I'm not versed in the ways of a 500-SEL; I suspect that in many ways my 300-SEL W126 and yours are similar with regards to tranny fluid change:
In my case, the following tools were required:
27mm six-point socket - 12 point will work if that's what you have
5mm allen head wrench/socket
Place front of car on ramps or as Larry Bible has done, drive car up on to 2x12 boards. You need to elevate the front end so you can crawl underneath it.
Use the 27mm socket to turn the engine over manually. Turn clock-wise facing the front of the car. You place the 27mm socket on the end of the crankshaft-front of car. Before you do this you will need to remove the large plastic shroud underneath in order to gain access to the under-carriage. In your case, I do not know what size bolts secure the shroud.
The is a cross-member that runs across the car that covers part of the auto-tranny converter. In my case, there are 3-17mm bolts holding each side in place. Remove these bolts and the cross-member. With this out of the way you can now turn the engine with the 27mm socket mentioned earlier. You'll do this until the converter drain plug is at the bottom of the converter. You'll see it through an opening at the bottom of the converter once the engine has been rotated enough to bring the converter drain plug to the bottom. The 5mm allen-head takes this out. The converter will now drain. I believe it's round 3.0 to 3.5 qts.
Further back is the tranny pan. 13mm bolts hold it up and access in my case is unobstructed. Before removing the pan, remove the drain plug on it - also uses 5mm allen-head to get it out. 3.0 to 3.5 qts drains out the pan - this is an approx. measurement.
Wih pan drained and removed, remove the tranny filter. I forget the number of phillips screws holding it, but it's around 3-4.
Remove and replace filter.
Be advised that the drain plugs have washers that should be replaced. One uses a silver washer and the other a copper-colored washer. I forget which uses which. I would definitely replace these.
Reassembly is reverse of this. Pour approx. 4 qts Dexron in through the tube where your tranny fluid level dipstick is. Start is up and slowly add the rest. Go slowly - do not overfill.
Remember that it takes about 15 miles of higway driving to warm up the tranny enough to check the fluid level. After this much driving, the fluid level should be up to the HOT mark.
This is an easy task.
1991 300-SEL - Model 126
M103 - SOHC
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