One rear wheel turns, the other doesn't

[KevAndAndi]

Until today, the Spider had been on stands for awhile. This was to address a couple of issues: The gasket on the lower transmission plate was shot and started leaking oil on the garage floor. Also, the bushing around the driveshaft's center support bearing was shot.

So I drained the tranny and removed the plate. Cleaned it up and painted it with POR-15. Replaced the gasket. Buttoned it up and filled the tranny with GL-1.

Bought a new center support bearing assembly, u-joints, and a rubber flex donut ("giubo"). Had a local machine shop R&R the bearing and u-joints. (Marked everything with a white grease marker when I removed the drive shaft.) Reinstalled the driveshaft.

At the same time, I also decided to install a starter relay, which went well - works great.

I finished up these tasks this morning, knowing that the unseasonably warm weather was providing the last top-down day of 2015. Started 'er up while she was in the air.

Noticed something peculiar: the passenger side rear wheel was spinning even though the car was in neutral. Put it into first gear. Passenger side wheel spinning, driver's side wheel spinning more slowly, and I found I could stop it entirely with my foot.

I know only the basics of how the pinion, ring, and spider gears work - I've never serviced a diff or even had this one open. So, for this newbie, can someone explain whether this is normal or not? The only topic I could find is the one linked below, and it didn't really come to a conclusion that I understood.

http://www.fiatspider.com/f15/viewtopic.php?t=15590

(We did have a nice drive. After just over a year of ownership, the car's owner was, for the first time ever, the driver rather than the passenger, and she overcame a two decade lapse in stick shift driving just fine. Not a single gear grind.)

[So Cal Mark]

it does that because you have an open rear end, if was positraction or a locker one wheel wouldn't spin

[KevAndAndi]

Thanks, Mark. So I gather this is normal, then? How does power go to the driver's side wheel?

[DieselSpider]

Thanks, Mark. So I gather this is normal, then? How does power go to the driver's side wheel?

It evens off as they both get power with the one with the least resistance spinning the fastest. When up on blocks with the brake pads and such in place one wheel generally spins more freely than the other so its the one that will be spinning with the other stationary or barely creeping. While on the ground it evens off between both rear wheels unless your on loose sand, gravel, ice, etc. This prevents the car from hopping around on corners with both rear wheels trying to spin at the exact same speed but covering different distances. I had a 4X4 with a fully locked transfer case and rear axle with a limited slip front axle and that truck would hop all over the place on dry pavement if you put in locked 4WD.

[KevAndAndi]

Thanks, Mark. So I gather this is normal, then? How does power go to the driver's side wheel?

It evens off as they both get power with the one with the least resistance spinning the fastest. When up on blocks with the brake pads and such in place one wheel generally spins more freely than the other so its the one that will be spinning with the other stationary or barely creeping. While on the ground it evens off between both rear wheels unless your on loose sand, gravel, ice, etc. This prevents the car from hopping around on corners with both rear wheels trying to spin at the exact same speed but covering different distances. I had a 4X4 with a fully locked transfer case and rear axle with a limited slip front axle and that truck would hop all over the place on dry pavement if you put in locked 4WD.

Thanks very much, DS. It definitely is counterintuitive to be able to stop one wheel from spinning at all, but that was just my ignorance. I hope this thread is a help to someone else who is equally clueless. I've now looked at animations of how the differential works, and it is quite an ingenious mechanism.

I'm still not understanding how the drive shaft and wheel were spinning when the car was in neutral, though. Is that indicative of a problem, or it is similarly of no consequence? (When the car is on the ground, it certainly does not move when it's in neutral.)

[131]

I'm still not understanding how the drive shaft and wheel were spinning when the car was in neutral, though.

Just rotating parts dragging on each other due to the viscosity of the fluid between them.

[DieselSpider]

I'm still not understanding how the drive shaft and wheel were spinning when the car was in neutral, though.

Just rotating parts dragging on each other due to the viscosity of the fluid between them.

As long as it required only minimal effort its just the tackiness of the lube in the transmission. On a positive note it shows that your output shaft/bearings are not binding nor is the rear axle.

[KevAndAndi]

Thanks, guys! That transmission lube sure is tacky. What a pain it was to pump it into the fill hole.

[707kevin]

This is, by far, the best explaining of how differentials work that I've found for explaining it to others.
https://www.youtube.com/watch?v=K4JhruinbWc