Is it normal to hear the limited slip differential on a M6 C5 lock and unlock in tight, low speed turns? Or is this a sign of missing/expired "friction modifier"?

Any particular reason to use the GM synthetic rear axle lube....is Mobil 1 75/90 ok? Is there a 3rd party substitute for the GM friction modifier?

thx...

Dan

 

Yes and no. Have you done anything to the rear end differential? If you have done nothing it would not hurt to bring it in under warranty. Depending on what you have done, you may or may not need more friction modifier or again it will not hurt to bring it in under warranty.

If you have not done anything, bring it in and if they say nothing is wrong you may want to wait till close to the end of the warranty period(app 32-36k miles) to make a differential oil change! First you already have a synthetic gear oil 75W-90 and friction modifier factory fill The friction modifier has a GM spec # and I know STA LUBE makes a GM spec equivilent. The reason why you might want to wait to do a differential fluid change is that your seals MAY start to leak and your covers might need retorqing or one or more of your seals might need replacing. Normally the rear differential fluid and F/M R/R are covered under warranty. My 2001 Z06 has 37k miles on it and the gear oil has been swapped out 4 times already! 1. Factory fill 2. is R/R that I had done 3. is the first time they swapped it out after they put trace dye on the first leak 4. is the next time they swapped the diff oil and friction modifier after a two seal change! :D


[Modified by ruking1, 5:05 PM 8/19/2002]

 

Thanks for the info. The car has 37k miles, and is therefore out o'warranty. The dif did indeed sprout a leak at about 10k miles and was repaired...and seems to have been oil-tight ever since.

Does the friction modifier deteriorate over time? Can one just add more modifier? Is this audible unlocking/locking normal?

thx...

Dan

 

Get ya some AMSOIL Series 2000 75w90 :yesnod: modifier is in it.
And be done with it 4 ever.

 

Here are some pics and tips for changing the rear diff fluid:
http://www.picturetrail.com/gmblack3/647207

I used the Amsoil series 2000 and the GM additive, no problems at all.

Here is some info that I gathered from internet sites and emails:

Amsoil and Redline claim that there fluids already have an additive in them. Amsoil says it both ways on there website:

AMSOIL gear oils are formulated with friction modifiers for limited slip differential applications. There is no need to add limited slip or additional additives.

AMSOIL Series 2000 Synthetic Gear Lube provides limited slip performance. Although generally not necessary, original equipment manufacturers' limited slip additives may be used in conjunction with AMSOIL Series 2000 Synthetic Gear Lube.

Redline says:

LIMITED-SLIP COMPATIBILITY Red Line Gear Oils, with the exception of the 75W90NS, already contain friction modifier required for operation in limited-slip differentials. Additional additive is not required.

I have emailed Redline and asked them if it would not hurt to add the GM additive to Redline.

Here is the reply from redline:

The 75W90 would be suitable for the limited slip in your Z06 it contains 5% friction modifier, typically what is needed to reduce the chatter from the limited slip when making a slow tight turn. Adding more friction modifier if it isn't needed reduces the lock up of the limited slip and reduces it's effectiveness, so I wouldn't add it unless it is needed due to chatter and then only just what is needed. The friction modifier isn't a lubricant but just an additive to make the fluid slipperier for chatter control. The 75W90 should be suitable right out of the bottle without the need for the additive. Thank you for your interest in Red Line Oil.

It states in the GM manual that no WOT for 300 miles after the diff fluid is changed.

 

I would say go with a complete change. Not sure what the chemistry is but too much modifier will cause your limited slip to turn in to a conventional rear end with little or no power going to the loaded wheel. You might want to consider going with Redline which has a 5% FM built in. I was geting noise out of my unit and it went away after I made the switch. 2 quarts of 75w90 synthetic gear oil should do it. http://www.myoilshop.com/SynGrOil.html

 

I'm running straight Redline in mine without issue.

 

If you are just getting the chatter in slow speed tight turns then you have the option of adding more friction modifier, changing to another oil with modifier or staying with GM.

If it doesn't chatter in normal driving, you also have the option of leaving it alone. The more friction modifier you add, the more slip you allow until you reach the point that you have defeated the limited slip capability of the differential.

If it were me, I would change it out and use straight Amsoil Series 2000 75W-90 with NO additional friction modifier. It should not be necessary. Then, after changing, do 10 tight slow figure 8's in a parking lot. The rear should be quiet after this.

Just my $0.02 :yesnod:


[Modified by vettenuts, 12:06 AM 8/20/2002]

 

Thanks all for the exellent inputs...

Sounds like a change to Redline would be the easiest way to go....

Dan

 

While you have it up in the air you might want to think about 4 quarts of D4 ATF for the MN6 transmission. While there has been some debate if it smooths out shifts when hot (I think it does) there is no question that it works better on cold days. That will give synthetics in all the running gear.

 

Had the same problem with my A4. Dealer drained, replaced the left seal and o-ring and new gear lube. Three weeks later the sound was back. I replaced the OEM stuff with AMSOIL and walla, no noise. :yesnod:

 

Thanks again to all. I ended up replacing the lube with the Redline stuff and the noises seem to have gone away. Not to be a critic or anything, but the correct spelling is voila. It's French for "there is".

thx,

Dan


Da