General Gearbox issue (4th gear)

[MOD500]

Morning all,
the fiat has developed a bit of a noise. It sounds like the start of a CV failure and since I have a set of those fitted instead of the Aluminium/rubber couplings , I thought that was the culprit.
However, as I've started to tune into it more, the noise only occurs in 4th gear. I can drive at 40 in 3rd and it's not there, even under high load.
As soon as I pop into 4th it re-appears.
I wonder if anyone is familiar with the innards of the gearbox, I have never taken one apart other than to swap the drive shafts. I believe there is a bearing right next to 4th gear, I wonder if that is the part that has started to fail.

 

[Bleeding Knuckles]

Morning all,
the fiat has developed a bit of a noise. It sounds like the start of a CV failure and since I have a set of those fitted instead of the Aluminium/rubber couplings , I thought that was the culprit.
However, as I've started to tune into it more, the noise only occurs in 4th gear. I can drive at 40 in 3rd and it's not there, even under high load.
As soon as I pop into 4th it re-appears.
I wonder if anyone is familiar with the innards of the gearbox, I have never taken one apart other than to swap the drive shafts. I believe there is a bearing right next to 4th gear, I wonder if that is the part that has started to fail.

Hi Mod, Did you have this noise before you changed the drive shafts???? If only slightly??? Without stripping the box and checking the individual gears for wear it is difficult to tell, but, if you have not disturbed the Pinion shaft during the drive shaft change and the noise is new it is possible that you have a worn 4th speed driven gear. There is also a possibility that the crown wheel is not engaging with the pinion properly after the work ( the backlash is critical ) and causing stress on the front bearing. The 4th gear is directly behind the bearing after the pinion shaft bearing shims so this too could be suffering stress. Otherwise it's a strip and check I'm afraid in any case - bummer
Ian.

 

[MOD500]

Thanks Ian,
The shafts have been in that box for 3 years so they are not a recent change. The noise only started a week ago, so definitely not related to that change.
I think you're right, I'm looking at a strip and inspect.
I have another spare box, I'll swap over to that one to keep the wheels turning while I investigate. the current gearbox also has an issue with the synchro so it's not the best box, although it does have the advantage of a good condition first gear.

I have been dodging a gearbox rebuild for years, I think it's time I bite the bullet!

 

[Bleeding Knuckles]

Thanks Ian,
The shafts have been in that box for 3 years so they are not a recent change. The noise only started a week ago, so definitely not related to that change.
I think you're right, I'm looking at a strip and inspect.
I have another spare box, I'll swap over to that one to keep the wheels turning while I investigate. the current gearbox also has an issue with the synchro so it's not the best box, although it does have the advantage of a good condition first gear.

I have been dodging a gearbox rebuild for years, I think it's time I bite the bullet!

Hi Mod, If you overhaul the spare completely first, you only have to change the unit once !!!
Ian.

 

[Toshi 975]

I have an odd issue with one of my gearboxes. A slight whine on 4th gear only and the box has only covered 8,000 miles from new. Has to be down to an actual gear rather than a bearing. MOD it is not like you to have not done a particular job on a 500 nor had I until I got fed up tripping over all the gearboxes I had accumulated of unknown condition so stripped down seven 126 gearboxes at the same time. Cleaned every single component and after a visual inspection I scrapped anything that was broken or obviously worn then with the addition of a few new components I rebuilt three whole gearboxes and one part gearbox using the Haynes manual as my guide. I have rebuilt a few other gearboxes since then but am still left with a whole pile of gearbox parts which I have dipped into from time to time to help guys out with parts. Think I also have brand new gearbox castings up in the loft.
I came across a range of problems in the gearboxes I worked on so you never know what you will find until you et in there. Apart from expected broken or worn gears and bearings there were broken selector forks, bent shafts, broken detention springs or rusted up detention balls, missing or incorrectly assembled parts etc. I would say think in terms of new bearings then inspect the rest. If anything looks dodgy I will probably have a good used part here apart from the dreaded 1st/reverse.

 

[MOD500]

I agree with Ian's point about rebuilding before swapping, but I don't think I'll get much more out of the gearbox before it fails. So I'll likely end up not suing the fiat for a while. With work and kids etc, I've always been a fan of swap first repair later.

The gearbox in the car has a dodgy synchro and now a noisy fourth, but it has a good first. I'm keen to strip this one since first gear is one of the best I've had.

If I have to buy a new first gear to build a second box then so be it. I'll be replacing bearings while it's apart. May as well.

Some jobs I am happy to do, but gearboxes do not appeal to me! I'll have to get over it though. I'm not paying someone else to do it!

 

[fiat500]

I'm not paying someone else to do it!

That's the right attitude to take!

The gearbox may look daunting but anyone who can build a Lego 500 could rebuild a 500 gearbox. The challenging part is getting your head round the crownwheel and pinion and its mesh and preload. But in theory you are spared that task as long as you re-use the differential bearings.

I've yet to try a Fiat 500 or 126 gearbox which didn't whine and I'm on my fourth version on this car already!, but the (presumably) clunking noise you describe seems a bit odd.....I suggest you double-check the oil level before anything else.

 

[the hobbler]

Peter's suggestion to check the oil level 1st is a wise one. It might be sensible to first of all drain the oil and see what comes out with it. If nothing untoward comes out, try just re-filling it with something like "Red-line" gear-box oil. Some of the products by this company are designed specifically for 'noisy' gear-boxes. If going this route 1st (and it won't cost a lot) produces no noticeable difference, THEN heave the gear-box out and overhaul it.