Replacing a fuel tank and sending unit, and getting the gauge to read right

Old fuel tanks rust from the inside, and once a tank is shedding rust it will keep destroying fuel filters, pumps and carburettors until it is dealt with. Lining an original tank is possible; replacing it is usually cheaper and always more reliable.

01Safety, briefly and seriously

Fuel vapour, not liquid fuel, is what ignites. An "empty" tank is the dangerous one because it is full of vapour at a burnable concentration.

• Outdoors, or with real ventilation. Not a closed garage.
• Disconnect the battery before you start, negative first.
• Drain the tank into proper sealed containers. Not buckets.
• No grinding, no welding, no drop lights anywhere near this job.
• A fire extinguisher within reach, not in the house.

If the tank is inside the cab behind the seat — as it is on many older trucks — take it outside before doing anything else with it.

02Removal

Support the tank before undoing the straps. They are usually the only thing holding it and a tank with fuel still in it is heavy and awkward. A trolley jack with a piece of plywood works well. Disconnect, in order: filler neck, vent lines, sending unit wire, ground strap, then the fuel line. The sending unit wire and the ground strap are the two people forget. The ground strap in particular tends to be corroded and is frequently the reason the old gauge read badly — worth knowing before you blame the new sender. Fasteners will be seized. Penetrating oil the day before makes a real difference. If a strap bolt shears, that is normal; buy the strap kit.

03The sending unit is where this job succeeds or fails

The gauge reads a resistance. If the sender's resistance range does not match what the gauge expects, the gauge will be wrong, and no amount of adjustment will fix it. Different manufacturers used different ranges, and they are not interchangeable:

• Many older GM applications use roughly 0 ohms empty to 30 ohms full.
• Much of the industry, including Ford and many imports, uses 73 ohms empty to 10 ohms full — note that this one runs backwards relative to the GM range.
• Later GM moved to a 40–250 ohm range.

Fit a sender with the wrong range and the symptom is a gauge that reads full at half a tank, or never leaves empty, or pins itself. It is not a faulty gauge and it is not a faulty sender — they simply do not speak the same language. Check the range before you buy, and confirm it with a multimeter across the sender terminals while you move the float through its travel. Two minutes on the bench saves dropping the tank a second time. Check the float arm travel too. It must swing freely through its whole arc without hitting the tank, and the float itself must actually float — foam floats absorb fuel over time and brass ones develop pinholes. A sunken float reads empty permanently.

04Fitting

New lock ring gasket every time. Never reuse the old one. The lock ring is turned with a brass drift and a hammer, or the proper tool. Brass, not steel — a steel drift and a spark at a tank opening is exactly the scenario the safety section was about. Set the sender to the correct depth for the tank. Reproduction tanks are sometimes a slightly different depth from the original, and a sender calibrated for the original will read wrong. Manufacturers usually specify this; if yours does not, measure the old one. New rubber isolator strips between the tank and the straps. Metal on metal chafes through a tank in a few years, and that is a fuel leak in the worst possible place. New filler neck hose and new clamps. The old hose is hardened, and modern fuel finishes off hoses that were fine in 1972.

05Grounding

The sender grounds through the tank, and the tank grounds through the straps to the frame — through paint, rust and rubber isolators. That is not a reliable ground, and a poor ground makes the gauge read low and erratic in exactly the way that looks like a failing sender. Run a dedicated ground wire from the sender flange or a tank strap bolt to clean bare chassis metal. It costs a few dollars and it eliminates the single most common post-installation complaint.

06Filling and checking

Put a few gallons in and check every joint before going further. Then check again after a drive, warm, with the vehicle on level ground. Expect the gauge to be approximate. These systems were never precise, the tank is not a uniform shape, and the last quarter is always optimistic. What matters is that it moves smoothly through the range and reads empty when empty. If it reads full permanently: usually an open circuit — a disconnected wire or a failed sender. If it reads empty permanently: usually a short to ground, or the ground problem above. Which of those two symptoms means what depends on your gauge's range, which is another reason to know it. Fit a new fuel filter at the same time. You have just changed the whole supply side; do not feed it through the old filter full of the debris you were trying to get away from.