Hey guys, I'm about to put the gas tank back in and just wanted to check and make sure I've got the right idea here on the wiring. The car is a 1969 Oldsmobile Cutlass, stock tank and sender. As you can see in the pics there is a pinkish yellowish wire with a black rubber cap that goes to a brass post on the top of the sender, theres a cable that goes from said post to the actual sender float reader inside the tank. I'm assuming this wire goes to the gauge, and is not powered, but simply reads resistance. There was also a ground wire which I am going to re-do as its gotten very dirty and nasty and oxidized over the years. If I'm thinking correctly, then I do NOT need a hot 12v positive wire going to the sender anywhere, just a ground and sender-output wire. Also, the fuel gauge when I first put a battery in the car and turned the key, did not read right. The needle moved all the way to the dummy light to the right of the actual gauge face (I think its oil pressure but can't remember), but the gauge did move. I'm assuming this is caused by a bad ground as the sender is like brand new on the inside and outside and the contact for the pinkish yellowish wire is clean. I'll redo the ground and see if it works.

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The sender is basically a variable resistor on one side of it it's connected to ground. The other side is connected to the wire with the black rubber cap. That wire runs to the gas gauge, and connected on the other side of the gas gauge is a switched 12v supply. If you remove the wire the gas gauge reads as you saw, passed full. If you ground that wire, the gauge reads empty.

 

Got it, thats exactly how I figured it worked but I just wanted to double check before I wrestled this thing back in there! Thanks!

 

Hooking up the sender wiring is pretty much foolproof. Orange wire connects to the orange wire coming from the gauge, usually in the area behind the license plate or in the trunk behind the latch, and the black wire goes to ground. An open circuit forces the gauge to go to F or usually well past it indicating infinite resistance. A grounded wire from the dash gauge sends the gauge to E as the resistance goes to zero. In fact, grounding and ungrounding the wire from the gauge is the first test one performs when the gauge doesn't work to rule out the possibility that the gauge itself at fault.

 

If you haven't put it all back together yet, make sure that crimp connector under the black cap is a tight fit on the sending unit stud. I had issues with the one on my '70 coming loose and I had to drop the tank to fix it.