I do not know how you are taking the measurements but the attachment shows the functions of the 4 pin pump connector. Pair 1/2 is for the tank float level measurement and the pump positive on pin 4 with the common negative on pin 3.
Incidentally the inertia fuel cut off switch interrupts the Negative to the pump pin 3 if it has been operated.
I am not sure but think the Positive on Pin 4 is only present for a short time when ignition is switched On and shut off unless the engine is started.
Whilst googling I found this comment on the Fiat forum that may be relevant to your intended use. I do not know if what is said is correct as it is contradicted in a later post:
"There are 2 fuel pumps.
1) A low pressure electric one on the top of the tank which you access from the smaller of the 2 'manhole covers' in the cab floor.
2) A high pressure mechanical one on the engine.
Once the engine has been primed with fuel by the electric pump, it is no longer necessary in normal temperatures because the engine pump will suck the fuel right through from the tank - (so the old anti-theft idea of fitting a secret switch for the electric pump won't work) Its only when you have run out of fuel, change the filter etc so the system needs priming, or it gets very cold so the diesel is too thick for the engine pump on its own, that you need the electric pump"