VDO Oil Temp Gage Question, 'cause I'm feeling really dumb Options

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(IMG:style_emoticons/default/headbang.gif) (IMG:style_emoticons/default/confused24.gif) I've been F*$#%ing this old dead dog for too long and need help with this. This is in my 73. I added the console with the correct harness. One of the gages added was the console Oil temp gage VDO 0-300' scale. I also installed the sender that was ordered with it. It would never give any reading. I checked volt & cont and found that the original 3 prong plug must be flipped so I thought that I probably fried it. I installed a factory gage and SoB it worked. To be sure I installed the factory sender in place of the newer one. (BTW, the factory gage read appx the same with either sender.). 12 engine hours later I buy another gage but before installing it I did 2 things. I flipped the plug and then I tested the new gage in place, jumped to the 3 prong plug for S & + but jumped the sender with a separate ground then placed the sender in a glass of boiling water boiling. It worked great. On a hunch I tested the "fried" gage whilst set up and SoB. It worked. (IMG:style_emoticons/default/WTF.gif) So I drain the oil, clean strainer, chase the threads to be sure there is no sealant to break ground, change out the sender, copper and paper gaskets and reassemble. Start the car and SoB, it doesn't work. (IMG:style_emoticons/default/WTF.gif) Do I need to run a separate ground from the gage to sender? Why wouldn't the sender ground thru the engine. Does it come down to those 2 small bolts eventually contacking the case half? Why does the factory style gage work without interference? (IMG:style_emoticons/default/blink.gif)