Replace the wheal bearings or look further? Differential?

[wwilme]

We've been driving our '06 XC90 2.5 AWD around for months under the happy assumption that the whirring noise coming from the rear was the wheel bearings going bad. They all do eventually, right?! A cheap enough repair and certainly something well within my typical Saturday afternoon time limits and mechanical capabilities. The noise was not there when I replaced the rear rotors about 5k miles ago or I would have just ordered the parts and just done the job while everything was apart. Anyway, I was going to place an order for parts last weekend and started checking around on the forums to see if anyone recommended one brand over another as FCP Groton has offerings over a wide price range. What I found was that some folks were saying that it could be the differential instead. In my case the noise is not different whether going around a right or left turn, but it is speed dependent, so I became suspect of my intitial diagnosis. I jacked up the XC90 one side at a time and tried to get some bad wheel bearing like movement out of the wheels. Nothing doing. So I spun the wheel and listened and I could hear a faint whir whir whir coming from the differential as I spun the wheel. So I dropped the car, jacked up the other side and listened again. Same noise coming from the differential. So I am now not so sure about my initial daignosis. My neighbor has a lift that is going to let me use this Sat afternoon 9/21 to check more thoroughly, so I am looking for you pros to give some advise on what to look for so I don't waste the opportunity. One thing I noticed is that when you spin one side, there is no resistance and the drive shaft up to the front will spin. If this is not normal, please let me know. The car was in park when I checked it out last weekend, so maybe the transmission parking cogs only lock up the front end. When the car is on a lift, what should I be looking for beyond the noise and location of the noise? Will the rear axle only engage when the front axle detects slip or is it always engaged? Not sure how these things are supposed to function here. Even if I end up having to replace the differnential, they don't seam that expensive. If replaced, do they have to programmed to the care like so many oher parts? What advice can you pros give so I make the best use of my time on the borrowed lift?

[hoonk]

Usually if a wheel bearing is bad it will be noisier when loaded (swerving left and right will change the noise). A differential usually makes a whining noise (much like the ex ) that increases in pitch with speed.

If you jack up one wheel and spin it, (leaving the other on that axle stationary, on the ground) you will hear the normal noises from the spider gears. (yes, confuses people who fix cars for a living also)

"The car was in park when I checked it out last weekend, so maybe the transmission parking cogs only lock up the front end."

The driveshaft to the rear wheels will not turn if the car is in park. It can't, the angle gear/driveshaft is connected directly to the front differential housing before the front spider gears. (unless your "sleeve" is stripped)

"Will the rear axle only engage when the front axle detects slip or is it always engaged?"

Yes the Haldex only "engages" when the front wheels are traveling at a speed "X" faster than the rear.

I have seen many cars diagnosed with noisy wheel bearings or bad differentials turn out to be bad wear patterns on the tires. "Scalloped" wear patterns usually on off-brand tires produce some really bad noises sometimes. Rotate/replace/swap the tires first to make sure it's not just bad tire wear pattern causing the noises.

A new/exchange replacement differential $$$$$ would be very expensive but would not require a software download. A dem?, differential control module would but I have never had to replace one.

[scirocco]

Reviving a dead thread, but i found it and it helped me.

Situation: 2003 xc70, 280k miles. Had replaced both wheel bearings a year to so ago. One Moog and one off-brand Autozone (only one moog in stock)

Low growling that starts at 30mph an continues, progressively louder and certainly speed-linked.

With the wheel off (but brake disc on, axle in) I could not detect any play at all in the bearing and got worried... what if it's the diff? Came here, found thread.

Went for a drive to see if tires? Current ones are very bad, because not spending $$ on car which may get dead soon. So i rotate the tires so less-bad ones on the front.

While on drive, no amount of left/right would change the pitch or noise, looking bad. Not the tires, noise exactly the same. Then i realized... critical differential diagnosis (hehe) includes varying speed/acceleration.

If the diff is bad, the noise should lessen or go away under coasting conditions, and possibly change a lot under engine-brake conditions. My noise did not. Feeling half-better, I replaced the bearings (and axles, struts and ball joints while at it) and voila. Noise gone. The bad bearing has as just very barely detectable noise/grittyness in in when turned by hand. Now I can buy new tires! (and do a timing belt... last done at 185).

TL;DR: If the differential is bad the noise will change between acceleration, deceleration and coasting in neutral. Bad bearings do not always change noises when turning.