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Old 06-30-2011, 01:13 PM   #1
BreakfastBurrito
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how to diagnose if axle or diff?

I got new tires, took it to get aligned. They said my tires are fine, the hum is coming from the rear end. I lifted my car and put it in gear but I can't replicate the sound when lifted. It sounds like a grinding hum and changes with the speed of the car. I changed the rear right bearing sometime last year so thats new. The gear oil is good.

How can I tell if its the axle or the diff itself? I thought about removing the axle and driving the car like that to see if I can isolate the sound. Anyone done that? I imagine the only thing will happen is the "spin out" light might go on.

help!
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Old 06-30-2011, 01:40 PM   #2
Steven747
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turn at low speeds and you hear clunking which i believe means diff is shot
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Old 06-30-2011, 03:38 PM   #3
Werkz
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Tight radius turning can diag an issue with the carrier or spider gears but not really the ring and pinion. Does the noise get louder under accel or decel? If it does you might wanna check your backlash and pinion preload. If it gets louder or "faster" when turning in one direction or the other you might wanna check the wheel bearings again. Then again noises are tough... could be the tires.

BTW what vehicle are we talking about? Assuming its a 325i, it has a open diff. If you took out an axle, which i would never recommend, the car wouldnt even move and the diff would be screaming.

If you cant replicate it with the wheels off the ground its probably load related. It also kinda rules out the axles cuz with the suspension down the angles of the pinion u-joint and c/v's are at their most extreme. Pop the cover off, take a look and change the oil.
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Old 06-30-2011, 05:15 PM   #4
BreakfastBurrito
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It is a 325i, and you're right about the diff just spinning if I released it on one side. Just realized that. Definitely not the tires.

I'll try the low speed turns but I don't know that it will work with a RWD car. This works with a FWD car because you're flexing the joint. This only happens when the suspension is compressed or extended. slow turning wouldn't provide much skew.

The hum is directly correlated with the speed of the car. *I think* it's the outer universal of the axle because it seemed to happen only when the car was on the ground and vertical load being applied through the axle shafts.
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Old 06-30-2011, 10:24 PM   #5
BreakfastBurrito
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so remember that the diff will just power the wheel that is "loose," I lifted one side of the car and ran it, I hear a hum/roar, did the other side and I hear the same noise.
so I conclude that it is my diff. I had checked the oil but I may have just felt "residual" , i'll have to check it again.

please let me know if anyone has any other ideas on how to check it.

If it is indeed the diff... well 3.64 here I come.
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