Classic/Antique Car Repair: How to fix a fuel guage, wound resistor, gas gauge

Question
The fuel guage on my 65 mustang stopped working recently.  It now stays on the E position.  Prior to this the gauge would never exceed about the 1/2 mark even if it was full.  Any suggestions as to where and how to trouble shoot this problem?  Tnanks

Answer
The problem is most likely the sending unit in the tank, but before you go to the trouble of dropping the tank to get at it, do this:  Find the wire that goes to the sending unit  - it is in the rear wiring harness, and will pass through the trunk floor somewhere and you'll see it heading over the top of the tank. Get access to that wire somehow, either by unplugging it at the other end, or by stripping a small part of it so you can see the bare copper.  Then, put a separate wire from that wire to a known good ground - like the bumper  - make sure you are making good contact.  Now, go turn on the igntion key - just for a few seconds - don't LEAVE IT ON!,  Just watch the gas gauge readout - it should move quickly up the scale in short jerks, once every second or so, until it reaches full scale or even beyond.  TURN THE KEY OFF QUICKLY now!  (If you leave it like this with the key on, you will damage the dash unit).

If that action is what you see, your dash unit is good, and your problem is back at the tank.  

Next step is to drain the tank and then remove the tank from the car.  Use only brass or plastic tools, once you have the tank loose - you don't want to strike a spark!   

When you have the tank out on the ground, remove the sending  unit from the top of the tank, and inspect it.  Take the float on the end of the rod and see if you can determine if it has fluid in it.  It shouldn't, but if it does, that's your problem (the "float" isn't floating!).  You can try to find a new float, or you can repair this one if you are handy with a soldering iron.  Or, if the float isn't full of  fluid, the sending unit may be bad internally.  It has a wire wound resistor in there which may be broken, or the moving contact may not be touching the wire - you'll just have to take it apart and inspect it.

There are vendors who rebuild the senders - check in Hemmings Motor news or online to find someone to fix it, or else call the Mustang parts houses to see if they can sell you a new one.

In the rare case that there turns out to be nothing wrong with the sender, your tank ground may have failed, or the wire may be making poor contact with the sender terminal - this ain't rocket science - you'll just have to inspect and test until you find the problem.

Dick