knocking sound, drive shaft and differential

[V12StealthHunter]

There is a knocking sound coming from underneath the car that seems to match road speed. Sounds like a bad U-Joint. Took the drive shaft off today and although the U-joints are pretty worn I think it might be the torn bushing in the center support bearing causing the knock.

When I separated the rear section of the drive shaft from the differential yoke, about half a pint of clean gear oil came out. I'm guessing the differential pinion seal is leaking and oil is getting past it into the yoke. Does that seem right? Will a new oil seal fix that? Or is there something else wrong?

Also how do I check if the splines on the forward section of the drive shaft that attach to the 3 point spider/flex disc are in good shape?

[redcars]

Oil coming from that location is not the pinion seal, but rather oil leaking past the splines on the pinion shaft. You can replace the seal as many times as you like and it will not stop this. If the pinion seal was leaking it would make an oil stripe on the under side of the car. What I do is take off the pinion nut and the washer. Then put a small amount of RTV behind the washer to seal the spline to shaft area. While you are there you might as well replace the seal.

[V12StealthHunter]

Hmm. Strange problem. I suspected it wouldn't be the pinion seal. But like you said, might as well do that while I'm in there.

In the Haynes manual they show a washer like thing called the oil flinger washer. Is that supposed to stop the oil?

Thanks for the tip.

[redcars]

The oil flinger is what troughs the oil when the seal leaks and makes the oil stripe on the under side of the car. Never could figure out why they are use. It is not just a Fiat thing. On thinking about it I may have put the RTV between the flange and the bearing. I has been a couple of years. There are write ups about it here on the form if you look.

[V12StealthHunter]

Great, I found a reference to the issue here;
http://www.fiatspider.com/f08/viewtopic ... ft#p114674

I fought the pinion seal leak for years and numerous seal changes, untill I finally realized that it wasn't the seal leaking, but rather the leaking oil was traveling down the splines between the pinion shaft and the pinion flange. The fix was to take it apart far enough to clean the splines (easier if the seal is out of the way, so new seal), then used a good RTV sealant on the splines and the washer and nut holding the pinion flange in place. I used a new crush sleeve and torqued it properly to crush, using a similar jig to hold the flange as you did. Hasn't leaked any since. You could probably do just as well with reusing the old crush sleeve as you did, but I went ahead and replaced it anyway.

As Alvon stated the oil will leak past the splines. It will get into the cavity between the two flanges and seep out the seam. I also sealed that interface with RTV or a homemade paper gasket. I don't know what the engineers were thinking on that design. The oil seal should have rode on the pinion shaft itself before the splines and not the separate flange piece.