In 1991 on 4.3 V6 - 5.0 - 5.7 V8.the pick-up coil and module are used for starting and timing along with injector signal
Sorry for the Misunderstanding, but as cctmstr last stated:
There is NO Crankshaft Position Sensor on this year Pick up.
The Ignition System is still off the Camshaft thru to Distributor, Coil & EST, with the Knock Sensor to help control Timing.
If this is a 1991 Chevrolet P.U, V/8..........There is NO Crankshaft Position Sensor.Crankshaft Position Sensor was not Installed on trucks until 1995
Is there a specific problem you are having that is making you look for a Crankshaft Sensor?