Today I started getting a thump thump (2 thumps) sound from the rear end when from a dead stop, making a left hand turn.
I have also experienced more of a growl rather than a grinding sound when pulling in and out of parking spaces and making turns in parking garages etc. especially when the car is cold.
Are these the typical differential symptoms that lead the dealer to change the rear end fluid and add the additive?

 

Yes. Mine did the same thing. The dealer changed the fluid and added some additive. That worked great for about a month. Now the sound is back and it's worse so back to the dealer it goes...

 

Your Limited Slip Diff is failing. Common problem. The dealer will change the fluid. Most likely it will fail again. This is also common. The dealer will then change the clutch packs. Unfortunately other damage may be happening that GM is not yet willing to fix. I am going through the same thing.

Good Luck.

 

Good info. Thanks.

looks like I got two threads going on this in error. My apologies...

 

Positraction rears have been doing this since at least 1965, so don't expect any permanent fix from GM. the problem is that "spirited" driving while the posi clutches are seating in can cause them to glaze. The best way to avoid a recurrence of the problem is to religiously follow the break in schedule when new, and again any time the Positraction clutches are replaced.

 

I get the groan noise too when backing out of the garage and turning, but it only happens the first time the car is pulled out of the garage for the day. I am guessing that once the fluid in the rear diff gets worked in after the car has been sitting the clutch packs get lubricated and the noise goes away. Do you guys think this is normal or will I be looking at repairs here soon?

 

Mine started the same way both times. It gradually got worse both times. I'd take it in.

 

Folks;
Only one comment-hope you followed GM recommendation for break-in.
Take Care.

 

Break-in has nothing to do with this. It is a known defect. And yes I followed the break-in procedure.

 

Same with me. I'll be scheduling an appointment with the dealer this week.

 

Thanks for the info... Sounds like this may be a common repair in the future. I am glad to know that the extended GM Major Guard warranty will cover this repair. And, yes, the car was broken in per the owners manual recs.

 

same with me. Off to the dealer I will go.

 

In the manual there is nothing stated about breaking in the rear end differential. All it says is that you don't need to add fluid unless there is a leak or problem and I hear it is a life time fill fluid. In the manual it Just mentions break in at 500 miles on the engine and brakes by keeping it under 55 varying speeds and rpm's and no hard braking.
I have never heard of breaking in a rear end diff. and auto transmission until recently

 

The owner's manual isn't exactly an in depth technical manual. It doesn't give detailed reasons for everything it tells you to do. But if you talk with automotive engineers, they'll tell you the primary reason for break in is to allow the differential and transmission gears/clutches to mate in properly.

 

So is that 500 miles also for the auto trans and rear differential and same technique for breaking in engine?

I did the engine oil change right after break-in period.
If this is all applicable then I better get my rear end diff fluid changed and auto trans fluid changed as I have 11,300 miles

 

The engine is broken-in when it leaves the factory. Moly rings seat in the first few hundred revolutions of the engine, or they won't seat at all. This is very different from the old days when it may have taken a hundred hours of engine operation to seat the rings. Forget the old ideas about engine break-in, they no longer apply. If you changed oil at 500 miles, you just wasted perfectly good oil.

The break-in procedure is just to let parts that are impossible to machine in exact mating pairs to come together smoothly. Primarily that's gear teeth. Matching hypoid curves precisely on two different gears is beyond the machining state of the art, so the parts have to be allowed to wear in together before being heavily loaded. Note that we're talking about less than 50 microinches here, there will be no gross wear particles that could cause problems. There is NO NEED TO CHANGE THE FLUIDS.

The transmission and differential fluids will normally last just fine for at least the first 50,000 miles, often far longer depending on how hot you run them. In other words, in an all out GT1 racing car, you might change the fluids as often as every season. But in a street car, this is totally unnecessary. The differential grease will last for years and years of normal driving, and automatic transmission fluid is good for at least 50,000 miles as long as its temperature stays below 300F. The very best thing you can do with transmission and differential fluids is LEAVE THEM ALONE, unless you have a leak, or obvious signs of burnt fluid.

 

This is great news to me as I'm at 11,400 miles and haven't changed any fluids on the transmission or rear end diff. I did change the oil just under 1,000 miles and the next change was at 58% on the OLM.-Waste of good oil again second time?
So why then would the manual say to vary our speeds up to 55 mph for the engine breakin is at 500 for engine and brakes?

 

Mine did the same. My dealer replaced the fluid and at the same time ordered the clutch packs knowing the change of fluid would not fix it. So far, after 200 miles, it is quiet.

 

The very best thing you can do with transmission and differential fluids is LEAVE THEM ALONE, unless you have a leak, or obvious signs of burnt fluid.[/QUOTE]

The engine is broken-in when it leaves the factory. Moly rings seat in the first few hundred revolutions of the engine, or they won't seat at all. This is very different from the old days when it may have taken a hundred hours of engine operation to seat the rings. Forget the old ideas about engine break-in, they no longer apply. If you changed oil at 500 miles, you just wasted perfectly good oil.

The break-in procedure is just to let parts that are impossible to machine in exact mating pairs to come together smoothly. Primarily that's gear teeth. Matching hypoid curves precisely on two different gears is beyond the machining state of the art, so the parts have to be allowed to wear in together before being heavily loaded. Note that we're talking about less than 50 microinches here, there will be no gross wear particles that could cause problems. There is NO NEED TO CHANGE THE FLUIDS.

The transmission and differential fluids will normally last just fine for at least the first 50,000 miles, often far longer depending on how hot you run them. In other words, in an all out GT1 racing car, you might change the fluids as often as every season. But in a street car, this is totally unnecessary. The differential grease will last for years and years of normal driving, and automatic transmission fluid is good for at least 50,000 miles as long as its temperature stays below 300F.

The very best thing you can do with transmission and differential fluids is LEAVE THEM ALONE, unless you have a leak, or obvious signs of burnt fluid.

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Today, 07:19 AM


This is exactly what the manual recommends.
Now with this differential problem some members are having this is even more so of a reason not to touch anything as it then may cause a leak when switching rear end differential fluids so "If it ain't broke don't fix it".

 

Bringing my noisy rear end to dealer Thursday. Service Rep said they 1st replace fluid with additive. Then if still noisy after 1000 miles they replace the parts inside.