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Old 01-25-2007, 07:48 PM   #1
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Default Diffrential question

On my 1999 Firebird 3.8l when I go around a turn, especially down a left hand hill turn, I hear a noise in the rear end. It's not a grinding sound but more like a tunk, tunk, tunk. But not all that loud. I don't hear it anywhere else, and I'm useually not accelerating. any ideas? I don't think its the bearings, but not sure...
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Old 01-25-2007, 07:57 PM   #2
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Bearings would be the 1st thought. It could get worse quickly though, so tear it open and see what's there(or broken).
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Old 01-25-2007, 08:49 PM   #3
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Make sure you've got enough fluid in there!!!!! Also, if you've gone through much water, you probably need to change the fluid because if water gets in it will turn the diff oil milky and it won't lube things as well.
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Old 01-26-2007, 05:55 PM   #4
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Thanks...I'm going to change the Diff. Oil. I imagine you have to just open the cover, than put a new gasket on and fill it throught the top...Never did much work on the drive train or Bearings...YET. I'm not sure on what kind of gear lube to use also, probally have Diff. Gear lube in the parts store. Also need to see how to do the bearings, what kind of tools, ETC..I've packed bearing myself before, and remember its kind of messy, but know how to do that...I might do all that, but some reason I thing there is something wrong with a bearing inside the pumpkin on a axel, or somekind of sleeve on that axel that is worn???..but would like to see some pictures...I'm waiting on a haynes maula I orderd...But I'm going to search here also...
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Old 01-26-2007, 11:54 PM   #5
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There will be NO packing of the bearings in this case!!

You'll need differential fluid/oil/lube. It's usually 75W90 or 85W140 and your owners manual or Haynes should tell you the specific version you should use, and even if you need synthetic oil versus standard. You'll know you have the right kind of gear lube because it will STINK! Two quarts generally does one car or small truck rear, particularly if any additive is used(ie limited slip additive).

I can't say I remember much about the rear for your car, but I would think the axles have bearings on the outer ends, which are pressed on. Inside bearings are side or center section(chunk) bearings and pinion bearings. The pinion uses one and the side bearings call for one per, so two are needed. Those are surely pressed on.

When you remove the cover, have something under it to catch your old oil so that you can inspect it later. Common tools should get the whole thing apart, but some bolts are stubborn and may require more attention. It's a good idea to inspect the ring&pinion too, so you'll know whether or not to replace those now(maybe with a lower set). If anything appears to be blue, replace it, gears included. Inspect the entire unit so you'll notice anything which may be the real cause of your problem.

The cover may need to be pry'd loose, but loosen all bolts 1st. Keep a bolt in the top for last. This way, you'll get leakage from the bottom and not the sides(at least not as much). Be sure to remove ALL non-metal from both the cover and the differential housing mating surface areas so you won't have a leak right away. That would suck.

If you end up needing new gears, either get someone who's done this before to help, or pay to have it done and try to watch and learn. Replacing a gearset requires setting lash. It isn't the most difficult thing in the world, but it's best to see it done or be a part of it rather than trying yourself without supervision. Wrong in either direction, and you'll know it very soon. And, you'll have to tear it all back apart to fix any mistakes.

When you go to put it back together, be sure not to over-tighten the pinion or cover, or under-tighten the ring gear. Be sure lash is correct and that you have all the parts needed to do it right the 1st time. You may need 5 bearings, 2 seals and a crush sleeve, up to 2.5qts of lube, a dyed lube and small brush to check lash, and you'll want some locktite handy. Nearly all you'd need will come in certain repair sets, but you can't find the right stuff at the general chain parts store, so be careful what you order, or find a gearshop or a place which specifically sells this type of thing. NAPA may have the right parts, but even they usually will have to order it. A hotrod shop or truck modification shop is the kind of place you'll want to look for this stuff.
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Old 01-27-2007, 03:26 AM   #6
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Uhhh...if it's going "tunk, tunk, tunk", it might be something as simple as a bad brake caliper on the outside wheel in the turn. The tunking noise could be the brake pad getting stuck, binding and releasing. I'm not saying a bad bearing is out of the question. I'm just saying not to over-think the problem. A brake caliper is subject to way more abuse than those axle bearings and it is more likely to fail. Personally, I'd rather replace a brake caliper or wheel cylinder than tear apart a differential to fix bearings. So in other words, check the easier stuff first. If it's bad, broke or worn, replace it and see if it fixes the problem. Either way, you'll have to fix that bad part eventually. Better to do it now before it fails and leaves you stranded than to wait and regret it. Especially if it fails in 18 degree weather, in the snow, befor ethe snow plows went through. Yeah, trust me, that sucks.

