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Fuel Gauge Sending Unit Calibration

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A funny thing happened on the way home from buying donuts with my 7-year-old son a couple of months ago, one fine Saturday morning.

I ran out of gas. The gauge indicated about 1/8th of a tank left. Fortunately, as the car began to sputter, I had an inkling of what was going on. This was the lowest I'd let the fuel level get since owning the car. I assumed that the gauge was working correctly -- and until I accidentally disassembled it more than I intended to while cleaning all the gauges a couple of months ago, it probably was. I had a bit of trouble with the gas gauge at the time when the face of the instrument fell off while I was trying to simply clean the glass on both sides. The needle got slightly bent out of whack and I carefully bent it back into place before putting the gauge back together.

I thought that it must be okay, since it appeared to be functioning normally. When the car was without power, the indicator would rest just past the "E" mark to the left. When the tank was full, it indicated full!

I was able to flip a U-turn while the engine was still sputtering and managed to point myself in the direction of a gas station just a few hundred yards away before the engine died completely. I had to push the car for about half that distance until I started going downhill and could coast the rest of the way. This episode wasn't as fun as it sounds. I just tried to keep the situation lighthearted though, so Mac wouldn't get nervous.

I've been wanting to take care of this problem but felt a bit intimidated about the prospect of playing around an open gas tank. Well, there was really nothing to it at all.

Today, I decided to check the sender unit in order to rule it out as a problem to be certain that the gauge was at fault. As it so happens, I recently acquired a NOS fuel guage on eBay, but it hasn't arrived yet, otherwise I would have simply checked the guage first. But still, the sending unit would have to be checked, regardless, unless I wanted to run the risk of running out of fuel again. No thanks.

Removing the sending unit was a breeze, thanks to my friends on the Spitfire list. There is a retaining ring that keeps the sending unit pressed flush across a rubber gasket that lines the opening to the fuel tank. This retaining ring has three prongs that protrude from it. To remove the sending unit, first remove the wires that go to the fuel gauge; make certain the ignition is off and using either a flat-blade screwdriver and a hammer or a stick of wood and a hammer (a better combination to avoid sparks and catastrophe) tap the easiest to reach prong in a counter-clockwise direction. Once loose, the three-prong retaining ring is easily removed and the sending unit will practically fall out! IMPORTANT: Although this should go without saying, before removing the sending unit, make certain that your fuel level is very low. The lower the better. My photos show a screwdriver against the prong on the retaining ring for illustrative purposes, but I actually used some scrap wood to loosen and then retighten the retaining ring -- just to be on the safe side.

Once I had the sending unit out of the tank (a few careful twists and turns are required to achieve this), I gave it a good once-over to see if there were any kind of calibation screw adjustments or anything of the sort. There are not. The fuel that had been coating the float quickly evaporated in this time. I reconnected the wires and turned the ignition key one position to the right to send power to the gas gauge. Moving the float assembly revealed that the gauge itself is in fact the problem. Even when the float actuator rod came to rest upon the little metal flap that extends from the sender unit body and acts as a stop, the fuel gauge registered an eighth of a tank left.

Since the gauge needs calibration and my replacement hasn't arrived yet, I decided to take an alternate path and with pliers in hand, carefully and slowly bent and angled back the stop on the sending unit (shown on the right of the photo, with the actuator rod resting against it), until I could achieve an "E" reading on the gauge when the actuator rod was at it's full (and now slightly extended) length of travel, indicating an empty tank. This flap is normally "square", parallel to the edge and sits at a 90 degree angle to the rest of the sending unit housing. You can clearly see in this photo the degree to which I bent the stop tab. It took a bit of trial and error to get it just right.

To compensate for the float mechanism being in the wrong position now that it was lower than intended, I bent the actuator rod up, ever so slightly, thus raising the position of the float, so that when I refitted the sending unit, it displayed basically the correct reading for the amount of gasoline still left in the tank. This also took a bit of trial and error. I realized that this step would be necessary when I refitted the sending unit and noticed that the gauge was reading 1/4 tank -- and I knew darn well there couldn't be that much left, since the gauge was indicating about half of that amount before I did anything.

I checked the fuel guage reading after reconnecting the wires, and when satisfied with the relative accuracy of the readings, turned the ignition back off, removed the wires again and making certain the sending unit fit flush against all parts of the seal, tightened up the retaining ring by tapping one of the prongs with a hammer against a wooden stick in a clockwise direction, making certain that it was good and snug, but not so tight that it would be difficult to remove again in the future. It shouldn't be.

On my way to the gas station this evening, the needle was registering just under 1/8th a tank of fuel remaining. Taking into account the 9.9 gallon capacity of the fuel tank, I subtracted the actual amount of gas I used to fill the tank, which indicated that I had roughly 1.5 gallons in the tank before filling up.

Dividing the 9.9 gallon capacity of the tank by 8 equals 1.2375 gallons, which in turn represents 1/8th of a tank of gas, so the fact that I was showing about 1/8th of a tank left with 1.5 gallons still left in the tank meant that I actually had just over a quarter gallon more in the tank than was being indicated on the gauge. An extra quarter gallon when the needle hits "E" is something I'll be glad to have. With the current mileage I'm getting, that will just mean another 5-8 miles I can travel once the guage reads empty, before actually running out of gas. Of course, I will never let the gauge read "empty". No need to push my luck.

TIP: I discovered that the sender unit really only fits in one specific position in the fuel tank opening. Though you can rotate the position of the sending unit however you wish, expect to have a boot full of gasoline when you fill up the tank if that sending unit isn't fitting perfectly flush against the seal.

One negative thing I noticed when everything was put back together is that gas gauge needle really bounces around from engine vibration when indicating near empty. It seems much more susceptible to bounce than my temperature gauge. It looks like I'll want to replace the fuel gauge after all -- and when I get around to that, I'll need to restore the float on the sending unit back to stock specs, or simply replace it with a new one to be absolutely certain of its' accuracy.

TIP: ALWAYS make certain that your ignition key is OFF before reconnecting the wires to your sending unit after it's withdrawn from the gas tank. Remember, all it takes to ignite gasoline fumes is a tiny spark -- like the one that could be generated when reconnecting (or removing) the sending unit wiring. Take no chances. Be certain that the power is off before removing or replacing the wires!

I was cautioned to replace the gasket at the same time I removed and replaced the sending unit, but the gasket appeared to be in perfectly good shape. A little extra care in removing and replacing the sending unit from the tank will go a long way towards preserving the seal. Just the same, after I made certain that the sending unit backing was flush against the seal of the gas tank before I tightened it back down. At the gas station and again back home, I made a visual inspection after filling the tank with gas and was pleased to see that nary a drop has leaked from the tank. The integrity of the seal was preserved.

Hopefully, I'll have no cause to ever push the car again because it ran out of gasoline. This project is finished!

Please. Always wear your seatbelt while driving -- and that goes double for your children if you have any.

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