One possibility is that one or more of the solder connections on the relay control module has come loose. An easy fix if your good at that kind of thing. Otherwise Tighed1 on this form will do it for you for postage and beer money, He does an excellent job, did mine in Feb. and no cel light since. Listen for the pump to come on when it is cold, it sounds like a small elect motor running in front of the engine and only runs for a minute or so. If it runs then it is a control issue. I would opt for the relays having a bad solder joint myself. if you don't want to get it resoldered I would get the relay somewhere else they are only about $170 at a MB dealership.

Over all read up on it like onehundred80 says good info there before you part with big bucks. Good luck.