Does Your Gas Gauge Lie To You?

by Ray Paulk

The gas gauge on a Fiero seems to be a PITA (Pain in the Butt) for all of us. I ran out of gas 5 times before I broke down and decided to fix it. It didn’t take too much investigation to figure out that the problem is with the 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 into this job. But when my fuel pump failed and I was forced to drop the tank, I figured that I might as well fix the sending unit too.

The fuel gauge sending unit for the fuel gauge is a simple potentiometer which varies its resistance with the level of the fuel in the tank. It’s supposed to read 0 ohms when Empty and 90 ohms when Full. The problem is that this “pot” never seems to get down to 0 ohms and always goes much higher than 90 ohms. Of the fuel gauge sending units that I’ve have removed, they usually read about 15 to 20 ohms with the float all the way down (Empty) and about 120 ohms with the float at the top. This means you’ll run for quite a while with the gauge reading Full and run out of gas when the gauge reads 1/8 to 1/4 tank. Sound familiar?

A potentiometer is a variable resistor. Its comprised of a resistance coil and a “center-tap” or “wiper” which slides along the coil. The resistance is measured between the center tap or wiper and one end of the coil. As the wiper moves along the coil, the resistance between the wiper and the end of the coil changes. Right at the end, under ideal conditions, 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 the pot is tied to the tank float. As the fuel level rises and falls, the wiper moves along the edge of the coil and varies the resistance between the wiper and 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 in and out to change the contact point of the wiper but its not enough to really fix it.

The problem is that the wiper never gets down to the bottom coils of the resistor to read zero. The fix is to “short” the bottom coils to meet the point where the wiper contacts at its bottom most position. Here’s what to do:

Remove the sending unit from the tank. (Not a really fun job but necessary.) The wiper mechanism and resistor board should be fairly obvious they are probably under an aluminum cover which is easily removed with bend tabs. Note the screws with the springs on them at the top and bottom of the board. These are the adjustment screws. Note that the last coil of wire falls under the spring on the screw. This is the contact point.

Although you can bend things to try to make it work, this might damage parts and is often ineffective. You may break something. Another “not recommended” technique is to remove some coils at the bottom. Here too you can get in trouble by breaking the wire or loosening the coils. Also, to get to zero ohms, the wiper would have to slide to the last coil and perhaps off the coils all together. Not a real good solution.

The better way is to first set the adjustment screws to a neutral position 1/2 way through their adjustment. Now mark the lowest point the wiper 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 coils because you are going to make a solder bridge from the last coil to the coil where the wiper last contacted. I either use a wire wheel in my Dremel tool or 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 are clean, coat them with solder flux (plumbers paste) even if you use resin core solder. This wire does not like solder. I even use Muriatic acid to help clean the wire. Then solder all the last coils together. Note, this is electronics work, not work for solder guns or torches. If you don’t have a “pencil” type, electronics duty soldering iron, you may be better off going to an electronics store or TV repair place to have this done.

Now remount the coil board. Naturally clean the springs and screws for better contact. (0 ohms is tough to get … when you want it). Attach ohmmeter to the gauge out put terminals (they should be pink and black, at least in the wiring harness. If not, they should be obvious if you’ve gone this 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 can get it. On mine I got 1.9 ohms. I turned the adjustment screw in on my units until I got a consistent low reading of 1.9 ohms. I then backed it out ’til the ohms started to climb. You want to set this adjustment right at this break point.
Now do the same with the lower screw ’til you get 90 ohms with the float all the way up. When you get this “Full” output set, go back and recheck the Empty set point. You’ll probably have to do this a few times as one setting affects the other. Remember that the bottom must be set right at the break point mentioned earlier. When you get both ends set, your sending unit is calibrated.

Now you can pop the cover back on the resistor coil, drop the sending unit/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 hands when I do this. It’s trying to get all those damn tubes and hoses back on that’s a killer.)

If you did the job right, your gauge will be correct. If you still have problems, 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, you can put a 90-ohm resistor in place of the sending unit. The gauge should read Full. Then put a solid jumper in and the gauge should read Empty. If it doesn’t, your gauge is messed up. But a bad gauge is seldom the problem.

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