Summary update -
Rear axle click was totally fixed with $100 service and no new axles. Totally solid after extended use.
If you’re getting a 2.5k estimate to replace them, probably not needed. Give your service tech the info below.
I bought two Model S’s a few years ago, and super happy with both cars.
One recently developed a rear axle ‘clack’ sound on acceleration from a stop, or on shifting between regen and acceleration.
Problem was diagnosed to be slapping of the rear axle splines that connect the wheels to the drive unit.
This occurs when the grease packing between the male and female splines gets compromised. The bare metal spline teeth hit their counterpart, and you’ll hear a loud metal-to-metal clack.
Potentially it can be caused by wear or damage to the spline teeth, which would require a replacement of the rear axles. That runs about $1,250 for each rear wheel.
The protocol is usually to first try re-packing the grease, and replacing the axle nuts that keep the grease in place.
That’s low cost at about $100 total for both axles, parts and labor all-in.
When that didn’t work in my case, I researched it to understand the failure mode, and worked with the service center to see if it could be solved without a full replacement.
What I learned pleasantly surprised me, and I thought I’d share it here to help others.
Here’s a summary -
1. The male spline fits into the female spline at about a 3.5 inch deep engagement.
2. When re-greased quickly (as on first visit), the fresh grease only gets about halfway in.
3. Corrosion at the deeper end of the splines can displace the grease, and give rise to the clacking.
4. To fully resolve, techs need to get solvent and long narrow brushes deep into the female spline.
5. The solvent has to sit there long enough so that all the oxide particulate is loosened and cleaned out.
6. Then fresh molybdenum paste grease is packed deep inside, and also brushed liberally onto the male spline.
7. Once assembled, the excess grease squeezes out, and the new axle nuts are installed to prevent water ingress, or grease leaking.
8. The specific grease is about 65% pure molybdenum. I brought them some Loctite LB8012 from Amazon @ $32.
9. They stock a similar paste at Tesla, Dow Molykote M-77. The loctite stuff had a bit more molybdenum, but also a small amount of microscopic hard quartz. That helps enforce a minimum spacing to prevent the grease from being forced out between the teeth.
It really worked.
The difference was immediately obvious as I drove away. The clack is totally gone, and the axle is now silent, even when subjected to strong acceleration / deceleration cycling.
I think the key to it is really getting all the rust out at the full depth of the female spline coupling.
That made room for the new grease, and then everything worked very smoothly.
Kudos to awesome shop foreman Alfredo at Tesla West Los Angeles, for patiently studying it to figure out what’s needed.
Since I had no luck the first time around, the first data was discouraging. But now those very same parts are performing perfectly.
I’m guessing a lot of techs and customers give up on the first attempt, and think the axles need replacing.
But it looks like this comes down to getting deep into the female spline to fully clean it.
If you have this issue, please share this info with your tech.
It can save time, money, and customer service workload for Tesla.