Fuel senders

I spent a few attempts regarding aftermarket fuel senders. I tested at the tank, with the wire, with the fuel sender out of the tank, and moved the float manually.....my wife watching the guage as well, at the driver side under the left panel, and in both cases, using an auxilliray/extra ground. Worked fine. Then hooked up the wiring, and did not work. I took the dash apart, added an extra ground to the dash frame, did not work. I looked more closely at the IVR, (Thanks 65 Dartman), and then I changed the IVR, and all worked. Ohming out the wires, insuring good grounds, discovering if the guage is good, is the dash circuit board functioning properly...no opens or shorts, and a good IVR. Maybe not an answer to your question, but these are what I looked at. I bought my last sender, after market, and it worked great. Bought it from Classic.