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  #1  
Old 07-24-2017, 11:45 PM
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How to diagnose rear end differential failure 1967 W108 250S

Hi all,

My mechanic says I am suffering from a failure in the rear end. Engine turns on, 4spd manual engages gears, speedo responds when in forward gears, and drive shaft (propeller) turns. But the wheels do not turn at all.

Unfortunately my mechanic does not do tranny/differential work. Its a 1967 250S W108 4spd manual on the floor. The differential case is stamped 092097 4,08. The places that he usually farms this sort of work to did not want to touch it because the car is so old and they did not want to get into a "waiting for parts" circus.

So, how can I go about diagnosing this so that I can have enough knowledge to make a decision to repair myself or sell the car? I see that the rear ends on ebay right now are all of the drum brake variety. I really don't want to make that sort of change; though I am willing to go the route of the 3.27 gear ratio.

If I could just know how to find out what is wrong, I would know where I stand (other than the bus stop).

Thanks in advance.

ps: I forgot to mention that the driveshaft appears to be rotating the pinion gear just fine. Something wrong is happening somewhere inside the pineapple - I think. This has been my daily driver. Hopefully to resume before the rainy season.
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  #2  
Old 07-25-2017, 06:58 AM
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The easiest thing to do would probably be to swap out the entire rear end. But I'm guessing that by your comment of "rainy season" that you are not in the US. So perhaps limiting the shipment costs by just sending the gears would be ideal. If that's what you'd want, I suggest determining the exact parts that you need, and if the gears themselves are replaceable.


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  #3  
Old 07-25-2017, 10:31 AM
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I forgot to mention in the post that I had the car jacked up, with both rear wheels off the ground, and turned the engine on and put the tranny into 1st gear. Driveshaft still rotated, but no movement in either wheel. However there was a difference - I could rotate by hand the Left tire much more easily than the Right tire. Loads of resistance on the Right. I just chalked that up to something in the diff since neither wheel was getting rotational energy. But, could this suggest something afoot in the axles?
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Old 07-25-2017, 10:35 AM
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Fonzi,
Thanks. I hate the thought of having to tear all of assembly apart just to figure out what is amiss. So, I am amenable to the swapping of the entire rear for a good one. The "rainy season" quip was in jest as I am in Northern California and we never know in we will be in drought or rain. Rain + bus= no fun.
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  #5  
Old 07-25-2017, 10:51 AM
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When these cars were new, and by that I mean when these rear ends were new, techs had to be trained in how to repair them since they were like nothing else on the road.

A variety of special tools are needed to put the insides together. The manual tells how to do this and it is complicated by anyone's standards.

Mercedes finally quit repairing these and started sending them out to some shop in California for rebuilding. Today I think the Classic Center can guide you in where to send it. They may be able to rebuild it themselves. If so this might be the way to deal with the problem.

If not then replacing the entire rear end assembly is the way to go. I should think that any rear end assembly from any 108 would work so you would not lose your rear discs.

Look this over carefully as there are two leading arms that bolt to the body right under the bottom of the rear seats. Note that there are two springs in there somewhere and these tend to, when the bolts holding the arms to the body are removed, expand at a very rapid rate. Rapid enough to take a finger off if you are not ready for them to release.

These are also quite heavy so borrow all the jacks you can find to lift this into place.
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Old 07-25-2017, 11:58 AM
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Quote:
Originally Posted by karshack View Post
I forgot to mention in the post that I had the car jacked up, with both rear wheels off the ground, and turned the engine on and put the tranny into 1st gear. Driveshaft still rotated, but no movement in either wheel. However there was a difference - I could rotate by hand the Left tire much more easily than the Right tire. Loads of resistance on the Right. I just chalked that up to something in the diff since neither wheel was getting rotational energy. But, could this suggest something afoot in the axles?
k:

Are you familiar with the general construction and operation of an automotive final drive and differential gearing? This question is asked to aid in advising you of check procedures that you could follow. Have a look at this in Wikipedia:
https://en.wikipedia.org/wiki/Differential_(mechanical_device)
If this is familiar, then you can proceed to first check for pinion and ring gear integrity, and then for damaged differential gears.
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  #7  
Old 07-25-2017, 07:48 PM
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A few things.

Pull the fill / drain plug and see if there is metal in the oil ( some slight metallic is normal. )

I don't recall if the rear end cover can be easily pulled on a low pivot rear axle. If is easy to pull, take a look inside.

