I just swapped in a 5.7 vortec into my 1999 Tahoe. The new motor had a 3 pin oil sensor plug.

The one on my cars factory harness has 1 pin. I went out and bought a 1 pin and installed it into motor its a bwd sensor not ac delco. When i went to start truck I get nothing but lights come on and gauges work but no crank no start. The s terminal on the starter is not getting power from ignition switch.

The oil gauge on the dash reads 0 when the key is in the off position and when i turn the key forward.

When I turn the key forward and then back to the off position the gauge will move up to about 20 psi.

 

Not the oil pressure switch, but the transmission selector switch (neutral/park) will keep the engine from cranking.
Also check the....
IGN A fuse (under the hood fuse block) ,
CRANK fuse (instrument panel fuse block),
STARTER RELAY (under the hood fuse block)

 

Thanks ! I'll double check those fuses again. I haven't checked the ignition fuse so I'll do that today . It be weird if the nuetral safety switch went out since it was last driven but anything is possible .