Does anyone know how to correctly check an oil pressure sending unit. The PO had an aftermarket sending unit and gauge installed which started to leak oil. I replaced it with a new sending unit from NAPA. The old one had one wire to the gauge and the new one has one spade for the connector. I installed it but it doesn't work. It shoots to 75 psi when the key is on and doesn't move. I measured resistance on the old sending unit and got 240 Ohms. I can't get any reading on the new one. Does this mean it is bad or does it operate differently?

ttreat
95 T-100 SR5 4x4
Pretty much stock, except for the goofy oil pressure gauge the previous owner installed.