Fuel pump has power but when 12 volts at harness is put under load from fuel pump it drops to 3volts and wont turn on.

new fuel pump. bench tested with 12 volts works good. harness at fuel pump shows 12 volts when ignition is turned on but when connected to fuel pump the 12 volts drops to 3 volts. checked larger ground wire connected to good ground on body of car just to check ground was not the issue, but still 3volts under load.

checked relay in fuse block good. fuse next to relay good.

I'm stumped to the drop in voltage.

Thanks for your help