DIY: W124 Differential Replacement - PeachParts Mercedes-Benz Forum

skiier3_9 · #1 04-26-2007, 10:05 PM

I did this project at the same time as I removed one wheel carrier (for wheel bearing replacement). This was a more difficult project - especially without a lift.

Special Tools Needed
- 10mm XZN Sockets (eBay ~$40)
- 46mm open end wrench or large crescent/pipe wrench
- 8, 12, 14mm hex keys (eBay ~$30)

I. Jack both rear wheels up, put the car on jack stands, and remove the wheels.

II. Remove the rubber exhaust supports at the rear muffler, as well as the next exhaust support (2x13mm bolts). See pic #1 and 2. I used a jack to lift the exhaust and a pliers/screwdriver to grab and pry the rubber mounts around the muffler. Don't forget to support the exhaust once its down.
**Note - on V8 models you've got a bit more work - according to the manual you need to remove the complete exhaust system.

IIB (with ABS only - no ASR) - remove the ABS speed sensor (according to the manual). It also says drain the oil - we skipped this, but it would make it a bit lighter.

III. Remove the center exhaust shield (3x 8mm bolts). With the exhaust lowered you'll be able to twist this around the exhaust and out of the way. I also removed the front exhaust support where the front of the heat shield screwed into to give myself a couple extra inches of space. I don't know if this helped or not - there were 4x 13mm bolts holding this on.

IV. With the exhaust lowered and the heat shield removed you can remove the two 13mm bolts holding the center drive shaft support bearing in place. See Pic #3. As a precaution I painted a stripe on my drive shaft with nail polish in the event that the shaft became separated. This is where you would be well advised to get a 46mm open end wrench. With the car in park you can use the transmission to hold the front portion of the drive shaft to loosen the rear bolt. It isn't on very tight, but its a big pain because there isn't very much room. Others here have said a 15" crescent wrench fits - I tried an 18" without any luck and used a pipe wrench.

V. Unbolt the 3 19mm bolts that hold the flex disc to the differential. I removed all of the bolts so that I could remove/inspect the flex disc. I didn't need to replace mine (although originally I planned to). Pic #4 shows the drive shaft bolts removed.

VI. Once you've loosened the flex disc you need to slide the drive shaft off the differential's centering pointy thing. I used an 18" pry bar - the manual suggests a "suitable drift" whatever that is

You can rest the drive shaft on the seatbelt bracket attachment (see the pic - its at the top).

VII. Now remove the axle bolts (6 per side) these require the 10mm XZN (or triple square) drivers. I purchased a set of 4 on ebay for about $40 shipped - not great quality but they worked. These bolts were extremely dirty on my car - I sprayed engine degreaser at all of these then inserted/removed the XZN a few times to remove some of the grease that had built up. I wanted to ensure that I got a really good seat of the XZN. I also had to use a breaker bar on my 1/2" drive socket (actually a 32mm box end wrench) to loosen these bolts. They are secured with blue lock-tight; heat might help, but I didn't have a torch and made due. See pic #5 - once I loosened the bolt I used a long extension to remove it all the rest of the way.

You'll need to use the parking brake to prevent the axle from turning while you loosen the bolts. You also can only loosen one or two in any one position - because the axle boot prevents you from getting the XZN securely in the head with the bolt at the bottom and the frame is in the way along the top/rear.

skiier3_9 · #2 04-26-2007, 10:24 PM

VIII. With all the axle bolts removed you can get your jack (and presumably your nice differential pick-up attachment if you have one) and take a little pressure off the differential. Now, loosen the two rear 12mm hex key differential supports (tightening torque = 120 Nm).

IX. Next remove the front differential support (on the passenger side). This requires a 17mm box-end wrench to hold the top bolt and an 8mm hex key/socket on the bottom. There are two differential mounts (one above, one below the mount/frame) here.

***BE CAREFUL***

This removes all attachments for the differential - you don't want to be under this thing if it falls. I had an assistant help me lower it and it still fell off the small floor jack that I had (luckily not on anyones fingers - we were well out of the way!).

