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I have what I think is a rear differential that is failing. I have a fairly loud whine that sounds as if it is coming from the rear of the car - speed dependent but not gear dependent so I think I've ruled out the transmission. I've looked for leaks and found none. All the joints seem to be in good shape so I assume the whining is coming from inside the differential. What parts would be most likely to fail? And is it common to replace parts or the entire differential?
 

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How about changing differential oil to see if that makes a difference.
 

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Also could be wheel bearings. I would investigate further.

If you can, get the car running and up on a lift to diagnose from under the car.
 

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Whining for the diff is normally a bad pinion bearing. It is always worst and loudest in 6th gear. Is there any oil leaking fron the pinion seal? They tend to eat seals as they get looser.
 

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I'd get it into a shop that knows what to look for. It could be any of the things mentioned but if it is the pinion bearing or anything else in the diff, you want to fix that immediately. The pinion bearing in mine went out but wasn't making significant noise. Eventually it broke teeth under load and the differnetial had to be replaced. That is not a cheap experience.
 

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I'd get it into a shop that knows what to look for. It could be any of the things mentioned but if it is the pinion bearing or anything else in the diff, you want to fix that immediately. The pinion bearing in mine went out but wasn't making significant noise. Eventually it broke teeth under load and the differnetial had to be replaced. That is not a cheap experience.
Used diffs show up from time to time on ebay and other sites. Price around $750-$1000 or so these days. Of course, there is never one around when yours breaks and you need it right away.........
Regards,
Jerry
 

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Used diffs show up from time to time on ebay and other sites. Price around $750-$1000 or so these days. Of course, there is never one around when yours breaks and you need it right away.........
Regards,
Jerry
Actually, I did have the opportunity to get a used one but being new to the car and not knowing what someone else's take off had seen in it's life I went with the Dinan. Either way it's an exercise that I would have preferred to avoid.

Greg
 

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750-1000 is crazy for a used diff. You can get a rebuilt one with a better lsd or a new one from bmw for not much more 1300 last I checked!

gzig5 when you purchased the dinan unit was it just the ring and pinion and you had to have some one install it or was it a complete unit. How was your milage affected by the gearing change?
 

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How was your milage affected by the gearing change?
Dood. You drive a $70K 400hp super car....I hope your mileage question is just out of curiosity as opposed to a genuine concern.
 

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Dood. You drive a $70K 400hp super car....I hope your mileage question is just out of curiosity as opposed to a genuine concern.
here in reality I drive a 16k sporty sedan. Which I love don't get me wrong but is not a super car. Even though most Super cars do get better milage than a stock e39 m5. I was just asking because I was curious.
 

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here in reality I drive a 16k sporty sedan. Which I love don't get me wrong but is not a super car. Even though most Super cars do get better milage than a stock e39 m5. I was just asking because I was curious.

Take all the fun out of it....yeah, i know. Point is (which you got) was that in this type of car mpg is not really THAT important.

When the e60 came out, people laughed at me because I was rather vocal about the MPG. They misunderstood that my disgust was not becasue of the actual mpg but more becasue of how that translated to range with the relatively tiny gas tank. If a car only gets 10mpg, put more than a 10gal tank on it. haha.

Bob
 

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750-1000 is crazy for a used diff. You can get a rebuilt one with a better lsd or a new one from bmw for not much more 1300 last I checked!

gzig5 when you purchased the dinan unit was it just the ring and pinion and you had to have some one install it or was it a complete unit. How was your milage affected by the gearing change?
not an M Diff
 

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750-1000 is crazy for a used diff. You can get a rebuilt one with a better lsd or a new one from bmw for not much more 1300 last I checked!

gzig5 when you purchased the dinan unit was it just the ring and pinion and you had to have some one install it or was it a complete unit. How was your milage affected by the gearing change?
When I had my issue two years ago a replacement differential from BMW was $3000+ so I don't know where you are looking. 700-1k is about right for what I've seen over the last two years.

The Dinan unit comes complete and you have to swap on your existing rear cover and fill with oil, then send back your core. I ended up getting in on the 20% sale and the Dinan unit was cheaper than the BMW replacement.

I really don't know how it affected milage because I got he car in October and the diff broke in December (at 104,000 miles) and I only had 5-600 miles mixed driving on it. I'm sure it hurt milage a bit but I get 23mpg or better cruising at 65 if I can keep it that slow. The nice thing is that I am up in the fat part of the torque band in sixth gear and there is zero hesitation if I need to pass someone. For the money it is one of the best bang for the buck performance enhancements available in my opinion and I'm glad I went with the 3.45 over the stock 3.15 gears.
Greg
 

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here in reality I drive a 16k sporty sedan. Which I love don't get me wrong but is not a super car. Even though most Super cars do get better milage than a stock e39 m5. I was just asking because I was curious.
The M5 gets considerably better milage than my 98 Dakota 4x4. It's about 17/23 mpg city/hwy for the M5 and 11/15 for the truck and about 100:1 more fun to drive.
 

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Even though most Super cars do get better milage than a stock e39 m5.
What supercars get better gas mileage than the 13/21 or whatever the M5 is rated at? Most super cars get low teens at best. I guess it does depend on what you consider a super car though.
 

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I had a similar experience after getting my 2000 M5 3 years ago with 100,000 mile on it. The PPI identified worn diff mounts and during the replacement, the local Dinan shop noticed the diff felt very tight, but smooth. We went ahead and just changed the fluid and put the new mounts in. About 3000 miles later, the whining started and got progressively worse. It was road speed related and not engine speed related.

The stock diff had eaten the pinion bearings and it was full of shaved metal. I went the route of a new Dinan 3.45 diff which was actually cheaper than a new stock unit. I love the better acceleration from the lower ratio, and I can actually use 6th gear at legal speeds now. With regard to gas mileage, it actually improved about 1 mpg average overall.
 

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I had a similar experience after getting my 2000 M5 3 years ago with 100,000 mile on it. The PPI identified worn diff mounts and during the replacement, the local Dinan shop noticed the diff felt very tight, but smooth. We went ahead and just changed the fluid and put the new mounts in. About 3000 miles later, the whining started and got progressively worse. It was road speed related and not engine speed related.

The stock diff had eaten the pinion bearings and it was full of shaved metal. I went the route of a new Dinan 3.45 diff which was actually cheaper than a new stock unit. I love the better acceleration from the lower ratio, and I can actually use 6th gear at legal speeds now. With regard to gas mileage, it actually improved about 1 mpg average overall.
That's interesting. Your diff had enough friction to decrease milage that much.

Last I check at my dealer with my account the diff was about 2k that included 650 core charge. There are some places that rebuild them that start at 1k depending on the options and gearing you want.

My other car is a viper now I don't know if you'd consider it a super car or not but it's pretty hard to beat in a straight line. Even with the extra 200hp and the two extra cylinders it still gets 25 mpg on the open road. Now my m5 is my summer dd and gets 19mpg with about the same driving style.
 
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