differential fluid: exact amount important?
 

When I do DIY rear differential fluid change, I put rear wheels on ramps.
But the fill plug of the W124 rear differential is in front of the differential.
So the fill plug is lowered, and thus I put less fluid than I would if the car were level.

Is this lower differential fluid level OK, or will it damage differential over time?

diff fluid level
 

It depends on how much of an angle. I assume you're doing this because you don't have much clearance to slide in there. If you can safely raise the front end also, that would take care of it. If it's just a few degrees then I would think you probably have sufficient fluid level when you refill.

I'm not familiar with the design of your MB, so what I'm going suggest may not work. Instead of attempting this job with the rear elevated the entire time, try this:

- If breaking the front plug loose is tough with car level, elevate it; then break the plug loose, then bring car back to a level position. You might as well do the same for the drain plug while it's elevated and DO NOT remove drain plug until you know the fill plug will come loose.

- If you're small enough, slide underneath from the rear or from a side and remove drain/fill plug. Once drained, install and tighten drain plug as best you can.

- Now take a length of heater hose(maybe 6ft-10ft) and put a small length of plastic tube in one of the end of the hose and clamp it down with a hose clamp. You can use the plastic tube that's in the hose that comes with a funnel/hose combination tool that you buy at one of the mass marketers like Auto Zone, etc. Fish the hose over the top of one of the rear tires, then pull it down and place tube end inside fill hole. Now place a funnel into the other end of the hose and slowly pour in the gear oil. Reinstall fill plug as best you can.

- Once refilled, put rear of car back on ramps and snug down the drain and fill plug as you'll low have greater access to provide better leverage for snugging down the plugs.

Also, drive the car for several miles before draining the gear oil. Like motor oil, it will flow faster if hot.

I've always added more or less what the manual called for and in doing so, noticed that fluid would flow out the fill hole afterwards.

This approach has served me well for many years. I drain and refill the differentials on all of my vehicles annually, irregardless of mileage.

That should be "make sure you can open the fill hole first before you open the drain hole".

I did mine the other way and overfilled.

Corrected my error - thanks for bringing it up.

I found it rather easy to change out the fluid in my diff, including the fill part by simply raising the rear of the car until just before the tires leave the ground. This way the differential is level and there's enough clearance to fill from the standard gear oil bottle.