignment has loaded a worn gearbox tailshaft plane bearing allowing the mainshaft to tip.
I take it you have just refitted the same large shim washers when refitting diff unit, you may like think about the old and new diff position with your new mounts.
Ron.

[MartinH]

Thanks Ron
I will look again at the diff location and how the prop is going into the gearbox tail.. I did pay careful attention to this to make sure it was well clear of any chassis points. However the new mounts i put in are heavy duty competition mounts and come with new shims/spacers which are much thicker than the old ones. Same thing for the torque rod bushes these are poly, competition bushes. It could be as you suggest that the prop is now going into the box at a different angle.

Martin

[Chancer]

Noise in all intermediate gears except top gear is usually layshaft needle rollers, noise in 2nd and 3rd but not 1st is usually the inner needle roller bearing (cant recall the term) a sort of spigot bearing that supports the first motion shaft where it enters into the mainshaft.

Neither of these usually occur immediately, they get progressively noisier so you are well advised to look for other misalignment/rubbing problems.

[Craven]

Hi,
With your heavy duty mounts etc. you now have a much stiffer, less compliant, drive chain. Perhaps the effect is to cause a sympathetic vibration elsewhere, the exhaust system for example, see if the noise is at certain engine revs or at a specific road speed.
Easier said than done, but check out the UJ?s in the prop shaft.

Chancer, If there is a difference in speed between the input shaft & mainshaft the bearing to which you refer must be rotating, in fact will be at its greatest speed in 1st gear. Is there an explanation for your comment?
FWIW
Ron.

[Chancer]

Layshaft bearings make noise in all intermediate gears because the drive is being taken through the layshaft, it is being forced away from the mainshaft and the thrust is against its bearings, top gear is a direct drive, the mainshaft and first motion shaft are locked together and the layshaft just idles.

its been d?cades and i might have got this back to front but IIRC when 1st gear is engaged which is at the opposite end of the mainshaft it causes noise if that central needle roller is worn, if I've got it back to front then the noise is more apparent in 3rd gear, once apon a time I could diagnose whatever bearing was faulty by the noises on a test drive, many years have passed!

In any case bearings rarely become noisy in any way but progressively.

[Craven]

Chancer, Thanks for your reply.
Ron.

[MartinH]

Craven

Thanks you are right, the noise is coming from the exhaust now being very close to the bell housing. I have spent hours today trying to adjust some more pace between the two. So far with limited success, it is better than it was but have not eliminated the noise completely especially under acceleration. The exhaust is a TTR manifold and system so was very close to the gearbox anyway. I have set the diff mounts and torque rods to the specified torque settings but will probably have to play with these a bit more. There is not a lot of scope to move the exhaust system sideways.
Martin

[Craven]

Hi,
Not really recommended but I have in the past used 2 exhaust gaskets under the manifold, surprising how much a 1/16th can make. Depends on how desperate you get!
Good luck
Ron.