The Stovebolt Site Forums The Shop Area Engine Shop Fuel tank level

Please remember the AD tanks include a horizontal baffle plate about 3 inches or so from the bottom. Even the replacement tanks include this from my experience. It wasn't a slosh baffle and interestingly with it in place your sender can't send a 0 ohm message to your gauge...the baffle prevents the wiper from reaching the lowest part of the range. One of GM's little mysteries, I suppose. So, with this baffle plate in place and a sender unit mounted from the top, your float can't touch the bottom of the tank nor register empty unless somebody has monkeyed with the float rod to make it register 0 when it hits the baffle. 4100 Fire Truck got your formula correct, and no matter which size tank you have at half tank the reading will be 30 ohms. As for the sender unit, the best ones by a very long shot are sold on eBay by a seller whose ID is renbusan. They measure exactly 0 when empty and 30 when full and the way they're made, they will last a long time. Image below. The wiper runs flat against the windings and the windings are molded into a nylon plate where they can't move nor touch each other, and it will take a long time before they wear out. Some of the senders sold by a popular vendor of parts frequently mentioned here are a completely different story. They are not 30 ohm senders but are 90 ohm senders onto which the assembler dribbled molten solder until the coil measured about 30 ohms. You can see a picture of this below. Obviously this isn't what you want.

Now about your original sender. What happens to it over time isn't pretty, and I believe this is the major cause of sender failure. The casing was made of pot metal and for a few years it was fine, but the single pivot point wears oblong and causes the wiper arm to travel all over the place. Images below. If GM had just made this with 2 opposed pivot points all would have been fine, but that would have cost more...