Cummins Generator 121 code puzzled.

I'm a heavy/automotive tech, but I was checking one of the generators at a water treatment plant. It starts, revs up slightly for half a second then dies with 121 a speed sensor lost code on the display. The motor is a 3.9L with a stanadyne pump and electrical governor system from barber colman. I replaced the magnetic pickup on the flywheel housing because it was cheap and most likely the problem but the engine is doing the same thing and throwing the same code. I set the sensor to 1/2 turn out, 2.1V AC while cranking, which is the same voltage as it's sister generator. The new sensor and old ohm out the same resistance (900-1000) as well as compared to a known good. I've check the wires from the control panel to the sensor for resistance and verified that they could carry a load with a test light, unhooked from the board. I also get the same AC reading back probing the control board as I get from the sensor itself. I sensor uses the teeth on the flywheel for its reluctor and they are clean. The control panel is displaying a RPM signal of 280rpm while cranking and 850 when it starts shortly before shutting down. We had Cummins come out to check the generator because I was suspecting a problem with the ECM and wanted them the verify before purchasing a new ecm and getting it programmed. They came to the same conclusion as me and replaced the ecm and programmed the board. The generator is still not staying running and throwing the same code, they don't know what's going on now either. I figured they would have had a trouble tree for this code. I'm wondering if anyone could spare their expertise with me as I'm not a generator guy. I'm curious if this code is not the problem but the symptom of another problem with this generator. Is it possible that a fuel related issue shutting this generator down would trip this code if the engine dies while the generator panel is still in run? losing the speed sensor signal. I believe the pick up sensor is the one of the only things telling the ecm that the motor is in fact running. I was curious if I should supply an alterative fuel source to see if the panel will throw the code when the engine is being forced to run? I checked the fuel filter and made sure the pump and injectors are getting fuel. I'm not familiar with the electronic control on this pump, it seems like it looks at the magnetic pickup and adjusts the rack in the pump by varying the current to the actuator by a throttle controller? It also uses this as the shut off the generator. Any info would be greatly appreciated