I was at a festival on Saturday, where parking was on a grassy slope. When I left it was raining and I had to drive up the grassy slope. I lost traction and gave it a little gas, but it resulted in some annoying chatter in the differential and not nearly the traction that I thought the limited slip should provide. I eventually took a more gradual route to get up the hill.

Question: Does that indicate that I need to add more Limited Slip additive to my differential??

Thanks in advance for any insights!

 

Hard to say. Grassy slopes, especially wet ones are slippery. Chattering might indicate a need for friction modifier, but if it were my car and I were concerned, I would change rear drive fluid and put in full synthetic 75W-90 and add appropriate friction modifier. This would give you an opportunity to check the drained fluid for metal. Then you would also be sure of the correct ratio--if your car has a stock differential with clutches.

My car has a WaveTrac aftermarket differential and requires no friction modifier.

You might also, assuming your car has a stock differential, jack up the rear and with the car in gear or park, try to turn one rear wheel by hand. A stock differential with clutches and Bellville spring tensioners in good condition will require quite a lot of force to turn one rear wheel--new condition will require about 100 foot pounds. If either rear wheel turns easily, it is likely that the respective Bellville spring for the side turning easily has broken and you will also likely find metal in drained fluid.

 

Possible, but from the description (and lack of any such indication during normal driving) I'd suspect there may be an issue with the clutches themselves. Have you checked those?
The way I've always done it is to raise one rear wheel, put a torque wrench on the axle nut and measure how much torque is required to turn the wheel with the other stationary/on the ground. I don't remember the correct amount without digging through the service manual.

 

How's the car in normal driving? Does the rear end chatter when making a tight turn on asphalt?

 

The noise you were probably hearing was the traction control kicking in? If the tires spin the system uses the ABS system to regain traction. I doubt it is a limited slip additive issue. I would not just go and add more as more can cause too much slip if it is not needed. If you still feel the fluid is an issue just drain and fill it with a synthetic fluid that already has the additive in it.

 

If it actually was the diff chattering, that's it not wanting to unlock. When you have a chattery diff that requires more friction modifiers, you will not lose more traction than normal, quite the opposite. Your chattery diff description and surprising lack of traction do not go hand in hand with a friction modifier problem in the gear oil in my experience.

 

A big THANK YOU to everyone who has provided help and insight into my chattering issue. The first (easiest) thing I plan to do is jack up each side in turn and see what the difference in effort to turn the raised wheel is. If they are roughly the same, I'll probably drain the diff and replace the fluid (synthetic is a great idea), and examine whatever drains out for metal particles or gold ().

The car is a 2003 vert, and I've never had any issues with it with respect to the differential and I do autocross it and (heaven forbid) occasionally drive it in rain. If it needs any rear end work, I may bite the bullet and trade the 3.15 automatic gears to something a little wilder, like 3.73. I do a fair amount of wrench turning, but like most Vette owners, I take it to professionals when it comes to the transmission or rear end.

Thanks, again!

 

Regarding going to 3.73 rear drive- late 2015 my 3.15 rear drive started to howl at Buttonwillow Raceway. Turned out that both Bellville springs were broken, metal in the drained fluid. I opted for a 3.73 rear drive with WaveTrac differential so I would have better acceleration on the straights and not have to worry about clutches in the differential and the WaveTrack requires no friction modifier. The new unit went in in April 2016 and I am happy after seven more years and twelve more days on road course tracks, Laguna Seca my favorite.

Keep in mind that the power control module will have to be reprogrammed if you go with lower ratios than 3.15, there are no factory programs available for those lower ratios and the C5. If you do not get the PCM reprogrammed, the car will hit rev limit before upshifting from 1st to 2nd or 2nd to 3rd when in 3 or D. Mine was reprogrammed by A&A Corvette in Oxnard CA along with fan settings. I only use 2nd and 3rd on road course tracks and EVERY shift, up or down on track is manual The car GOES!

If you let the A4 downshift automatically coming out of a turn in 3rd at high lateral G and high throttle you may be in for a taste of wheels in the dirt- ask me how I know.

Find a shop that KNOWS how to program the C5 PCM for lower ratio rear drives.

 

Thanks for the info! I have a great Corvette shop nearby that can handle those details. The problem is how much of my plasma am I going to have to sell to afford the upgrade!!
Thinking aloud: Maybe If I determine the stock diff is toast, I can buy the WaveTrack diff and install it in my garage. I have skills as long as I don't have to crack the diff open and dig around inside.