Mine didn't make noise right away. I first changed the fluid 20kmiles ago and noise started between 5-15Kmiles.

Brian

jcd,

I cannot imagine that changing the fluid after 18 years could do any further harm to the differential. If you do get some whining it is likely some wear from the previous abuse and you might want to look at some of the Redline differential fluids if the noise bothers you. My (now my son's) 190E 2.3-16 has had the differential fluid changed, and I have been using Redline synthetic stuff since the first change, every 65k miles or so. We just did it at 200,000 miles as the differential was getting a little noisy at highway speeds. I used the "Shockproof" stuff and the noise has gone away. The stuff is more expensive than most oils, but for the transmission and differential I figure an extra $10 every 65,000 miles is ok if the equipment lasts longer.

Good luck, Jim

I agree
 

I was being "overly dramatic" about my concern as a lame attempt at humor. However, I am going to follow "someone's " advice about swapping out the stuff I just put in in about 2,000 miles or so. I can get Redline at the local autoparts store, so I might just put it in during that change and be done with it.

JCD

What is all this crap about a mallet?

If the plug is stuck, you need to set up some vibration to help seperate the plug from the rest of the unit.

You can hit the plug directly with a hammer to "ring" the plug and set up vibrations to help seperate the threads. Using a soft mallet to reduce the shock to the threads, you just have to use more force and increase the chance of stripping the shoulders of the plug.

If you heat, do it carefull. Work carefully and quickly. Use a oxy acet torch, not a Bic lighter. Spending a long time with too cool a torch will only cause misery. If you have to heat, get in and get out quickly. This is a place where experience counts. If you do it enough, you learn to read the color of the metal. If you are not qoing to do it a lot, let someone else screw it up.

To loosen that plug--

place your wrench / breaker bar at a 45 degree angle, handle pointing towards the ground, get a floor jack under it and start raising the jack to put upwards pressure on the wrench.

Worked great for me on both our 123s, letting the weight of the car work for me.

md21722 said: mine started to make noise. This may be because of my use of synthetic
 

(following is long verbage on why I do NOT like synthetic diff lubes.)
I had the unfortunate experience of letting a shop owner talk me into using synthetic diff oil in my 220. I found that this action almost immediately initiated some bearing noise from the rear end.
We had the car on a hoist with a pickup microphoone listening device applied at various points until it was certain that the noise was coming from the differential, not the bearings or axles. I saw the thin looking fluid drained from the diff and witnessed the new thicker fluid that replaced it.

I replaced the fluid with dino hypoid (85W90) and the noise was diminished but it did not go away completely. It is still there to this day. One thing that I recall that may have a factor, is I happened to have the rear seat out, and there is some dampening provided by the seat materials so the noise was probably louder than it would have been with it installed (rear seat is still out - don't ask!)
Since then I have heard mechanics warn against using synthetic differential lubricant on old Mercedes, and believe I know why.
Was it md21722 that had some feelings on this subject (no this isn't another oil thread I promise!)
Afterwards, I bought some MGL (?) Multi Gear Lube additive for differentials that seemed to reduce differential whine in another application (my '83 300D with 265K miles on it) - the REAL solution was to replace it with a low mileage diff from a 1985 300D (the 2.82 ratio!) and I got a whole new ride as a bonus!

What I believe is that once a synthetic is used it cleans out the bearing races and allows the bearings to move more than they did with some gunk build up in them, and the result is an increase in noise.
Synthetics have their place especially in cold weather where starting an engine can be very different than on a warm day, or where hydraulic lifters are used.
I don't see any merit to using synthetic in a differential used for non-racing applications. I question the stated increase in fuel economy (edited here) although some say that new car manufacturers had to use synthetics here to get higher fuel economy numbers, I question even that. I believe that driving habits have more to do with fuel economy than a small change in diff fluid.

Contrary to what is felt by some here in the forum, sticking with the OE recommended lube might be the best advise to follow with an old differential.

This is only my opinion based on my experience in daily drivers in fairly warm climates, but as always, YMMV.
I hope this helps someone avoid what I went through. I wish I had never heard of synthetic diff lube!
DDH

I changed mine back in April and I am sure the oil was original, the plug just looked like it was never moved. But the oil that came out was nice and tan looking it makes me wonder if you ever realy need to change it? I put Mobil 1 75w-90 in and it has been fine for 3k miles no leaks, noise etc.

this is on the list of things for tomorrow. i have also bought lucas synth additive to add the .2 qt to the diff after the mob 1.

this has been a great read BEFORE attempting this. i didn't realize previously i would need a hex head to get the plugs out and maybe didn't think to take off the fill head first.

For those of you in
Canada , I just purchased a set of 6 hex sockets including 14, 17,19 mm from Princess Auto on sale for $12.00..1/2 inch drive and worked great. I used Pennsoil gear lube and just squeezed the bottle. I added transmission treatment to soften the seals and hopefully reduce leakage. I will drain and refill with Mobil 1 syn soon