The W201 differential refresh thread

[layback40]

Originally Posted by Stretch

X2



X2

Yeah add 'em in John it might make me feel a bit better about my purchase after diff doctor layback40 says his piece!

Your the only doctor here Stretch !!
With all your dis-assembly, we should talk of you as a surgeon.
Surgeon Stretch !!
That dull surface looks like an oil problem. Normally you get a polish mark showing the contact point on the tooth. In my limited experience, noise can occur when the contact point varies as you move around the crown wheel.
maybe other members have more experience with this.

[Stretch]

Originally Posted by layback40

...
That dull surface looks like an oil problem. Normally you get a polish mark showing the contact point on the tooth. In my limited experience, noise can occur when the contact point varies as you move around the crown wheel.
maybe other members have more experience with this.

Thanks for your input. That oil is a problem - I might take my other differential to bits to see if that can shed any light on the problem.

I've been reading through the FSM this morning trying to figure out a way in which this contact can be adjusted / corrected. I think I've got it sussed - it is surprisingly complicated for a system with such simple geometry...

...I'm gonna do some drawings to explain it.

[Stretch]

It is worth noting that although the position of the pinion and crown wheels is a three dimensional problem the adjustments in practical terms can only be made in two dimensions =>


If the holes for the axial bearings in the differential are not aligned properly then there's little we as mechanics can do to correct for this error. If the centre line of the axial shafts are not at the correct height with respect to the height of the centre line of the pinion then again we as mechanics can't really do anything about it.


You might want to let that sink in! (If you really want to understand what's going on and what could go wrong)




Adjustments are made to the positions of the pinion and the crown wheel by the use of shims.


Shims are placed in three areas in the differential (for the adjustment of the pinion and crown wheel)


The position of the pinion is adjusted by placing a shim behind the inner most tapered roller bearing cup.


The position of the crown wheel is adjusted by adding shims behind the axial tapered roller bearing cups. These shims are circlips!





The circlips are shims!




The position of the pinion is probably going to be relatively easy to measure and adjust. The position of the crown wheel, however, is complicated by the pre-load of the axial bearings.


Bearings need to have a pre-load to stop them from sloshing about within the differential case. Sloshy bearings = noise {because there will be uneven contact between crown wheel and pinion}


At no expense spared here's a scanned in sketch of what is going on.





The position of the pinion with respect to the centre of the crown wheel is distance “x”


The position of the crown wheel with respect to the centre of the crown wheel is distance “y”


Think of the differential case as a great big C clamp. By stretching the C clamp within its elastic limits you can essentially use the springiness of the clamp as a way of pre-loading the axial bearings => you make it bigger than it is – you stick the bearings in there and release the pressure. It is a bit like making an interference fit – you stuff something slightly larger into a slightly smaller hole.


The problem is – and here's where you move from the realms of theory into the realms of practice – the differential case isn't a beautiful uniform homogeneous lump of metal. It is a roughly cast bell shaped thing with holes drilled in it. This means that when you stretch the opening (to get your pre-load for the bearings) one side of the case (y1) might not move as far as the other side of the case (y2).


This is a total bugger when you are trying to set your distance “y”...


...to be continued.

[spock505]

I have just re-read your informative thread again and wanted to ask a third (but probably daft) question.

Assuming it's just the axle bearings causing miss-alignment of the crown gears due to wear. Would it not be possible to order new MB bearing using original shims (circ clips) to bring the unit back to factory settings?

Originally Posted by Stretch

It is worth noting that although the position of the pinion and crown wheels is a three dimensional problem the adjustments in practical terms can only be made in two dimensions =>


If the holes for the axial bearings in the differential are not aligned properly then there's little we as mechanics can do to correct for this error. If the centre line of the axial shafts are not at the correct height with respect to the height of the centre line of the pinion then again we as mechanics can't really do anything about it.


