Well, i doubt its the pump. They didnt even mention the auxiliary pump. Could be the monovalve, the temp. sensor on the dash, the ACC, or any number of things. She didnt mention overheating, so I would thinnk the pump to be the least likely suspect.
OH, yeah, $500 for a pump? Someone needs to email them a link to fastlane!!

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Paul
2004 E500 4matic; 72,000mi