In addition to all of the above...I am having fits with the AC compresssor on my '93 500SEL. Aparently there is a sensor that compares the engine speed with the speed of the compressor. If they don't match the compressor shuts off. So far I have replaced the drive belt and tensioner and the only way that I can get it to work is to keep spraying STP belt conditioner on it every few days. In the spring when I get my income tax refund I plan on taking it in and just telling them to replace the compressor at a hit of $1150 (it's a nippondenso).
I love my W140, but you do need to have a fairly substantial bank account to keep up with the repairs!