NOTE: I define the cogging sound as a metallic / crunchy / semi-snappy sound as if two large pitch toothed gears are trying to mesh and they not quite in proper sync / phase when they do mesh.


Long question, lot's of detail, but if you know LSD operation, I'd appreciate you taking the time to read and comment.

My car exhibits the following and I'd like your thoughts;

1. Driving off straight from a stop = no unusual noises
2. Driving off 90 right from a stop = slight "cogging" noises, (probably the diff trying to make each wheel turn at the same RPM.
3. Driving off 90 left from a stop = slight cogging noises.
4. Reversing from a stop = noticeable cogging noises, louder than driving forward.
5. Reversing and turning 90 right = noticeable cogging noises, louder than driving straight back.
6. Reversing and turning 90 left = noticeable cogging noises, louder than driving straight back.
7. Reversing while coasting backwards in Neutral = no cogging noises.

I could convince myself that all of the above is normal operation for a normal LSD except, why would it make noticeable cogging noises when reversing straight???

I know that a diff's job is to try to get the wheels to rotate at the same RPM, but mine acts more like a locker / spool instead of a limited SLIP diff.

I drove a babied 997.1 GT3 and a stock but often tracked 996.2 GT3 recently and they do not exhibit any cogging noises.

I changed the transaxle fluid which made no difference and also added grease to each of the 4 CV joints on the off chance that it was a dry or bad CV. No change with greasy joints.

There are several threads (6 speed / Renntech) where others have asked similar questions to mine and the general consensus = that this is normal.

I'm not so sure since I keep going back to why it makes noticeable cogging noises when reversing straight???

My next step is to remove and inspect the LSD unless "you" convince me otherwise.

Thanks for your time.

ps.

1. My car = 37K miles, babied, 3 DE days in it's life.
2. Driveline exhibits no unusual noises / lash when on the lift and I shake / spin things.
3. LSD seems to work correctly on the lift, both wheels spin in the same direction when rotated by hand and I can stop either wheel with light hand pressure when rotating the opposite wheel.
4. I do hear gear meshing noise from the diff area if car is running on the lift at idle in 3rd or 4th gear. I have no idea if it is normal or noisy.

Craig

 

Never heard the term cogging, when referring gear related noise. Usually in an open diff a whine is a result of not enough lash (clearance), where quiet but a clunck when loaded or unloaded is excessive lash. This is between the ring and pinion. LSD's generally have clutches and many require a special oil and some an additive to make them work as designed. Many can be perfect and exhibit a somewhat jerky action that may make audible noise as well as be felt in the controls and seat of the pants when turning or changing directions.

How long have you owned your car? How long has it been making unusual noise. If you are mechanically inclined, change the fluid. If really a good wrench, pull the diff, check it out. There have been various bearing failures reported in trans and diffs. Could be this be what you are hearing?

A diff's job is to allow wheels to rotate at different speeds when turning. An LDS is to equalize power to both driven wheels when going straight or turning.

Any noise when driving doing tight esses in 1st, 2nd, 3rd gears? Let us know what you find.

 

I've had a Guard Billet 40/60 LSD in my car for 4+ years. I recently had it rebuild with an aggressive 80lbs preload. The OEM GT2/3 diff by comparison is a measly 7.5 lbs and as such typically wears out after a few track days. In any case, my Guard make no noise whatsoever. If your OEM is noisy, something is not right.

You state that:

"LSD seems to work correctly on the lift, both wheels spin in the same direction when rotated by hand and I can stop either wheel with light hand pressure when rotating the opposite wheel."

This indicates that your LSD is shot. Your preload is essentially nonexistent due to the weakness of the OEM clutches and belville washers. Time for a rebuild. Call Guard and Matt can set up with new guts.

 

Thanks for taking the time to reply.
As I said above, I have changed the fluid with no noted change.
I plan to remove this weekend an perform a visual inspection.

Cheers

 

Thanks, I'm in contact with Matt.

 

Update.
I bought the rebuild kit from Matt @ guard.
(clutches / plates / bellvilles)
Installed same.
all is good.
Saw no obvious issues with previous parts.