1990 Ram 50 tachometer problem

I have a 1990 Dodge Ram 50 2WD pickup. I purchased a used combination speedometer & tachometer unit and just installed it in my truck. I've read how it is basically a "plug and play" change to make requiring only a change of the oil pressure sending unit. I installed a genuine Mitsubishi oil pressure sending unit. My issue is upon running the truck since installation all the gauges work fine, but the tachometer does not work. Is there something I'm missing? Was I sold a bad unit? Is there a way to trouble shoot the tachometer? Any help will be greatly appreciated.

I think the tach signal comes off the (-) terminal of the ignition coil, see if you can find whether that's connected or not.

Not sure about FI models like yours, but on carb'd models that would be a white wire, and that (-) coil terminal would have one other wire connected, from the ICM/ignitor sensor in the distributor.