87 SDL rear end
 

Me again. Liesl is close to 100K. I suspect rear end issues:
* Shocks probably haven't been replaced
* She sits a bit low at back, empty
* Back wheels don't seem vertical - they're a tiny bit like so: / \
* 4 rubber booties on rear axles are starting to crumble
* Underside of car in diff area has grease/oil stains
* I sometimes hear a slight clunk during off-power shifts

What diagnostics can I do (i.e check gear oil level, drive shaft bushing). What shall I tell my shop to do? Note that she's getting a nice big car trip with fam/kids/luggage to Colorado in two months.

Thanks.

CRote,

Changing your shocks should help with the rear end sag and / \ arrangement of the wheels. The rubber boots over the constant velocity joints in the rear axles are pretty rugged. The exterior can look a little worse for wear, but if they still seal the grease inside, and keep the dirt and water on the road outside they are as functional as they can be. If grease is leaking it is apparent. 100k miles is not an identified endurance limit and changing the boots is an ordeal, so I would tend to leave it alone until you see leakage.

If you don't know when the differential fluid was last changed, and you go through the trouble to check the level, just change it. It can only help.

Clunking can be from a variety of things, and I guess I can't point to anything in particular. Hope this helps, and good luck, Jim

Changing diff fluid/shocks
 

I'm spoiling for a set of manuals, but need to ask directions in the mean time.

How to check/change diff fluid, and is there a good synth fluid to use?

How to change shocks, and is MB the best place to get them?

Thanks for assistance.

W126:88 420 SEL differential
 

On this vehicle, what size rear differential hex wrench do I need for fluid removal, and which fluid (not limited slip car) is best in your experience?

CRote and Robert Boyer,

Sorry, I missed your response and request for information nearly two months ago, CRote, but just in case you are still interested, here are a couple of suggestions.

I believe the differential fluid drain plug and fill plug are 14mm Allen head plugs, which means you will need a 14mm Allen wrench. These are available from a variety of sources, and I believe Sears now carries them. I bought mine in 1976 or so, from Snap-on, and it was about $15 or so then. The last time I bought Allen wrenches from Sears they were around the same price, a little less.

You need to get the car up on a lift, or jack stands, or ramps and jack stands to get enough room for you to get under the car and gain access. Always remove the fill plug first, as is you drain the fluid out, and the fill plug is stuck, so are you. So, the idea is to loosen the fill plug and remove it, then the drain plug. I think the plugs have tapered threads, and they can be very tight. I use a hammer to rap the wrench to get the plug to loosen. Works relatively easily, and avoids making room for breaker bars and excessive tooling and grunting. I will put them back in by hand then tap them about a quarter turn further to make sure they do not vibrate loose.

After the old stuff is all drained out, you put the drain plug back in, and, hopefully the car is pretty level, and you pump (I have a little hand pump that pumps the oil out of the container and into the differential) or pour or squeeze the containter to get the new oil into the fill port. You fill it unil it drains out the fill port, so the car being level has some influence on doing the job correctly. Then put the fill plug back in.

I use RedLine Oil Co. synthetic lubes for the transmissions and differentials. They are pricey, but they only get changed every couple years, and the RedLine products have worked very well in my experience. Go to www.redlineoil.com to check what is available. In my 1986 190E 2.3-16 I recently developed a whine at around 65 mph, and the RedLine "Shockproof" fluid quieted it right down.

The rear shocks are available from many aftermarket stores, including FastLane, right on this site. Go to the top of the page and click on the "FastLane" tab, then click on the "on-line Catalog" button and the rest is pretty self explanatory. The windows will lead you to car and make, year, etc. then down to the part you are looking for, by its grouping (engine mechanical, engine electrical, body, suspension, etc.).

I have never changed shocks on a W126 body, but imagine they are similar to the W123 series. Post another question or do a searh on the subject. In general the job is pretty simple, a couple of bolts on the bottom, and a pair of concentric nuts on the top. Pay attention to how the parts came off, and get the sequence correct. The shocks come compressed, and have a tie wrap type restraint on them. New shocks are very stiff and you should not take the tie wrap off until the shock is in postion. You may have to soak the old bottom fasteners in WD-40 or the like to help loosen them. They live in a potentially corrosive environment and I have broken them off before, requiring me to drill out the tapped hole, and use a bolt and pair of nuts to lock the assembly. Major effect on how quickly the job gets done.

Once again, sorry about the delay, and I hope this helps, Jim

Jim:

I think I read somewhere that 10 mm is the size for the hex nuts on the W126. Can you verify?

thanks.

PS: although I do not have any whine noises, are there any noticeable improvements from changing the differential fluid?

thjanks again, Jim.

In addition to ........
 

I would also recommend checking the condition of the lower control arm's in the rear. The (W123/126) control arms are a popular item that go bad.

Robert Boyer,

I no longer own a W126 body machine, so I cannot go out and check it out. But, I doubt the differential housing design changed much, as the 14mm Allen hex configuration has been used on MB drivelines on 220D and 240D cars that I have worked on since 1971, and the 1986 190E. I am presuming you are talking about the differential plugs.

The fluid change is necessary on a regular basis to remove wear and chemical breakdown contamination of the lubricant that will build up in the differential. This is a unit that is cooled and lubricated by the fluid, as it is splashed and sprayed all over the inside of the housing by the driveline turning. There are no filters or other means to get rid of wear particles, and when it is hot and the lubricant is loaded at the gear tooth interface by shear and pressure forces, the oil molecules break up and become something other than oil, with no where to go. I believe regular changes at 30,000 miles is recommended in your manual. I go for 60,000 miles using RedLine products. When I change it, the oil is darker, but not ugly. No problems to date. That whine started after hittng 200,000 miles, and it went away after changing the fluid.

Hope this helps, Jim

To Potomac Pat:

Regarding the rear lower control arms, what should I look for re condition, and is there anything I can do other than replacement for a quick fix?

In reagrds to the lower control arms. They are steel & rust out over time due to the amount of road salt & grime, etc..... that build up and cause them to rust out. The only way to correct it is to swap the control arm out complete, but only after installing new bushings. (if used) Again, the W123/126 and other chassis share the same lower control arm (rear) and we sell a few a day for the reason I had mentioned. Just a thought, check it out & keep us posted.:D

Pat:

Mine's a florida car, and has no rust on those parts.

Nevertheless, can they loosen up over time, and can anything be tightened?

HHHHHhhhhmmmmmmm!
 

Just a thought, but if your car is still sagging in the rear and your control arms are in good shape.....? I would recommend checking the springs and/or the shocks.:)

Pat:

Car is not sagging at all. It has, in addition, relatively new Bilstein HD's in the rear.

I've heard that springs on this car rarely if ever need replacement. Any thoughts?


It's just that car seems a bit unstable from rear at highway speeds, and seems to sway a bit too much.

Bump on a Log
 

Job done - I used a small fireplace log to bump the wrench to get the nuts off. Almost didn't work, but glad I perservered. No jacks necessary - just chocked the wheels.

My 6yrold son crawled under and "fed the baby" with 1.5 bottles of Mobil 1 gear oil ("feed her until she spits some up, big guy"). He's in love with that he now calls "his" car.

Had shop do Mobil synth atf - shifts so smoothly now its captivating. What a drive, what a drive.