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. 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. 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?

The sender has its value (100c) stamped on the hex.

The gauge has 3 connections. S-source (sender), + for 12v and - for ground. The sender does not need to be grounded as the gague is grounded.
My guess is that you have a ground or voltage issue with the 3 prong connector. Does the factory connector fit the aftermarket gague and are the poles in the same location?

Brown is the ground and the red/white is the 12v. I forget the factory color for the sender.

Correct. The S and the + are opposite and have been flipped over. The 3rd wire or ground stays at the same position on the plug. The grn/blk is the sender wire. If I reinstall the old gage (after crossing the S & + wires back to the original configuration) it will work. Shit, you just got me to thinking. I'll be right back.........

Did you ground the gauge? No ground no workeeee! Gauge does not ground thru the sender.

I just remembered that the bulbs are two different styles. The old gage has the metal sleeve. I wonder if the gage gets its ground thru that whereas the new gage has the plastic bulb holder with a 2 wire connection. I'll try bringing a separate ground to the gage and report back later. And now I see Ron's remark. Keeping my fingers crossed.

The sender does have to be grounded. The gauge measures resistance across the sender to ground.