mercedes 1979 450sl Fuel pump?

Question

why are both wires to fuel pump grounded?

Answer 1

the brown wire is the ground the black is the hot , when the relay turns off the "rest position is a ground to prevent high voltage spike [collapse of Field coil windings from pump motor] from damaging relay, computer, other electronic devices. if stays ground while cranking suspect pump relay [make sure relay is getting ign. pulses] ie: check for spark first.before condemning relay