Most cars, including vintage Mustangs, have a fuel-tank sending unit that conveys to the fuel gauge the amount of gasoline in the tank. Sometimes they quit working and need replacement. No need to worry though, as the R&R procedure on a vintage Mustang is quite easy.
The following photos and captions show exactly how we replaced the sending unit in our '69 Mustang project car. Our friends at Mustang Country International had our car up on the shop hoist, the fuel tank drained of about six gallons of gas, and the new sending unit installed in our car in about a half hour. We acquired a sending unit from Mustangs Plus, a Northern California vintage-Mustang parts house whose print catalog and Web site offer just about any part you can think of for '65-'73 Mustangs, as well as a few late-model bits, too.
While the sending unit in our car still worked fine, we wanted to show you how to do this basic but still fundamentally important repair.
 This is the new sending unit...  This is the new sending unit we installed in our car. Available from Mustangs Plus, it's a direct replacement for the original. Basically, '65-'73 Mustangs came with three fuel tank sizes: '65-'68s had a 16-gallon tank, '69s had a 20-gallon, '70s had a 22-gallon, and '71-'73s went back to a 20. We installed a larger 22-gallon tank in our '69 for an article in the June '04 issue, but at the time, we reused the car's original 20-gallon sending unit, which affected gauge accuracy. We wanted to install a sending unit for use with our 22-gallon tank. Also note that it is possible to install the 22-gallon tank in any '65-'70 Mustang. The '71-'73 tanks install from underneath the car so the upgrade to a 22 isn't possible. |  |  Have as little fuel in the...  Have as little fuel in the tank as possible before starting. We only had about six gallons, which wasn't too difficult to handle. If your tank doesn't have a drain plug, siphon the gas directly into a gas can. We drained the gasoline into this plastic bucket, and then immediately poured the gas from the bucket into a plastic six-gallon can with a funnel so it could be poured back into the tank after the installation. |
 Once the tank is drained,...  Once the tank is drained, removing the old sending unit is easy. Access the sender's location underneath the tank just behind the car's rear axle. Begin by unplugging the wire for the gauge from the sender. A gentle tug with your fingers should facilitate its removal. |  Next remove the fuel line....  Next remove the fuel line. Once the small hose clamp is loosened with a screwdriver, the hose can be pulled off. Have an absorbent towel or rag handy to catch any excess fuel still in the line. |  With the hose and wire removed,...  With the hose and wire removed, use a large flat-head screwdriver and a hammer to carefully tap the sending-unit retaining ring counterclockwise to loosen it from the tank... |