mpyne makes an interesting point - his profile indicates 29 yrs. MB experience - can't argue with that.

I'll throw this in for what it's worth. It's what I call a poor mans electrical system test. It does not "load" test the system, but gives some indication as to the general health of the charging system. If you do not understand what I'm about to describe, find someone who does and to the originator of this post, this would not include the mechanic you've given 8 grand to.

Buy a digital voltage-ohm meter, more commonly known as a VOM. The ones sold at Wal-Mart are pure crap as most everything else they sell is.

Set it on the 20 scale - DC side. Start car. Put red VOM prong on the + terminal of the battery and the black prong on the - terminal of the battery.

What voltage reading are you seeing? Your MB is likely equipped with Bosch electrics and this sytem for whatever reason doesn't put out what Asian and many Americam models do. Your MB may only be putting out 13.5 or so. When the brushes in the voltage regulator start to wear, this reading starts to look more like 13.3 or lower.

If you perform this test on a cold engine and all is well, you may see 13.95 or slightly higher, but not much more. The reading drops when the engine has been driven. I get suspicious if it falls below 13.5 when testing a warm engine.

One of these days I'm going to experiment with the adjustable Transpo regulators. They make a model that an be turned up in .5 volt increments, all the way to 14.4. This sort of reading is what you see on Asian cars.