If the noise is accompanied by the entire car shaking, I'd be inclined to suspect a bad clutch pack in a limited-slip differential before a bearing. Bearings will take alot more abuse than most people think. The clutch packs are comparativly squishy and will wear down much quicker. That is if your Firebird has a limited slip diff. I think it does though. I might be wrong.

As far as internal bearings, I don't think they are pressed on anything but the pinion. If the pinion is bad, it would make a catastrophic noise and it'd be hard to move the vehicle. However, it is unlikely that the pinion bearing is bad.

Check your owner's manual for what lube to use. If you don't have an owner's manual and you don't want to wait for the Haynes manula to show up, call the service department at your local Pontiac dealer. They can tell you exactly what lube you need to use and how much of it. It's important because if your diff takes 90 weight and you put 75 in, you'll tank it faster than a baby can shit a clean diaper. Also, some rears have been known to use odd stuff like ATF. Most of them are foreign cars though.
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Old 01-27-2007, 09:08 PM   #7
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Wow...Good stuff...I'm going to look at it tomorrow. The car does not shake or anything, so thats good...I'm going to check the caliber as you said...Might be just that...Will I know by looking at it if it is messed up, or maybe will it move or shimmey? I guess maybe worn pads might make that noise also? the binding and relasing action...or no? Also will I know when the wheel is off, if it is a bearing, by looks or feel, or something besides taking it out...Thanks again for all the help, I really appreciate it!
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Old 01-27-2007, 10:51 PM   #8
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If you want to know if it's a bearing, get teh wheel in the air but leave it bolted on. While the wheel is in the air, grab it and see if you can't move it in and out or wiggle it side to side. If it doesn't move, it's probably not a bearing problem. If it does, make sure that it's the wheel moving and not something like a suspension part too. If a suspension part moves, you have a bad ball joint, bushing or even shock mount.

To check teh caliper, give it a look-see. Take it off and examine it. Don't undo the lines though. Just take it off as if you were replacing pads. If it is stuck in a position and doesn't budge at all, it's seized. If it moves but it scapres or the piston boot is all busted up, you probably have a sticking caliper.

Another problem could be a broken shock. If the shock is broken, it can cause an ubeven undulation and clunking noise in a turn. Check teh shock mounts. If the rubber bushings and pds are worn, that's probably your problem. Personally, I'd use the excuse to go with performance versions all around but you can just replace the rear shocks if you want. BTW, ALWAYS replace shocks and springs in pairs. If one side of an axle is bad, consider both sides bad and replace them both. Otherwise things wear unevenly and teh car can actually be dangerous to drive.
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Old 01-28-2007, 01:48 AM   #9
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The problem couldbe any number of things, but if he feels the rear end clunking, it might really be the rear. Sure, a shock or other suspension part could cause it, but the difference in feel should be all but immense. No ball joints in the rear of that car, but there are shocks and probably bushings for a rear sway bar, control arm, torque arm or something. But I don't remember enough about GM's wonderful 7.5" rear to explain it. This suspension is quite different from a Mustang, I remember that.

Axle bearings need to be pressed or bolted onto something, btw. They won't just sit in place unless they're either pressed or bolted(hence C-clip eliminators need bolt on replacements). Some are pressed directly onto the axles themselves(often called semi-floating), others into the housing and still others are bolted or even pressed into the backing plate iirc. Roller bearing styles go into the housing(always, I'd think). These are open bearings and used with c-clip style axles/housings Most sealed, but round bearings go onto the axle and are also used with c-clip style and those retained inside a larger housing with bolt holes mount to the backing/retainer plate. If 11secondranger stops in anytime soon, I think he could 'splain it very well. I suppose there ARE a few bearings in vehicles which aren't pressed to anything, but it would seem to me, they should be. There are 3 basic types on most vehicles. Semi floating, full floating, and C-clip. There are also hub bearing assemblies, used mostly on cars with FWD or ISS. No doubt, I got some part of this wrong, but the general thought is, a bearing not purposely held in place will simply move around and that can't be good inside a rear end. It's one thing to have sloppy axles which can move back and forth... Entirely another to have axles able to simply slide out of a housing and fall away from a vehicle on their own.

If your car has C-clip style axles(and yours probably does), expect inward/outward movement. It shouldn't be much, but there will likely be enough to make you think about it. If it goes side to side as you pull/push at the sides of the wheel, you probably have a need for further inspection.

Now, are you confused yet? And also, after all this, I don't really think you have a bad bearing either. Perhaps something within the rear, but it doesn't "sound" like a bearing. At least not an axle bearing. sounds more like you broke something along the lines of a control arm or torque arm to me. Just inspect it before you drain that stinky gear oil.
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