LSD Burnout?

Bags · 04-28-2003, 07:36 PM

I'm looking at a 97 C2S with option 220 which is LSD. With the rear wheels off the ground, when I turn one wheel the other does not turn. This is the same on both right and left sides. This is with the car out of gear. In gear (1st) truning one rear wheel has the other turning in the opposite direction. I thought with LSD that both wheel should turn in the same direction? Could the LSD be burned out? Is this a common problem? What would the repair cost me?
Thanks for the help
Mike

Jim Richards · 04-29-2003, 12:12 PM

Mike, I'm curious about this, too. I have a click (not too loud) whenever the momentum changes on my car at lower speeds. It's probably there at higher speeds but I can't hear it then. Also, it's not a loud clunk, like some have mentioned when describing a DMF failure. So, I'm wondering if this could be attributable to a bad LSD.

Geary · 04-29-2003, 01:00 PM

Mike

Your test #1 tells you that there is a working LSD in the car, although it may have somewhat worn friction discs.

In test #2, you have virtually locked the mainshaft with the engine. In this situation, the wheels would only turn in the same direction if you had a spool, or an LSD locked up so tight that the compression of your engine could be overcome.

Bags · 04-29-2003, 01:30 PM

Geary;
Thanks for the reply, I'm still a bit confused (as usual)! Out of gear, rotating a rear tire, I thought with LSD the other rear tire would rotate in the same direction? Are you saying because the other tire does not rotate that it also shows the car has LSD but possibly a little worn? How do you know when it is worn out to the point of needing replaced? Secondly on both my 993 and 911 when in first gear and I rotate a rear tire, the other tire rotates the opposite. I did not understand what you are saying in your explaination, sorry...
Mike

Geary · 04-29-2003, 03:22 PM

The other tire rotating in the opposite direction (turned by hand, trans in neutral) likely indicates that one of these is installed: an open diff; an LSD with very worn friction discs (or some other problem); another diff type that acts like an open diff when near zero torque is applied.

The other tire rotating in the same direction likely indicates a snug LSD (but also could indicate a spool or a welded diff).

The other tire remaining stationary is probably a variation of the tire rotating in the same direction, but COULD indicate an LSD with worn friction discs. But because there is nothing "precise" about this test, actual inspection of the diff would be required to determine whether or not this was the case.

Bags · 04-29-2003, 07:21 PM

Geary;
Thanks, thanks, thanks, I got it, makes sence now. It appears there is work to be done!!!
Mike

ChrisBennet · ChrisBennet's Garage

Go to the Rennlist Web Forum on Performance Modifications and do a search on "Why are the 993 LSD bad".
Here are some excerpts from what Steve Weiner wrote:
"OMG, You would NOT want the 993's LSD,...its a real POS. Stay away from those."
"1) The number of drive & driven plates is 1/2 of the early ones and they are normally all worn out by 25K miles.
2) The 993 LSD cases are cast, not forged, and are prone to break."

-Chris

///alpinepower · 05-14-2003, 06:47 PM

your diff is fine. when the car is in gear, the wheels should spin in opposite directions when you spin one by hand. if your diff were worn and the car in gear, the other wheel would not turn at all.....that's how diffs work.

Geary · 05-15-2003, 07:08 AM

When one tire is turned, something somewhere else has to turn. If the trans is in gear, with an open diff, the turning is in the spider gears --- thus the other wheel turns. When the trans is in neutral, an open diff STILL offers less resistance than the pinion shaft, and thus the other wheel still turns in the opposite direction.

If you have an LSD and the trans is in neutral, the diff will no longer yield, and it will be the pinion that rotates, with the opposite wheel rotating in the same direction as the tire you're turning. Put the trans in gear, and nothing wants to turn -- but with increased muscle, a worn LSD will let the other tire turn in the opposite direction. A tight LSD won't allow the other tire to turn in the opposite direction until the breakaway torque is achieved, which is likely over 50 pounds.

In the case of the other tire remaining stationary when the car is in neutral, something on the other side is offering more resistance to turning than the pinion shaft, so either the pinion shaft alone is turning, or the pinion is turning AND the diff is yielding (possibly a worn LSD), or possibly there is merely brake drag on the opposite side.