There's a slight chance that something may be disconnected under the hood. I doubt it, but check that all the little plugs and wires to the right of the alternator are connected. Sorry I'm not being too technical when describing this, but I recently changed spark plug wires on my 88 420SEL and had to disconnect something to run the wire. When I started the car, I forgot to reconnect this little plug near the alternator, and the car idled at 1500.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino