Technical Transmission or Differential thumping noise

[ride106]

Hi folks - did some searching but not sure anyone here has something quite like my issue. I hope I am not looking at a transmission overhaul. I have a noise that is best described as a thumping sound - rhymical as speed increases, almost like a wheel out of balance (but they are not) - when accelerating even modestly. Does not occur every time I increase speed, but when it's at its worst, you can feel the thunk through the chassis. I could be imagining it, but it could be worse when accelerating in turns, making me think differential could be problematic.

I went to change the gearbox oil (I have no real history of the car, acquired it last year), and to my dismay, almost none came out. Maybe a few ounces. Unclear how many miles it had been driven like this. Filled the box back up per "spec" as detailed here on the forum. Sounded better immediately after, but it seems to be back and persistent.

Maybe the oil I added has worked into the gears and I need to top up again, but I'm afraid it's more likely what damage is already done. I will check that when I find some more time and a few old syringes.

I should mention that the car is now using an electric drivetrain after converting it over the last few months. I did not notice the noise during it's last gas-engine drive, but I think that's because the engine was so noisy. The electric motor has enough torque to start off (gently) in 3rd gear. Obviously if I give it the beans the clutch slips so I am driving very gingerly, generally not taking off much faster than when I had the old gas engine. The symptom is also there if I take off in 1st or 2nd.

Any thoughts? If it's differential related, are those replaceable parts in the US?

 

[Bounding Bambino]

Does the sound vary with engine/motor speed. Or vehicle speed. This could help narrow down where its coming from in the trans. If the speed is directly related to motor speed regardless of what gear it is in. Then its isolated to the top half of the transmission. If the noise varies proportionally with vehicle speed regardless of gear. Then it could be narrowed down to a few areas (ring and pinion), differential assembly/drive shaft joints/driveshaft knuckles, or lower shifting gear assembly.

Also are you running a non syncro, or upgraded 126 syncro transmission?

 

[Goldnrust]

I chased a similar sounding noise for ages in one of my other classics, turned out to be a wheel bearing race with a chip in it. So might be worth inspecting the rear wheel bearings?

 

[ride106]

Thanks for the replies- I’ll try to get better diagnostics, all helpful. This is a non-synchro box.

 

[the hobbler]

Thanks for the replies- I’ll try to get better diagnostics, all helpful. This is a non-synchro box.

Check the drive shaft rubber/alloy couplings---they don't last for ever, even with the low power that the Fiat engine chucks out (and the 'pattern' couplings don'tlast as long as the factory original). One of your problems ir that the drive-train of the 500 was never designed to take the torque that the electric motor produces----even the Abarth 695SS produced only about 42ft lb

 

[ride106]

Right - agree re: torque. The system I am using is fairly true to original, peaking around 40ft-lb, I didn’t want to risk total gearbox liquefaction…

 

[ride106]

I replaced the flex bushings at the axles - they were definitely shot, so I’m glad the replacement was done, but did not have any impact on the thump. It varies linearly with wheel speed so I assume it’s something loose or worn in the differential.

I’ve seen the flickr album showing how to disassemble the transmission and it certainly does not seem like a simple task. Any honest opinions on difficulty level? I am certainly handy but worried I would muck something up worse. Not that I am made of money, but I sort of would rather replace the whole transmission rather than monkey around in terra incognita, you know?

Tear down album for easy ref: https://www.flickr.com/photos/ricambifiat500/albums/72157615692546231/page2

 

[ride106]

New transmission is in and the noise and thumps are gone. @ Goldnrust, I think I may indeed also have a tired or worn rear wheel bearing, but everything is holding up nicely now - maybe a job to investigate this winter. It zips right along!

FYI - to anyone who might buy a replacement transmission for the Giardiniera from MrFiat, they appear to be bellhousings from a 126 (bis?). That's fine, but the starter motor bracket shape is effectively flipped upside down. This means my beautifully machined adapter plate no longer matches properly... but the rest of the bolt holes line up. Pics attached. Maybe someday I will swap the bellhousings but for now it's time to drive the car and enjoy.