I vote for low voltage -- caused by radiator neck failure (cracking).

Coolant could be spraying onto the belt, causing some slippage over the alernator pulley (thus low voltage) and water pump pulley (contributing to elevated temperatures because of less efficient cooling.) If the radiator neck (or other part) is cracked, you will additionally suffer low system pressure, further reducing cooling capacity.

Squeeze the top hose when the engine is hot; it should be basically non-compressible. If you can sqeeze it significantly, and if you hear hissing, your neck likely has a hairline crack.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black