380sl won't start

I purchased this car six months ago and it have had NO problems at all. Runs great until yeterday. I tried to start it yesterday and it turned over great but just wouldn't catch. I tested the battery and it is
fine, however the lights dim a bit when I turn on the ignition. I don't know much about cars, but am wondering if I am missing something simple?
Any help would be much appreciated!
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Fuel pump relay... check the date stamped on the side... if it is more than 10 years old... it's probably bad. Give it a whack and put it back in.
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