David, try this.. Disconnect the lead from the sending unit, turn on the key, check the reading, start the engine, check the reading. If it reads max all of the time, then you have another problem. May be the gauge itself or a short in the wiring. You can also try grounding the lead wire to the block. If this changes what you are seeing, then it may in fact be the sending unit. The sending unit presents a variable resistance to the gauge, however, I am not sure at which end of the scale this occurs.
2000 Dodge Grand Caravan SE