gauge or sender problem?

I have a 1973 Mercruser 120HP engine in my boat. I am trying to determine if the problem is with my trim sender or gauge. My gauge responds when I turn the ignition on. Then as I lower the stern drive it does nothing until the unit is at the bottom of the stroke. Then the gauge indicates the unit is down. I have opened and cleaned the sender. It seams to operate properly from a mechanical standpoint, indicating something electrical. I have a test meter but I am unsure what to look for to test the sender or gauge.

measured resistance between sender wire at the gauge and ground. Found open circuit throughout travel of lower unit until it reached the bottom,then resistance went to 1-3 ohms. Looks like a sender problem to me

Bingo you found it, the sender is bad, not working throughout the range