Oil Pressure and temp gauge

I thought I had a bad combination oil pressure and block temp gauge, OMC application. I purchased one to replace it and was told the gauge was tested and working. I installed it along with new sending units. I did a continuity test by making a wire with alligator clips on both ends. I hooked one clip to the gauge terminal wire and back to the sending unit wire. Both sending unit wires show continuity, from gauge terminal to sending unit so I know the sending unit wires are good. It appears there is a ignition wire to the gauge and it has a jumper to the oil pressure function to the temp function. There is also a black ground terminal. The 2 other gauges, tach, and combo gas and amp gauge work fine. All the lights to each gauge work also. Could I have a bad ground connection or ignition connection. I notice on gauges when you turn on the ignition key they pulse slightly with the surge of current. The gauge in question does nothing. What to do next.........Help.