front differential on an '03 suburban

(warning - i am no car expert by any standard). we just took our 2003 suburban in for it's 45000 checkup to a local shop and asked them to also check out a whine we hear whenever it's in 4wd-hi and also 'auto-4wd'. the guy said there were shavings in the pan, so he didn't look any further. he said we needed a new front differential -$1700 to $2000 for a used one, installed. yikes! he said it was quite unusual for this to happen so early in the life of the car.

after looking into this more, i fear i may have caused the problem unwittingly. we do a lot of mid-range (100 or so) mountain driving through, snowy and icy roads. when the temp get's into the high 30's, i would switch it into the 'auto' setting. when it drops below 35 or so, i'd switch it into 4wd-hi if the road was wet and i thought there was a chance for black ice. well, after reading up on this all now, i'm realizing that i should have left it in 'auto' until there was snowpack on the road. is this a reasonable hypothesis for the cause of the problem? also, does the mere presence of shavings mean we definitely need the differential replaced, or could we perhaps need more limited work on the existing differential?

and one last question - there is no apparent problem with the rear differential since 2wd works just fine. if we decided to wait for a few or several months before fixing the front diff (and just used 2wd only), could we cause more damage to something else on the car? we aren't using the 4wd or auto in the meantime as i know that could be disastrous.

thanks for any input.

These mid-distance trips (I assume you meant 100 miles), did they include highway driving (speeds over 50 mph)? Every 4wd vehicle I've owned wasn't designed for highway speeds while in 4wd. I believe this has to do with the front dif not getting as much wear as the rear so at higher speeds and on hard surfaces (where both axles have to spin at the same rate) the two difs are trying to rotate at slightly different rates which puts pressure on one or both to compensate. I'm not familiar with the auto-4wd but I'm guessing that it would disengage the front axle at speeds approaching highway, for this reason. That is the only reason I can think of where switching between the two would be a problem. Someone else may have more experience with this, but I wouldn't suspect that a vehicle with low mileage would have as big an issue with the difference in wear.

I don't know a ton about differentials, but what I do know is that the gears within them mesh with a very high level of precision. It's possible, but probably not as likely, that there was a defect in the front end dif assembly.

I wouldn't think that postponing the repair would harm anything else in the drivetrain, but again, my experience here is limited. Meant to ask, lots of shavings?

Hope that helps.

thanks fcdgmc. yes, 100 mile trips or more, and yes, i would drive at highway speeds. from what i read, you're basically right in your assumption about the auto setting. as i understand it, it runs in 2wd until a wheel starts to slip, and then it distributes power to all 4 wheels. this is where i should have been keeping it i'm afraid during bad but not snowy conditions. thanks again for your input - much appreciated.

oops, i almost forgot. yes, they said a good amount of shavings. the 4wd still engages though. (we aren't using it)

i bought a new 02 sub 4wd, at 20k miles i noticed a noise in front diff, the dealer replaced everything in front end because i was told that the factory overtorqued the pinion bearing, mine was under warranty. still have it a 43k miles have had no problems>?

mosesdog, you need to get a second opinion from a trans specialist either independant or dealer type. Incidentally, four-wheel drive vehicles in 4 HI are designed to be used at highway speeds. Wear and tear on the 4WD units mostly occurs(in my opinion) when they are used on "dry pavement" where there is no "slip", but this is debatable since the tires will wear before the transmissions(in my opinion). Also, neglected transfer-case fluid can increase wear on the diff. This is about all I know 4WD systems.

the shavings are from worn side bearings and I used replace them in about 2-4 hours, depending on where Murphy happenned to be.

the front diffs are toys compared to the rear so 4wd should be used when neccesary, just like the good ol' owners manual says. if you use auto4wd too much there are bearings in your tranfer case that are similar to the side bearings in the front diff. in both appearance and durability and you'll replace those.

your guy probably has the last "bad" one he replaced unneccesarily out back and figures he can make some cash, or he just doesn't do diffs.