Knowing How Much Gas You Have in the Tank Is Pretty Important

While 16 gallons of gas might have been plenty for a 200 cubic inch six, but when Ford decided to put a 390 4V in the 1967 Cougar and Mustang, gas stops became far more frequent. Even the 289 and 302 were relatively thirsty, and gas stations are not on every corner the way they used to be. In 1969 capacity was increased to 20 gallons and then up to 22 gallons in 1970. Still not much range with a 351, 390, 428, or 429!

Fuel Sending units came in two varieties. The low fuel sender has two posts and a built in thermistor. The standard sender has a single post until 1971. The sender for each year is different from ’67 to ’70 and then the same from ’71 through ’73. In general they are not interchangeable unless you use the matching gas tank.

Many of our cars are still running the original fuel sending unit. Over time gasoline residue, a sort of shellac, causes these sending units to progressively read lower. If you fill up the tank and the gauge is only reading 3/4, this is most likely the problem. Of course the gauge and instrument voltage regulator are also 50 years old, but inspection of the sending unit will quickly determine the fault.

All Cougar XR-7s and Eliminators were equipped with a low fuel warning light. Standard Cougars and Mustangs were optionally equipped with the Safety Convenience Panel that also included the low fuel warning light. Chances are that you have never seen this light work properly. When you turn the ignition key to the start position, the light should come on while the engine is cranking over. This is called the prove out function. It proves that the light bulb still works. The system uses an electrical device called a thermistor that detects the fuel level. The one that Ford used seldom lasted more than a few years. Back in the day, the fix was a new sender with a new thermistor. Today a New Old Stock sender can cost over $400, IF you can find one. And even if you do, that 50 year old thermistor may not last long (like 15 seconds on one I just bought!)

There is a fix. Unless your old sender is rusted through or otherwise mangled, they usually can be rebuilt.

The sender is first cleaned up and evaluated. This may involve some quality time in the parts washer and then into the blast cabinet for careful hand cleaning. The main tube is cleaned internally with solvent, wire brush and then blasting. After testing to ensure that there is no rust thorough, the sender is disassembled, the resistor housing opened, the thermistor housing unsoldered and then completely disassembled. The internal wire wound resistor is wet sanded using very fine grit emery paper and contact cleaner. The sweep arm is filed back into shape to remove any sharp or jagged edges. The disassembled parts then go back into the blast cabinet for very careful internal cleaning. The thermistor bullet is disassembled. The internal screen is removed, wire brushed, and cleaned with solvent. The internal housing is cleaned with a wire brush and solvent. The bullet is them polished to insure a good electrical connection. A new more durable thermistor is installed and carefully soldered into place. The bullet is then soldered back on to the main tube bracket. The resistor enclosure is reassembled and then the first round of testing begins.

Calibration of your sender is done by hand inside the resistor housing. Initial testing is done with a multi-meter but the proof of success is putting your sender under load in the original Ford circuit, powered by an original Ford Instrument Voltage Regulator, driving a Ford gauge. We can supply you pictures of your sender under test.

The low fuel function is also tested. The way the thermistor works is that it is self heating. When it is immersed in gasoline the gasoline acts like a heat sink and keeps the thermistor from warming up. In air the thermistor warms up, and as it does it’s resistance goes down, eventually triggering a relay to light up the low fuel warning light.

The thermistor that Ford used has been out of production for many years, but as it turns out that is not really a bad thing. It was not very durable, barely lasting out the warranty period, and it is also not compatible with modern solid state voltage regulators. The original electro-mechanical voltage regulators were calibrated for about 13.8 volts of output. When you car is running this is the maximum voltage it is operating at. Most were below that, and if you were running the lights and heater it would barely be above battery voltage at about 12.6 volts. The 42 amp alternators our cars came with seldom even put out 42 amps. A modern solid state voltage regulator typically tries to hold voltage at close to 15 volts, and new alternators are far more capable. While the difference in voltage may sound trivial, to the thermistor it is not. All of the other instruments in you car run off of a regulated 5 volt supply. This means that they won’t be effected by changes in the operating voltage of the car. The thermistor sees whatever the alternator can put out. After much research a new thermistor has been located that can accommodate the higher voltages.

Each rebuilt sender is returned to you with a new brass float, new filter, seal ring, and a new thermistor if originally equipped. Return postage by Priority Mail in the USA is also included.