Help with differential problem...
 

My car has slowly developed a knocking sound from the rear end. At first it was intermittent, and only when accellerating. Now it's all the time on accelleration and when climbing hills. I had my indy mechanic check it out, and he said the differential is failing. Actually, he said the problem is the pinion bearing, but that on these cars it's actually easier to replace the whole diff. They also said it doesn't need to be replaced imminently, that I should just monitor the noise and replace it when it gets real bad.

Well, it sounds pretty darn bad to me already. My wife and I are moving up north in a couple weeks and I'm wondering if I should get it replaced before then for safety's sake.

I've read a little about this problem in the archives here, but wanted to get a second opinion from you guys. Am I really better off replacing the whole diff.? Should I do it right away, or wait?

I cannot help you with the noise situation as I have never heard what a bad diff sounds like on these machines. However, I would be very cautious about repairing your own diff unless you can be absolutely sure that you have a qualified mechanic who works on these units. I would hazard a guess that they are few and far between. I have had virtually no luck finding a qualified shop, locally, to recondition the head on the SD. I may end up taking it down to North Carolina to a shop recommended by my indy. Much less risky than the alternative.

My experience has been that a knocking sound from the rear end of the car, which can also be described as a clanking / banging when it gets worse can be from a CV joint gone bad. This is a fairly siimple DIY chore though not trivial. The cost with new half shafts would be way less than a new differential, especially if it really was a CV joint.

I agree with Lycoming-8 on that. The first thing I though of was the half shafts, I had one go out knocking. it made a "ka-whunk, ka-whunk" noise very bad.
The remedy for a bad differential, if in fact that is what is bad, is to swap one from a wrecked car. Replacing the gears is difficult to do right, even if you find an experienced shop, torquing the pinion lock nut is very touchy.

You sorta take a chance on getting a whiner if you get one from a high mileage car. For turbo diesels, I recommend finding a differential from an '85 car, the same model car as yours, in '85 Benz went to a 2.88 ratio. The cars really take on a "cruise" mentality with one of them instead of the 3.07 stock ratio. My '83 300D was "updated" to an '85 diff. I really like the results, and the fuel economy is slightly better too, with hardly perceptible loss of acceleration.
Watchout though, the SD differential uses a case that is slightly different in dimensions from the 300D and you could use it in a 300D provided you had the right shims to keep the caster to spec.

I'm no MB guy but a pinion bearing is a pinion bearing and a pinion bearing going south will not make a knocking noise. If the bearing is shot the pinion shaft will have wobbled around enough to destroy the seal and you'll be leaking gear lube big time but it won't make noise especially a knocking noise. Sounds like your wrench is trying to pay for his kids Christmas presents early by selling you a rear axle job. Look into the CV/half-shaft thing as the others have said.

Thanks for all the tips. I asked the mechanic to consider that it might be a bad CV joint, and the answer I got, again, was that the pinion bearing/diff is bad. I'm inclined to think it's a CV joint, too, because the noise is definitely worse when the rear end is squatting from accelleration or climbing a hill. If my undertanding of the driveline layout is correct, the angles between the driveshaft and the differential do not change under these conditions, but the angles between the halfshafts and the diff. definitely do change. Am I right in thinking this might be a reasonable way to diagnose the problem?

Assuming it's one of these problems, do you think I could safely put up to another 1,000 miles on the car before getting it fixed, or should I not wait?

Your are correct.

I'm quite sure that you could put another 1,000 miles on the vehicle without getting it fixed. Of course, you already know the points of operation where the symptom is exaggerated, so, avoid these if at all possible.

Followup: Just wanted to let you all know that since first posting this, I have moved and had the car looked at by a different Indy mechanic. This guy guessed that the grease in the rear CV joints may have dried up, and suggested pulling back the boots, cleaning up the joints, and packing in new grease. That seems to have worked, and for just $125 of his time, the noise is gone. Thanks!

SWMBOs car has a badly torn CV boot on the left side. I knew they were in bad shape but didn't realize how bad until preflighting her car prior to a quick trip to OKC. The thing was ripped completely open.

I drove to OKC with crossed fingers. It wasn't until a week later on the trip home that the thing started clicking and popping. For 50-75 miles, I could get it to quit just by reducing power and unloading the driveline but finally it wouldn't quit. So, I stopped at a truck stop and tried to buy some 90wt hypoid oil to squirt into the boot and on the joint...the only thing I could find was something called 'fifth wheel lube'...ick, nasty stuff. Anyway, I squirted it in through the ripped boot and in the joint and rocked the car back and forth, and squirted some more.

That was last spring. It hasn't popped since.