I have a 1985 190E that is not starting.

I put in a used fuel pump and the car started and ran for a couple of days.
then it died again. this time though i hear the fuel pump energize when i turn the key to the on position, but when i crank it it does not go on.I have tried starting fluid in the air filter and the car starts and runs until the fluid runs out.
Is thier a secondary circuit that runs the fuel pump after it energizes?
What should i be checking if thier is not a secondary circuit?

Thanks for the help

BIG AL