Fuel Gauge Troubleshooting.
 

Needing some help figuring out whats up with my fuel gauge.
I have a new 19 gal tank with a new sending unit( which i've Ohm ed twice and according to this site, is in the required readings).

I've tried two different Gas Gauges and both read about the same.
When i fill up, it seems like within a hundred miles I'm on empty.
So when i went to fill up, i put in 10 gals and it was full again.

I had to run a new wire from the sending unit to the fire wall because the old green one looked like somebody took a test light to every inch from the cab back and it had holes like crazy.

I took my Dremal and ground down to fresh metal for the ground, and when i ground the sender wire to the frame, the needle moves past full, so i know I'm good there.

But what would be causing my gauge to read half?

Mine does this too, but I never found the motivation to figure out what was going on. So, I spent about fifteen minutes thinking about it, and here's what I came up with.

I think the sending units are supposed to read 10 ohms full and 72 ohms empty. However, there may be a problem with the "new" aftermarket sending unit float arms.

For example, if the float arm was bent slightly downward (rainbowed) - the gauge would read artificially high when the tank is overly full. And you'd think that it would also read artificially high as the level dropped. However, the float arm swings through an ARC - meaning as the fuel level dropped, the arm would move MORE than it's supposed to as it got closer to the bottom of its swing.

OR, if the float arm was too SHORT the same thing would happen - as the level dropped the shorter float arm would move further (and faster) through its arc, showing an empty tank when it's really not. Basically, the arm is resting on the hard-stop at the end of it's travel, and the float isn't in the gas anymore, even though there's still a lot of fuel left.

If I had the motivation, I would take it apart, and extend the float arm by about an inch and see what that does for you! I haven't messed with it simply because there are so many other things wrong with my truck that have priority!

I considered that the wiring could be bad. An open circuit in the wiring (broken/corroded wires) would make the gauge read artificially low. However, I suspect that bad wiring would make the gauge ALWAYS read low - even after a fresh top-up. The fact that it's starting off really high, but then crashing, means that this isn't probable.

I then considered the voltage regulator in the instrument panel. It doesn't work out that if the regulator was bad, it would go from "over-full" to "empty" - it would read low all the time, or high all the time.

Lastly, I wondered what if the Bi-metal strip in the gauge itself was bad. It seems unlikely, because if they went bad frequently, we'd hear about it a lot more.

The most likely culprit is that the aftermarket senders have the float arm too short, or that their rheostat isn't calibrated linearly (or non-linearly) like the OE units.

If you have the motivation, try extending the float arm (well away from the gas tank!!). If it works, I'm glad to have helped - if it doesn't work...I don't claim any responsibility for anything that may happen (like running out of gas on the side of a dark road in the middle of nowhere next to a chainsaw murdering psycho!)

My gauge is doing the exact same thing. I tried bending the float arm to compensate and it just stays reading full longer. :-X04 I checked wires, grounds, the gauge itself and all is well. I found the following which seems to explain the issue perfectly!
Auto Meter

I replaced my sending unit with one from LMC and it's been like this since. Seems the only solution is to try and find a working OEM one. I've verified the resistance at the different points the article describes and I found what it says to be dead on with my replacement sender.

You've verified that your ground is good, and the sending unit rheostat is good. The last thing to check would be to make sure you're getting 5 volts out of the ICVR to the gauge. If that checks out, then I would suspect a pinhole leak in the sending unit float, which would make it stay toward the bottom of the tank (happened to me with a brand new sending unit).

I'll have to check the voltage on the ICVR, I didn't think that would be the issue, since all my gauges work.
I had to take out the new sender to replace the tank, and I didn't notice any gas in the plastic float then, and it was reading wrong then to.

Thanks for that link, that sums everything up.
Now I wish I hadn't thrown away the stock sender, it was still good, just the float was bad.
O f course I found out after I threw it away, you can buy replacement floats.

Glad I could help! :-X22

I said it first :-P

I think a longer float arm would also help "slow" down the issue without needing to resort to buying an OE sending unit, but that's a stop-gap. It doesn't mean that it would fix it 100%. If I ever have to pull the tank again I'm going to try doing it!

-Brad

How long would we need to extend the arm?

Would this sender be a better option?
19 Gallon Sending Unit 73-79 Ford Truck

I installed that exact same sending unit (from JBG) in my '79 three years ago and it failed within one week (pin hole in float, float sinks, gauge permanently reads empty/70 ohms). I could have just been unlucky.

Really?
Well do you know if it was readying accurately?

For the week that it operated properly, it read accurately. But by the 4th or 5th day, the needle started wandering, then slowly dropped to empty and stayed there for good.

That's too bad about the float. If it read accurately, you could've just pulled the sender, slathered the float with epoxy resin, let it cure, then reinstalled.

Good epoxy resins (not the polyesther resin most places carry) is impervious to gasoline/oil.

-Brad

It's still in there; haven't pulled the tank a second time. For the last few years I've just written the mileage at the last fillup on the dash in chalk; it hasn't been enough of an inconvenience to do anything about (yet).