You might be able to look through the fill plug and see if the pinion turns while turning the drive shaft by hand. DO NOT put a finger in the hole and turn the drive shaft. . .

One side of the axle is going to have a rubber boot covering the drive flex joint. I'd unclamp this and have a look.

Others with 80's cars have reported the trans output flange wearing out splines resulting in loss of drive, perhaps this has occurred on the rear end side.
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  #8  
Old 07-28-2017, 12:42 AM
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There will be binding in the axle if you attempt to turn it with the axles dropped down from the wheels being off the ground and could futher the damage. You need to diagnose it with the wheels at 0 camber.
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Old 08-07-2017, 01:43 AM
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Thanks Idle,
I hate it when a company quits repairing it's own product. But am glad that they did adopt the removable rear cover plate. I'm sure it makes a world of difference. Looks like I'm going to give the diagnosis a go before getting a replacement unit. Thanks for the info.
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Old 08-07-2017, 01:48 AM
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Hi Frank,
Thanks for the info. Have been looking at some youtube vids on differentials, that have been of some help.
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  #11  
Old 08-07-2017, 02:16 AM
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Update:

Drained differential - 2.5 quarts of dark brown/black oil of medium viscosity. Sediment, but not bits of metal. I have no idea when the oil was last changed. I have not done it in the 3 years since I've owned the car.

Took the left axle off. Nothing remarkable. The axle is intact - not visible breaks, cracks or bending. No oil leaking out of axle tube. Axle bearing seems to spin well, without friction or debris. And the splines appear healthy and straight, even wear.

I have not removed the bearing since I don't have to tools. I might leave the bearing remove/replace to a machinist. Already have the Febi kits (for both axles).

Tomorrow, on to the right axle.

Note: Tires/Wheels off and both axles elevated by jack stands. And just to "see what would happen" (before removing the left axle) - I turned the engine on and ran through all the gears with short taps of the gas to take it past idle and filmed the left and right axles for corresponding response. The left axle turned through all of the gears and acceleration. Absolutely no movement from the right axle, even when in 1st gear and I held the left axle still by my hands, there was no movement by the right axle.

Point well taken: Torque arms/rear spring assembly deserve much respect b/c tremendous stored/potential energy there. Treat with care.
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Old 08-07-2017, 12:01 PM
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So if I remember correctly, the right axle has a very particular removal sequence or you will break the clip that holds it in place. Speaking from memory, you have to remove the shocks and springs (and hydraulic compensator/spring) from both right and left sides. Then elevate both hubs to the bump stops. That allows the right shaft to be removed from the spring clip without breaking it.
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  #13  
Old 08-11-2017, 11:58 AM
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Update:

Ah, got the right axle out. Jacking it up to near level was necessary to take pressure off of the joint containing the sliding sleeve/bearing assembly covered by the flexible rubber boot. Thanks for the heads up about that.

Nothing was remarkable about the right axle. The bearing appears and spins just fine. The splines are all there and without damage. The end of the axle at the splines does show impact marks, but I don't know if those happened while driving or during some previous service/maintenance - see photo.

Considering removing Bolt #72 and the jointed assembly #60 for inspection, but not sure if that can be done without removing the right axle tube?

At what point of the disassembly/diagnosis should I stop and just look for a replacement differential pineapple? Or entire rear suspension and differential assembly since they often seem to be sold as entire units. I see few if any of the differential components on ebay, so the smaller elements seem to be scarce as stand alone items for sale. Am I right?
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  #14  
Old 08-11-2017, 12:02 PM
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Ooops. Forgot the pix.
Attached Thumbnails
How to diagnose rear end differential failure 1967 W108 250S-cat-page-112_2652.jpg   How to diagnose rear end differential failure 1967 W108 250S-right-axle-end_2646.jpg   How to diagnose rear end differential failure 1967 W108 250S-mbz-parts-cat-12_110.jpg  
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  #15  
Old 08-11-2017, 02:44 PM
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Quote:
Originally Posted by karshack View Post
Update:


Nothing was remarkable about the right axle. The bearing appears and spins just fine. The splines are all there and without damage. The end of the axle at the splines does show impact marks, but I don't know if those happened while driving or during some previous service/maintenance - see photo.

Considering removing Bolt #72 and the jointed assembly #60 for inspection, but not sure if that can be done without removing the right axle tube?
k:

Note the deformation of the root of the axle splines at the end. The internally splined tube into which the axle slides is likely cracked or broken, resulting in wobbling of the tube on the axle, and hence the deformation of the axle splines.
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