Wiring HMMWV tachometer in M1028 truck

[Sharecropper]

I'm attempting to install a HMMWV tachometer in my M1028 truck. I'm installing a new 6.5 diesel engine, therefore I don't need the 6.2 vacuum pump anymore, so I purchased a new HMMWV tachometer and sending unit. But before I go to the effort to design and fabricate a custom bracket in my truck, I wanted to temporarily wire it up and spin the sending unit to make sure everything worked. I wired 24.2v to the appropriate posts on the rear of the tach, and then rigged a redneck pigtail to the wires of the sending unit. I then chucked the drive rod of the sending unit in my Milwaukee drill, and spun the sender to 1700 rpm. But the tach needle never budged. What am I missing? It is my understanding that the black sending unit is the correct one for this tach. I reworked the connections on the pigtail and verified continuity, but for the life of me I cannot get the tach needle to move. Is the tach case supposed to be grounded also? Thanks in advance for your help and comments.