I have a 96 Dodge Ram 2500 4x4 that use to be a v10. I bought a 98 12v cummins, wiring harness an ECM. But I have a issue with no first gear an no overdrive an the gauges aren’t working other then the fuel an voltage. My question is what would cause my speedometer to rev up like the tach is supposed to an why my transmission won’t shift through all the gears.

 

Because a LOT changed from 96 to 98. Wiring harness etc, from a 96 or 97 diesel truck, along with the ecm, is likely to be your best solution.

 

96-97 wiring works, if OP has engine with P7100 injection pump, but if it's with VP44 then it's a whole other story. Both were offered in 98, pre 98.5 had P7100.

 

12 valves have the P pump the VP44 was on 24v trucks.