ok so i have a 92 sc300 na-t. had all the factory wire connected and the fuel ecu is bypassed just by connecting the black n red with the green wires. worked fine although the pump was LOUD. On monday i went out to start the car and take the kiddo to school and the pump wouldnt turn on. so i popped the trunk and realized i forgot the plug the connector on top of the tank. so i plugged it in and the pump started right up.... i forgot something inside shut the car off and the pump was still running.... couldnt figure it out so i unplugged it. and walked the 1/8th mile to kiddos school with him.
went to the store bought two new relays plugged one into the efi relay and other into defroster. boom the fuel pump stayed off. i try to start the car and the pump doesnt turn on. i messed with the plug on top of tank and it turned on but sounded weak.

ended up completely bypassing that connector on top of the tank. and hardwired the fuel pump with 8 gauge from battery, and a 30 amp relay. everything it hooked up right. but the pumps not kicking on and its a brand new walbro 255. i went to where the fuel pump ecu is and where they had by passed it. wires were loose. fixed them by soldering the wires together. tried starting car again and the pump still dont turn on.

any ideas? the key is somewhat loose but everything besides the pump works.

 

Since you checked the 2 most common spots. Best bet would be to check your ground, then start jumping things in the circuit as a test to see if it is malfunctioned. Are you getting 12v at the pump? Very low ohms as a ground? if not then got to find out where your loosing your voltage starting at the battery towards the pump.