Here are some general close-ups of the in-cab fuel tank sending unit. Fig. 3 and 4 show two slightly different styles of heads, but although slightly different in appearance, they both work the same and have the same electrical resistance values.

Even though it probably doesn't seem like it to someone who's having problems with their fuel level gauge, these are actually very simply electrical devices. Understanding how they work makes it much easier to diagnose problems. Here's how the fuel gauge works:

The fuel gage circuit consists of the sending unit in the fuel tank, the gauge in the dash, and the wiring and dash wiring (or circuit board) between them. As with most sensors, the tank sending unit is simply a variable resistor. Its function is to resist the flow of current from the ground to the gauge depending on the position of the float within the tank. The higher the fuel level, the higher the float raises, which decreases the resistance between gauge and ground. The more current that flows through the meter, the higher the gauge needle moves.

The sending unit operates off a 5-volt pulse supplied by the instrument panel voltage regulator (IPVR, Fig. 5), so just running a 12V hot wire to the unit won't work, and in fact will fry the sending unit. See Fig. 6 for the typical location on the backside of the instrument panel.

Fig. 7 - '67-'69 vs. '70-'72 style regulator. The later style was for use with a printed circuit board. (click to enlarge)

You can test the IPVR without pulling the instrument cluster. Tilt the seat forward and pull off the orange wire from the fuel tank sending unit. When you probe the connector, your test light should flash on and off. You can also do the same test under the hood. Pull off the sender wire from the temperature sender and/or the oil pressure sender and probe those connectors...they should flash too.

With the ignition switch on, remove the wire from the fuel tank sending unit and ground it to the unit housing. This removes the resistance supplied by the sending unit, and should cause the gauge to swing over to the 'Full' position. Does this happen?

The problem is with the sending unit ground. Either the unit itself is bad or the float/arm is binding/missing. The sending unit must be removed for further testing.

The power supply to the gauge has been interrupted, (check fuses), the gauge is defective, or the wire from the gauge to the sending unit is open or has a break in it.

Testing the Sending Unit

After the sending unit has been removed from the tank, check to make sure the float is present,  intact and still floats and that there is no binding with the float arm. Reconnect the gauge wiring to the unit and ground the unit. Move the float arm up and down. If the gauge reads full in the full up position and empty in the full down position, then the sending unit is good. Verify by hooking up an ohmmeter to it and making sure it reads 8-12 ohms full and 70-73 ohms empty. If not, the sending unit needs repaired or replaced. You can easily disassemble the head of the sending unit to clean and/or repair...see the photos below.

(1) If you have dual tanks and a non-operative or sporadic fuel gauge, and grounding the sending unit wire on each tank pegs the gauge, you probably don't have problems with both sending units. Take a look at your fuel tank selector switch as a culprit. Test the switch for continuity. You might find you'll need to carefully disassemble and clean the switch internally and lubricate it with dielectric grease.
 

Fig. 9

Measuring the resistance in the 'Full' position of the float gave readings between 9-11 ohms.

Measuring the resistance in the 'Empty' position of the float gave readings between 67-70 ohms.

As mentioned above, a fuel tank sending unit is nothing more than a variable resistor, and the fuel gauge is simply measuring the resistance between the hot wire and the ground. The fuel tank sending units used on all Ford products up through 1986 have a resistance range of approximately 8-12 ohms full and 70-73 ohms empty. (I tested my factory 25-gallon in-frame auxiliary tank's sending unit, however, and got readings of 20Ω full and 85Ω empty.)

Fig. 5 - To open the head, simply straighten out these metal tabs holding the unit closed and gently pull the two halves apart.

Fig. 6 - Once the unit is opened up, you'll see the thin wrapped wire which make up the resistor. The thin metal wipe glides along the wrapped wires, and the resistance is measured between the end of the resistor and the end of the metal tang.

Fig. 7 - Here's another view of the exposed resistor, with a better angle showing the metal wipe inside. This wipe can be carefully bent, if needed, to make better contact with the wound resistor wire. Occasionally the wipe has fallen off the shaft and can be soldered back on. You can also gently clean the internals with fine steel wool, but be VERY careful not to break any of the wires.

If you're unable to find an exact factory-style replacement sending unit (or unwilling to pay the prices they're getting these days), there's another option. You can simply go to your auto parts store and tell them you need a sending unit to work with those resistance readings mentioned above. If you can only get one which similar resistance values but not exact, you can usually bend the float rod and/or the tangs on the sender body to alter the resistance to suit. Most sending units have universal flanges that will work in any tank unless it is home made.

If you're going an aftermarket gauge, depending on who you get it from, you might have to use their sending unit. Most aftermarket gauges are not for use with a factory Ford sending unit. However, Autometer does have special gauges which work with the factory sending unit. From the Autometer website: