I have an oil pressure indication issue. Mechanical test confirms oil pressure. Done. Ground grey wire unhooked from sender and turn key on, gauge sweeps smoothly to high then fluctuates, I assume the fluctuation means that the voltage reg is working. Multimeter at terminal end, key on, unhooked from sender, moves to 4-5v, again, fluctuating. Sender (for dash gauge, not idiot light) grounds fine to the engine and body with a continuity test. Sender (new) ohms test shows 19-22 at idle. But put it all together and dash gauge doesn't move. All other gauges operate, temp, fuel and amp. All operating. What am I missing?