by Ray Paulk
The gas gauge on a Fiero seems to be a PITA (Pain in the Butt) for all ofus. I ran out of gas 5 times before I broke down and decided to fix it. Itdidn't take too much investigation to figure out that the problem is withthe sending unit which is part of the fuel pump assembly inside the tank.Having dropped Fiero gas tanks before I was none to anxious to jump intothis job. But when my fuel pump failed and I was forced to drop the tank, Ifigured that I might as well fix the sending unit too.
The fuel gauge sending unit for the fuel gauge is a simple potentiometer whichvaries its resistance with the level of the fuel in the tank. It’ssupposed to read 0 ohms when Empty and 90 ohms when Full. The problem isthat this "pot" never seems to get down to 0 ohms and always goesmuch higher than 90 ohms. Of the fuel gauge sending units that I've haveremoved, they usually read about 15 to 20 ohms with the float all the waydown (Empty) and about 120 ohms with the float at the top. This means you'llrun for quite a while with the gauge reading Full and run out of gas whenthe gauge reads 1/8 to 1/4 tank. Sound familiar?
A potentiometer is a variable resistor. Its comprised of a resistancecoil and a "center-tap" or "wiper" which slides alongthe coil. The resistance is measured between the center tap or wiper and oneend of the coil. As the wiper moves along the coil, the resistance betweenthe wiper and the end of the coil changes. Right at the end, under idealconditions, it reads zero.
The potentiometer of the fuel gauge sending unit is made from components.The resistance coil is literally a 1/16" fiber board about 1/2"wide with a resistance wire wrapped around it. The "wiper" of thepot is tied to the tank float. As the fuel level rises and falls, the wipermoves along the edge of the coil and varies the resistance between the wiperand the top end of the coil. (The float goes down and the wiper goes up.)There are fine tune adjustment screws which actually tilt the coil board inand out to change the contact point of the wiper but its not enough toreally fix it.
The problem is that the wiper never gets down to the bottom coils of theresistor to read zero. The fix is to "short" the bottom coils tomeet the point where the wiper contacts at its bottom most position. Here'swhat to do:
Remove the sending unit from the tank. (Not a really fun job butnecessary.) The wiper mechanism and resistor board should be fairly obviousthey are probably under an aluminum cover which is easily removed with bendtabs. Note the screws with the springs on them at the top and bottom of theboard. These are the adjustment screws. Note that the last coil of wirefalls under the spring on the screw. This is the contact point.
Although you can bend things to try to make it work, this might damageparts and is often ineffective. You may break something. Another "notrecommended" technique is to remove some coils at the bottom. Here tooyou can get in trouble by breaking the wire or loosening the coils. Also, toget to zero ohms, the wiper would have to slide to the last coil and perhapsoff the coils all together. Not a real good solution.
The better way is to first set the adjustment screws to a neutralposition 1/2 way through their adjustment. Now mark the lowest point thewiper goes on the coil (mark with something like a "Sharpie" pen.)
On the side of the board, you will have to clean the surface of the coilsbecause you are going to make a solder bridge from the last coil to the coilwhere the wiper last contacted. I either use a wire wheel in my Dremel toolor a fiberglass burnishing brush which I got at Radio Shack (Cat. No.64-1986). When you have the varnish removed from the coils and they areclean, coat them with solder flux (plumbers paste) even if you use resincore solder. This wire does not like solder. I even use Muriatic acid tohelp clean the wire. Then solder all the last coils together. Note, this iselectronics work, not work for solder guns or torches. If you don't have a"pencil" type, electronics duty soldering iron, you may be betteroff going to an electronics store or TV repair place to have this done.
Now remount the coil board. Naturally clean the springs and screws forbetter contact. (0 ohms is tough to get ... when you want it). Attachohmmeter to the gauge out put terminals (they should be pink and black, atleast in the wiring harness. If not, they should be obvious if you've gonethis far.) Hold the float as low as it can go and run the upper adjustment(empty position) screw in and out to see what the lowest ohm reading you canget it. On mine I got 1.9 ohms. I turned the adjustment screw in on my unitsuntil I got a consistent low reading of 1.9 ohms. I then backed it out 'tilthe ohms started to climb. You want to set this adjustment right at thisbreak point.
Now do the same with the lower screw 'til you get 90 ohms with the floatall the way up. When you get this "Full" output set, go back andrecheck the Empty set point. You'll probably have to do this a few times asone setting affects the other. Remember that the bottom must be set right atthe break point mentioned earlier. When you get both ends set, your sendingunit is calibrated.
Now you can pop the cover back on the resistor coil, drop the sendingunit/fuel pump back into the tank and "throw the tank back up in place.(Ya, I wish it were that easy. I always tend to cut the heck out of my handswhen I do this. It’s trying to get all those damn tubes and hoses back onthat's a killer.)
If you did the job right, your gauge will be correct. If you still haveproblems, you can check the gauge with a 90-ohm resistor and a solid wire.Remembering that the pink and black wires are the sending unit wires, youcan put a 90-ohm resistor in place of the sending unit. The gauge shouldread Full. Then put a solid jumper in and the gauge should read Empty. If itdoesn't, your gauge is messed up. But a bad gauge is seldom the problem.
Now if you run out of gas, it’s your own darn fault! I speak fromexperience.