And if you have the Smiths BT2240-00C I'm pretty sure it has to be fed by a voltage stabilizer. So, that means one of the feeds is going to have to be the
regulated ten volts (not twelve), another lead is going to be your ground, a third lead will be the 12v rheostat controlled feed for the lights. And then there's the feed to the sender. What I would first do is put a VOM on the wires and see if one of them was feeding me ten volts when I know the battery is 12.6 volts. Then I would disconnect the wire at the sender on the intake manifold and run a direct wire from the sending unit up over the windshield and in to the cockpit and see if I could get the gauge to respond using that 10v feed and the short cut wire to the sending unit (with the engine hot, of course). If you can, and you have the lights already working, then I would trace the leftover wires in the harness to see which one connects up to the sender, since I can make it work with the shortcut wire. Maybe the whole problem is there is no connection between the sender and the gauge. But, I have all mechanical Smiths gauges... they're pretty easy to wire up.