[69 Code51 Z]

Had a similar problem a few years ago. I have a friend that was a long time Ford mechanic and he had a tool from Snap On that checked gauges. It looked like a small box maybe 4" square with a rotary switch on top. Thew positions of the switch to the best of my memory was OPEN - 0 ohms - 75 ohms. He hook one lead to the lead that hooked to the sender and the other to a good ground. As he rotated the switch the gauge would go up and down. You have to make sure the sender is grounded to the head and that the engine is grounded. If you sender is wrapped with teflon tape or the threads in the heads have heavy paint that could be your problem. Hope this helps