The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel gauge reading incorrectly

Just wanting some input for a starting point maybe.
So when I got the truck I had to free up the sender. Anyhow using ohm meter it checked ok. Installed and been working fine so I thought. Never really ran below 1/4 tank. Today I ran out of fuel. Gauge shows 1/4. Put around 8-9 gallons in and shows little over 1/2. So float need adjustment or what other opinions does anyone have. Gauge goes to E with key off so I'm kinda lost. Thanks

When you pull out the sender, hold it next to the tank wear it would sit if installed. Move the arm up and down and see where the float is. If the top of the arm's travel is at the top of the tank and the bottom of travel is near the bottom of the tank, then you probably need a new send. Make sure it's oriented correctly with the arm facing the driver side.

If you need a new one, Jon G recommends a really nice one from a seller on ebay from Taiwan

I want to say it points to the passenger side. I had it out a couple weeks ago for a new gasket install. Maybe that's the problem. Thanks

As long as it's moving up and down unobstructed. Now you have me second guessing which way is right. I think the screws are set in such a way that it can only be installed one way.

Well that's what I thought also. Was oriented at parallel to tank. I thought. Second guessing my work is a daily experience.

Check to make sure the float isn’t rubbing on the side of the tank. A gentle massage of the arm may take care of your problem.

Probably wont get to it today. Has to be something simple. 18 gallon tank. Best I can figure I put around 11 gallons in it and it's a little over 1/2 so seems correct just not reading below 1/4 with key on.

Do you have the old sending unit handy? Compare it side by side with your new one. I just installed one in another car and the new unit had the wrong angle on the float arm. Bend the new one to match the original one.

Didn't get new unit
Gasket was leaking

Gotcha. You may still be able to make it more accurate. I think the top to bottom on an AD Fuel tank is about 12 inches. Adjust the arm so that when the lever arm sweep is exactly halfway between full and empty, that the float is 6 inches down from the top of the tank ( or whatever you determine the middle of the tank is.

That way, even if the travel isn't all the way full or all the way empty, you'll at least know that the halfway mark is exactly 9 gallons.

Well I guess my gauge is bad. Pulled sender wire and went to full. Grounded it and showed 1/4 tank. This sound like a correct diagnosis?

The gas gauge is a little volt meter. Very small current. So, not much to move it. The problem sounds like a bind in the gauge. Might be some dirt or it might touch the face of the gauge. I have seen both. You can get it out. But it will take a new gasket for the glass front. It is working. Just not correctly. Should have gone to Empty when grounded. Full when open is just right. The idea to set the 1/2 tank is a good one. Look close at the needle and see if it rubbing or sticking on any thing. A very small spring is on there to get it to empty when key is off. I hope that helps.

Will this Tech Tip help with this? Troubleshooting a Gas Gauge on a Stovebolt written by Phak1 and tech review by Jon G.

"One of the most asked question, on this forum is how do I fix my inoperative gas gauge. I wrote this procedure first as a "Sticky Thread", then I just reformatted it into a PDF so pictures could be inserted into the procedure. This was written for 47-55 first series trucks but may apply to many more years as the systems are very similar." ~ Phil H.

It goes to empty with key off not sticking that I can tell. Could it have something to do with the 12 volt resistor device.

The gauge is supposed to go to empty when the key is off. Gravity takes over and drops the needle. Have you removed the sending unit and operated it outside the tank while watching the needle? You’ll need to ground the sending unit in order for it to work.

Also watch the needle to note full range. My gauge only goes from the three lines never touching the E or F marks on the gauge.

My gauges work just like Phil's. When my needle stops wiggling from sloshing fuel, I need to find a gas station within the next 29 miles.

May have found my problem but I'm unsure. I converted to 12 volt and this seems to be when the gauge started reading incorrectly. I tested the positive input at the gauge. I have almost 8 volts. So would that cause the gauge to be off? Have a speedway voltage reducer installed. Anyone know of a better solution or source on a voltage reducer that will be correct. Thanks

Finally getting back to this. After repairing our dryer circuit board I ran across a voltage reducer on the board. 5 volts. And it was free. And my wife gets so upset when I don't throw anything away. Hopefully get installed this weekend. I will update if the lower voltage solves the problem.

