I am testing a new 3/8"npt sending unit. The ohms appears ok. Problem is when I connected the guage assembly to a 12v power pack (made up of AA batteries) I used for light checking. The guage will react to the 12v on quick contact but will not react to the curcuit with the sending units installed. I have the units setting in the opening of a clean pop can with water. There is a food thermo in another hole. I warm the water with a propane torch to different temp and check each sending unit ohm reading. Both the original with the 1/2" npt and the new 3/8" units check very close at high temp. Taking the sending unit out of the line and jumped the guage reacts but with either unit in - nothing. Does the battery pack not have enough strength to operate the guage with the sending unit resistance?