What would be the outcome of installing an in-tank pump on one of these units? I have read about it causing the pump to run "backwards" and I want to know: Would that mean the pump would pull fuel away from the main pump or would it just reduce the volume pumped by the in-tank pump?

 

a reversed in tank pump would try to suck gas out of the fuel line back into the tank, and fight the main pump, and likely cause major running problems. the main pump likely would be noisy as its having to work extra hard.

 

The aftermarket unit I purchased not only had the plus and minus wires running to the pump reversed, it also swapped the pump positive wire switched with the fuel sending unit wire. I had to swap them at the black connector.

Also, just to let you know, I don't know if I would buy another aftermarket sending unit as the one I installed a year and a half ago is no longer working. Fuel pump still runs but the gas gauge is reading empty all the time. I haven't yet pulled it out to determine the cause of failure as it has been too cold to work on it.