My first reaction is you have the wrong sender.
The easy way to find out is to measure the resistance of both senders as you are heating them in water.
They can be connected together for the ground. And a small wire on the sensor stud will enable connection to the ohmmeter.
The resistance of both old and the new senders should be the same throughout the temperature changes of the water.
The gauge going to full pin says that you have a switching sensor (used with idiot lights) and not a resistance sensor and the threaded connection into the head is being insulated by the pipe thread sealer.
Last edited by vicrod; 05-02-2013 at 11:35 AM.