Please let me see if I can help. You may be dealing with a sender that isn't doing its job right. The keys there are (1) good float (2) float arm adjusted properly (very important...essential in fact) (3) range of 0 ohms (empty) to 30 ohms (full) and a float that doesn't strike the sides of the fuel tank and bind up and that moves easily throughout the entire range. If you have an aftermarket gauge, I can't stress enough about the float arm adjustment. Please let me know if you need more about that. For most gauges the figures are 5.5" down from mounting surface to the arm pivot point and 5 7/8" between that pivot point and the middle of the float.

If you've been using your fuel gauge with 12 volts, it is unlikely a voltage reducer will solve anything. Inside your fuel gauge you have an hourglass shaped permanent magnet and two electromagnets of varying designs. You can see in the image below. The permanent magnet in the gauge can be affected by higher voltage in that the strength of it can change. Nobody here has the equipment to measure that so nobody really knows how long that change will take but if it happens, your 6 volt gauge will not read correctly at 6 volts any longer. 8 volts might not be a bad number for you. Years ago I built a voltage regulator which is adjustable to any voltage (up to around 12 of course) but I stopped the project. Way too many people have converted to 12v and I'm not convinced a voltage regulator will do a lot for them now. Although the adjustable feature does allow you to vary voltage to see if adjusting it will help you make your fuel gauge work better.

But the thing is your gauge will care more about current (amperage) than voltage. Good luck. If you understand electronics even a bit, you can use common resistors to see how varying resistance to ground will affect your gauge's reading.

Edit: the 7805 chip you found on the dryer circuit board is working fine, by the way...the "5" in the number tells you it should output 5v.
Second edit: the arm adjustment figures I gave you are for a 16 gallon tank which will normally be 11 inches down to the bottom (or the lowest point the float can drop down to). If your tank is higher (say 12 inches) and you need those numbers please let me know. A chart I prepared is attached:

Haven't been running 12 volts. Used a runtz from speedway. It's showing 8 volts. Gauge worked fine with 6 volt set up. I'm thinking the 8 volts is my problem from research I have found. The 5 volts may show a little low but that's better than to high. Thanks for your advice.

Your 7805 (5 volt) regulator may be free, but 7806 (6 volt) regulators are cheap and would provide the proper voltage.

7806

The Runtz device is showing 8v because it uses a 7808 chip. An LM317 is adjustable, linear and would have been a better one to use, but they didn't. Good luck and please keep us posted.

Good news and bad news. Good news is it worked. Bad news is I broke it before completely installed. To much bending the wires around I guess. But it did read about 3/8. With the runtz at 8 volts it was reading 1/2. Back to the drawing board.
So if you think voltage doesn't effect it I'm satisfied it does.

Well, I don't have any 7806 chips handy (to output 6v) but I do have one of the voltage regulators I made (adjustable). The Runtz device used the 7808 simply because your generator will provide 7 to 7.2v through your voltage regulator so your gas gauge won't see 6v...but a lot of folks don't know that or maybe they do know it but they don't always remember. Important...it won't output 6v unless something is wrong with it. Runtz used the 7808 chip because it was cheap, 8v is close to 7.2v and they didn't want to make an adjustable gizmo. There is no LM7807 chip...not a popular voltage choice. If your 6v gauge has not been exposed to 12v it may not have been re-magnetized and if that is the case voltage may make a bit of difference...it also depends on how the electromagnets in your gauge were wound (a complicated swamp we're going to avoid today). Explaining the variables of electromagnetism can head into deep water quickly.

If your needle moves correctly and through the entire range and your sending unit is good for 0 to 30 ohms, you can (a) fill your tank (b) disconnect your battery (c) install the device I made (which installs just like the Runtz thing) and (d) carefully adjust it using a tiny screwdriver until your gauge reads F. Tip: you can connect a wire or patch cable to the fuel gauge and then to the device I made so you can do this without having to crawl under the dash and then install it after you've adjusted things. You will need to connect it to the power wire going to the gauge using the thumbscrew connection, connect it to ground (using the black wire and connector) and then connect it back to the gauge using the patch wire (connect to the back side of the device and then connect the patch wire to the correct gauge post). I can't offer anything for sale on this forum, but if you'll please send me a PM I can tell you more. There are some images below that explain this a bit...

Very nice Jon. I just broke down and ordered a 5 volt buck converter from Amazon. Should be here Saturday. Yours would be how they should be built though. I would rather it read a little low then run out of fuel. Not impressed at all with the $26 runts crap. Live and learn

Thanks! A buck converter should be fine. The circuitry in the one I made is based on the buck design but slightly different and adjustable to allow it to be used for other devices. It will handle 1.8 to 2 amps without a heat sink. Just let me know if it doesn't work out as you hope.