you need to find a wiring diagram for it and trace the wires and test the components.

mine had a ground wire from the - side right to the frame like 6 in of wire when i first bought the truck it had not been started in over a year i opened the hood and saw that and said look at this hillbilly shit and riped it off the owner had lost the keys so i hot wired the pump and the starter then drove it home where i started it this way for about a month then put in an aftermarket ignition switch and then found out that the headlights were dim the gauge light were dim and it wouldn't start or run anyhoot i chased my tail on this for 4-5 days and finally figured it out i cant explain why but it works and has for the last 2 years i owned it

91 Dodge Cummins Speedometer
 

I recently changed standard Transmissions on my 91 Dodge 1 ton diesel truck.
It now has a 1995 NV4500.
The old trans had a 2 wire Speedometer Sensor the new one 3 wires.
I bought an aftermarket Speedometer that can use either.
I bought a VSS103 and a SC105 sender that fit but have no wiring diagram.
I do not even know if it has to go thru an ECM to work.:scare2:

How do I get one of my Speedometers to work?