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  #1  
Old 11-18-2003, 09:49 PM
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Limited Slip question

I have the limited slip diff on my 126. However, over the weekend it was raining. Every time I would come off a light, it would break the left rear tire loose (perhaps it was kicking down to first, I don't remember it ever spinning the tires so easily). But each time it would break it loose, only the left rear would spin. With limited slip, shouldn't both of them spin? Could there be something wrong with my diff??
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  #2  
Old 11-19-2003, 05:05 AM
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mbtjc,

Sounds horribly like you need to get your hands dirty.

The LSD is behaving like an ordinary one.......both wheels should definately spin together if it was working ok. For the amount they cost they should work until the end of time, but sometimes you're unlucky.

It could just be that the clutches are worn, but the diff will need to come apart to be sure.

Start giving-up something now, that way, when the bill comes in you'll have some cash towards it..........
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  #3  
Old 11-19-2003, 01:09 PM
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Greeeeaaaaat. Can it actually be rebuilt or does the thing have to be replaced? Any clue as to what something like this might cost? I'm assuming this is beyond a DIYer with good mechanical skills (did my timing chain) and MB manuals....??
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Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."
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  #4  
Old 11-19-2003, 01:12 PM
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The mechanical bits are straightforward enough, but itt usually involves specialist tools.

I've never rebuilt a MB axle, so this may not apply, but I'd be surprised if you can do it with just a spanner and a big 'ammer.......
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  #5  
Old 11-19-2003, 04:55 PM
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Could it be the wrong oil?
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  #6  
Old 11-19-2003, 05:04 PM
PaulC
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If the diff fluid had been changed without using the proper additive, there could be a problem. However, I was always under the impression that the symptoms would be vibration/grabbiness while turning. This sounds a little different.

The clutches on limited-slip diffs are wearable items, just like brake pads or a transmission clutch. Could be that their time has come.
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  #7  
Old 11-19-2003, 07:53 PM
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do you have the right oil in there. If you do not have a specific limited slip oil in there the limited part is not effective.
I use mobil1 with a BG additive for limited slip diffs.
m
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  #8  
Old 11-19-2003, 08:42 PM
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As far as the oil....it has not been changed since I have owned it. And since it was bought new by my father, I doubt it has ever been changed, maaaayyybe once. So although the oil is without a doubt old, I'm confident that it's correct. Will just old oil cause the limited slip to quit working? Assuming the answer to that question is yes, could it actually cause damage? Or will a fluid change perhaps cure the problem?
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Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."
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  #9  
Old 11-19-2003, 08:57 PM
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The real test is...

to lift up both back wheels, and with the parking brake off and the transmission in park, turn one of the back wheels. A conventional differential will have the other wheel turn the opposite way. A posi-traction will have the other wheel turn the same way. A limited slip will have the other wheel turn the opposite way with effort.

Does everybody agree?
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  #10  
Old 11-19-2003, 09:01 PM
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Isn't posi-traction just a fancy name for limited slip? Same thing I believe. For them to turn the same way, wouldn't it have to be a locker like in 4x4s or a spool?
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Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."
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  #11  
Old 11-19-2003, 09:59 PM
PaulC
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Posi-Traction is an old GM trade name for limited-slip. I recall reading an old road test on a then-new 1958 Corvette equipped with this feature.
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  #12  
Old 11-19-2003, 10:45 PM
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With a limited slip both wheels should turn the same direction when jacked up. The clutch pack in the diff. acts as a lock but allows the wheels to slip slightly (limited) to make it easier when going around corners. I would say that if one wheel is spinning under power that the clutch pack is going out. I have never been into a MB diff. but common sense tells me that the power coming into the diff is not being distriputed equally to both wheels. The way it get distributed is through the clutch disks. If they slip power only goes to one side.

ps. mbtjc- Remember, not all wine gets better with age-some turns into vinegar.
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  #13  
Old 11-19-2003, 10:48 PM
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Whoa!! Sounds to me like your diff. is doing what it is supposed to. Before you start tearing stuff apart, consider the following: A limited-slip or positive traction (yes they are the same thing) differential does not mean that when one wheel loses traction, the other wheel takes over. When one wheel loses traction, the other continues to move forward only under the torque required to overpower its internal clutches (if that's the type in an MB), not at regular speed. Were you not still moving forward even though one tire had no traction? Furthermore, if MB uses a Torsen type differential, they can lose traction altogether as the traction wheel gets a certain ratio (called the bias ratio) of the spinning wheels' torque. If the spinning wheel loses all traction, your sol because the torque=0 and anything times 0 = 0.


Check out this thread...
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  #14  
Old 11-20-2003, 12:39 PM
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MB limited slip diffs from the eighties are clutch type - just like GM's classic positration, which was first offered in 1957. The clutches have preload, but with use they wear, which reduces the preload and it eventually behaves just like an open differential. Most manufacturers publish a minimum preload torque, which can be used to determine the health of the unit. A subset of the clutch type is the "cone type" used for light duty applications. Instead of a clutch pack, the differential side gears are tapered and bear against a similar taper inside the differential case.

Clutch and cone type limited slips need a "friction modifier" to prevent "clutch or cone chatter", which is a form of stick-slip friction that can make the diff noisy or jerky on tight turns. Most of the OEs sell a limited slip additive (basically all the same), and it should be added to straight 80W-90 GL5 gear oil in the proper concentration.

Replacing the limited slip clutch pack is probably a pretty pricey operation and may not be worth it on an older car. If the clutch pack is worn the differential will continue to function as a normal open diff with no ill affect other than loss of the limited slip function.

Since clutch wear particles get into the oil I recommend diff lube changes every 30-60K miles for clutch and cone type limited slip diffs, however, on a open diff, after Mercedes' recommended break-in change (not sure if they recommend this now, but they did in the eighties), it's not necessary to change the oil on an open diff.

Normally, with about equal traction on a slippery surface, both wheels will turn at equal speed, however if one wheel has traction and one doesn't sufficient input torque can overcome the preload and allow one wheel spin by overcoming the clutch clamping force, and this can also quickly burn up the clutches. Clutch type limited slips are "torque sensative" in that they respond to axle differential torque. They are usually defined by "percent limited slip", and a "30 percent limited slip" would "lock up" at a differential torque of 30 percent. Differential torque creates an end thrust on the differential side gears, which adds to the preload to create more clamping force to "lock-up" the diff. Street cars are usually set up with about 30 percent limited slip. Higher values can make them cranky when going around corners - chatter and jerking.

Torsen limited slip diffs use gears and are based on the principle that a worm can drive a worm gear, but a worm gear cant drive a worm, and they respond to differential torque. One advantage they have is no internal wear items and no need for a lubricant additive.

Another type of limited slip is the "viscous type". This type has a silicone fluid sealed up in the differential case whose viscosity varies with the rate that it is sheared. This type is sensative to differential axle speed, not torque, and, like the Torsen, there are no special maintenance requirements.

BTW, I have never understood why the German OEMs charge so much for limited slips diffs. On both my '63 Corvette (clutch type) and '76 Cosworth Vega (cone type) the option cost was between 40 and 45 dollars. I don't recall Mercedes even offering the option on US models from the eighties (my 190E 2.6 could sure use one), but it was standard on the 560 and 16V. BMW offered optional limited slips on some models in that era, and the cost was about 500 bucks!

Duke

Last edited by Duke2.6; 11-20-2003 at 02:51 PM.
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