If you have ASR you'll have to disconnect the two speed sensors under the plastic enclosure hiding the fuel pump/filter on the passenger side (now would be a good time to change that filter if you haven't done that in 60k miles). See pic #1 for the connector. You need to pull the outer ring straight down, then the connectors will slide out easily. Also, the two ASR connectors are labeled "D" and "E" to correspond to the connector block labels. The other two connectors in the picture are for the rear brake pad wear indicator lights - it doesn't matter if you get these reversed if they fall out (just make sure they get plugged into the two sockets with connectors there).

You'll also have to remove a metal cable support (3x 13/10?mm bolts + 2x 8mm screws). This guy was REALLY rusted for me!

Pic #1: I circled the ASR connectors in yellow and the brake pad wear indicators connector sockets in red.

Pic #2: Front differential mount. Cable support that you'll have to remove.

Pic #3: More of the cable support that needs to be removed.

Pic #4: XZN close-up.

Pic #5: Passenger side axle removed from differential - you can also see the 14mm hex drain plug at the bottom.

skiier3_9 · #3 04-26-2007, 10:32 PM

Differential Installation Notes

Before installing the new (salvage take off) differential I loosened both the fill and drain plug (beacuse I would be changing the fluid once the diff was installed). This requires the 14mm hex/socket. It is much easier to loosen these with the diff out of the car - trust me!

It was difficult with a regular floor jack to get the differential in place because you had to balance it and clear the centering pin (on the diff) with the drive shaft. I had an assistant help me get it into place and we eneded up lowering one side of the seat belt attachment bracket to lower the drive shaft even more and give ourselves more room for the differential. We also put a piece of plywood on the jack to have something more to balance/grab onto to maneuver the differential.

Once we got it into place (after much sweating a few 4-letter words) I got a few bolts through the flex disc/drive shaft, then the two bolts in the rear. Once we re-secured the seat belt attachment/bracket and got the front mounting bolt in place I re-seated the rear mounting bolts to prevent cross-threading or stripping.

Torque specs:
Rear mounts: ~120Nm
Front mounts: ~45Nm
Axle shafts (with blue locktight re-applied): ~70Nm

Any thoughts/suggestions/comments? It was quite a project!

Hirnbeiss · #4 04-27-2007, 06:36 AM

Great post, both here and on your wheel carrier project. Give us an update on noise levels after you road test it.

skiier3_9 · #5 04-27-2007, 08:23 AM

Everything is quite. The scraping noise that I heard before is gone as is the 'whine' when accelerating at 40+ mph that was likely coming from the old differential.

whunter · #6 04-30-2007, 01:16 PM

I sent this to webmaster for the wiki page.

Thank you, and have a great day.

Bill Wood · #7 04-30-2007, 03:41 PM

This DIY is now in the Wiki:

CLICK HERE

bob_98sr5 · #8 07-20-2008, 02:13 PM

great writeup, thanks skiier.

after reading numerous threads on diff swaps (now they are all mush in my head) today, i have a 2 questions. first, i am replacing my rusted out rear subframe with a junk yard, rust free one. i've installed all new bushings in the donor subframe with the exception of the rear bushings that hold the diff onto the subframe because i read that it requires a special tool and that you need a special alignment tool (read/seen it on gsxr's website) to do a diff swap properly. the diff swap will be done on my 87 300DT: id like to swap my old 300DT diff onto the donor rear subframe.

skiier: if you are still around or for anyone else who has done a diff swap, here are my 2 questions:

1) this writeup does not mention like other threads on the forum about getting on the diff bushings which i recall needs a special mbz tool. is there a DIY diff bushing tool like there is for the DIY subframe bushing tool?

2) im drawing somewhat of a blank here, but i recall too that there is some sort of alignment tool that you need to make sure the diff is in perfect alignment (someone help me here). there is no mention of that here in this writeup.

So is #2 above really necessary? if not, i will try to attempt this myself. i spoke with my mechanic who said he'd do this, but he's now on vacation for 2 weeks, so if i can, i'd like to do this provided i find a solution for #1 and if #2 is really unnecessary.

any input from people who have done a diff swap would be greatly appreciated.

thanks,
bob

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