You might want to let that sink in! (If you really want to understand what's going on and what could go wrong)




Adjustments are made to the positions of the pinion and the crown wheel by the use of shims.


Shims are placed in three areas in the differential (for the adjustment of the pinion and crown wheel)


The position of the pinion is adjusted by placing a shim behind the inner most tapered roller bearing cup.


The position of the crown wheel is adjusted by adding shims behind the axial tapered roller bearing cups. These shims are circlips!





The circlips are shims!




The position of the pinion is probably going to be relatively easy to measure and adjust. The position of the crown wheel, however, is complicated by the pre-load of the axial bearings.


Bearings need to have a pre-load to stop them from sloshing about within the differential case. Sloshy bearings = noise {because there will be uneven contact between crown wheel and pinion}


At no expense spared here's a scanned in sketch of what is going on.





The position of the pinion with respect to the centre of the crown wheel is distance “x”


The position of the crown wheel with respect to the centre of the crown wheel is distance “y”


Think of the differential case as a great big C clamp. By stretching the C clamp within its elastic limits you can essentially use the springiness of the clamp as a way of pre-loading the axial bearings => you make it bigger than it is – you stick the bearings in there and release the pressure. It is a bit like making an interference fit – you stuff something slightly larger into a slightly smaller hole.


The problem is – and here's where you move from the realms of theory into the realms of practice – the differential case isn't a beautiful uniform homogeneous lump of metal. It is a roughly cast bell shaped thing with holes drilled in it. This means that when you stretch the opening (to get your pre-load for the bearings) one side of the case (y1) might not move as far as the other side of the case (y2).


This is a total bugger when you are trying to set your distance “y”...


...to be continued.

[Stretch]

Originally Posted by spock505

I have just re-read your informative thread again and wanted to ask a third (but probably daft) question.

Assuming it's just the axle bearings causing miss-alignment of the crown gears due to wear. Would it not be possible to order new MB bearing using original shims (circ clips) to bring the unit back to factory settings?

Yes it could be as simple as that.

It might, however, not be as simple as that!


##########


The big long winded process in the FSM is designed to get the pinion and crown wheel gear in (about) the right position. A decent differential rebuilder would then check the wear pattern with engineer's blue. Apparently the trick to getting a representative wear pattern is to check this under load - this is hard to do for the DIY mechanic.

maboyce over on Benz World made a rig to try and do this for his W123 differential =>

1983 300TD - Mercedes-Benz Forum

(That's a massive thread - it will take ages to find the post - when I do I'll update the link)


####


I think the problem with the small case W201 / W124 differentials is that the case is not rigid enough: In service life it relaxes and so this compression across the axial bearings dies. This increases the backlash and so you end up with miserable output to input shaft play and a noisy differential...

[spock505]

Thanks Stretch, that may well come in useful for future rebuild if needed.

I ended up getting a replacement differential which is the exact same match to my own. Believe me this was no easy task with one MB parts dealer listing a very used diff (once he knew I needed one) on eBay for £300 plus delivery and ..ouch!

The main problem is everyone lists second hand diffs using the number stamped on the cover, I searched using this too but couldn't make sense before finally realizing it's on the cast part.

Anyway, fitted the replacement diff only to find it made virtually no difference at all, by this time I had already replaced the front wheel bearings and center bearing. These parts needed replacing (center bearing rubber came away from surround, wheel bearings yellow with no play, over-tightened)

After this I was sure "it must be the diff" given the amount of oil leaked and then distributed around the underside. Unfortunately not, although I do now have a spare which can be worked on at leisure or sent away pending tools/difficulty level.

So onto the rear wheel bearings which can be a tad more difficult than standard W124 sedan (we are on a American forum - that said I have found a tool which should be okay for bearing removal but the **puck is too small (W124 small diameter bearing hole) to hold the back whilst pushing in flange plate.

I have the measurements so shouldn't be too hard to find additional **pucks.

Cheers for now.

** no idea what those bearing plates are called.