Sometimes they draw too much current. Consider what M.B.DOC had to say in this thread:
http://www.peachparts.com/shopforum/showthread.php?t=20307&highlight=fuel+pump
Not sure if the '91 190E has a MAS controller(many functions in one to include fuel pump relay). If so, you can measure the current between pins 1 & 2 using the 10 amp scale of a VOM. I'd use a cheapy and not my pride-n-joy Fluke as these have a max. capacity of 10 amps.
If you don't have a MAS, then use the SEARCH facility to determine where/what pins, etc. for your fuel pump relay.
Just something else to check.