fuel gauge reading high with new senders

[ratdude747]

This drives me crazy. Why is this and how can I fix it? I didn't order them, so I can't verify that they are the right units but I obviously have to assume so.

I went through this with my '84, but mine wasn't reading over-full, it was reading under full...like 1/2 or 3/4 tank when full. This drove me crazy because it would show empty but then only tank a half a tank worth of gas to fill.

Anyway, I took the sender out, and I just kept bending the float arm and testing it over and over again until I go it close. What I did specifically was start with the tank empty, then put in 10L (2.5 gallons approx), and kept adjust the float arm until it sat right on the "E", or a hair below it. Once I did that, it also read full properly. The down side of this was that I bent the float arm downwards enough that I ended up having to also bend the pickup tube down even lower as well (or the truck would have run out of gas before it read empty). After all that, it worked great.

That was a fairly tortuous path to take to get a gas gauge reading properly, but they were all brand new parts, and I was lucky enough to have made all of these float adjustments while the bed was removed, so I didn't have to drop the tank.

I thought I was being a real keener by checking the new sending unit before installing it...I made sure that it worked with my gas gauge before putting it in the tank. What I didn't know then, but I DO know now, is that I should have also compared the new sending unit to the old sending unit to make sure the float range of sweep was relatively the same between the two.

I could be way off base here, but I think that some of the aftermarket senders are fine electrically, but just not bent/formed correctly.

Mine (a Spectra OE sending unit) seems to be right at the top and bottom of the scale... but it's like the sending unit has an exponetial response curve compared to the gauge.

At full, it's full. But it drops off very quickly to 1/4 (roughly half a tank), and then slowly drops to E. Which would fit with an exponential output feeding a linear gauge (1/2 squared is 1/4, after all). Short of brute forcing an arduino based custom board, I don't know of a good passive way to fix that... and I'm drawing a blank on how to do it with single-supply OP-amps.

[Gary Lewis]

Mine (a Spectra OE sending unit) seems to be right at the top and bottom of the scale... but it's like the sending unit has an exponetial response curve compared to the gauge.

At full, it's full. But it drops off very quickly to 1/4 (roughly half a tank), and then slowly drops to E. Which would fit with an exponential output feeding a linear gauge (1/2 squared is 1/4, after all). Short of brute forcing an arduino based custom board, I don't know of a good passive way to fix that... and I'm drawing a blank on how to do it with single-supply OP-amps.

The thread Bricknose Sender to Bullnose Gauge Interface has an overall diagram awa an outline of the sketch.

What hasn't been included in the sketch is reading the output of the tank selector switch and then choosing which of two different curves to use.

And, speaking of the curve, it appears to use a straight line, but we need to allow an exponential curve. So the equation needs to be a bit more complex. But that's not difficult.

HOWEVER, I'm running a MeterMatch. For $84 it does a pretty decent job of interfacing a Bricknose sender to Big Blue's Bullnose gauge. And while it doesn't truly allow for an exponential curve it does provide 4 points on the curve to calibrate it, so you'd have three straight lines that could approximate the curve.

 

[Gary Lewis]

The thread Bricknose Sender to Bullnose Gauge Interface has an overall diagram awa an outline of the sketch.

What hasn't been included in the sketch is reading the output of the tank selector switch and then choosing which of two different curves to use.

And, speaking of the curve, it appears to use a straight line, but we need to allow an exponential curve. So the equation needs to be a bit more complex. But that's not difficult.

HOWEVER, I'm running a MeterMatch. For $84 it does a pretty decent job of interfacing a Bricknose sender to Big Blue's Bullnose gauge. And while it doesn't truly allow for an exponential curve it does provide 4 points on the curve to calibrate it, so you'd have three straight lines that could approximate the curve.

Update: I bought from Technoversions, and I believe they designed it. And it is $75 inc shipping. Plus there's a good writeup on how it works.

[Machspeed]

Update: I bought from Technoversions, and I believe they designed it. And it is $75 inc shipping. Plus there's a good writeup on how it works.

Interesting conversation and a project I will be delving in to soon.

Larry, my gauge, when it was working, was just as you described.....seemed to stay on full for some time then drop quickly to a quarter and hold there for awhile. When that needle hit the line, you were out of gas. Ask me how I know!

 

[Gary Lewis]

Interesting conversation and a project I will be delving in to soon.

Larry, my gauge, when it was working, was just as you described.....seemed to stay on full for some time then drop quickly to a quarter and hold there for awhile. When that needle hit the line, you were out of gas. Ask me how I know!

Dad loaned me his '72 Mustang when our Citation was in the shop for one of its many recalls. I pulled into the driveway as the needle touched E. The next morning it would not start. He said "Sorry, I forgot to tell you that."

With the MeterMatch you fill it to the point you want it to show Full and hit the button to calibrate for Full. Drive it to where you want it to read in the middle of the scale, say 2/3's, and hit that button. Then maybe 1/3 and hit that button. And finally where you want it to be for Empty and calibrate there.

But, you only have one "map" and both tanks read off that. Unless you buy two MeterMatches and wire them in ahead of the switch instead of after.

[Machspeed]

With the MeterMatch you fill it to the point you want it to show Full and hit the button to calibrate for Full. Drive it to where you want it to read in the middle of the scale, say 2/3's, and hit that button. Then maybe 1/3 and hit that button. And finally where you want it to be for Empty and calibrate there.

But, you only have one "map" and both tanks read off that. Unless you buy two MeterMatches and wire them in ahead of the switch instead of after.

Thanks Gary, won't be buying two MeterMatches so will just figure out a way to make the sending units work.

 

[FuzzFace2]

With the MeterMatch you fill it to the point you want it to show Full and hit the button to calibrate for Full. Drive it to where you want it to read in the middle of the scale, say 2/3's, and hit that button. Then maybe 1/3 and hit that button. And finally where you want it to be for Empty and calibrate there.

But, you only have one "map" and both tanks read off that. Unless you buy two MeterMatches and wire them in ahead of the switch instead of after.

Thanks Gary, won't be buying two MeterMatches so will just figure out a way to make the sending units work.

If the gauge reads right at empty but not full, over full, bending the float arm is not going to get it right as pointed out the empty reading will now be off.

You need to move the full level arm stop lower so the arm will not go up as far.

Again I dont care were it reads when full but I dont want to be running out of gas when showing 1/8 tank. Had a Toyota LC that happened all the time on after the 4th time, each heading to a station, I bent the arm to read empty with a few gallons in the tank. Dont remember what it read when full as I cant remember ever filling it, kid and no money LOL

Dave ----

[delco1946]

If the gauge reads right at empty but not full, over full, bending the float arm is not going to get it right as pointed out the empty reading will now be off.

You need to move the full level arm stop lower so the arm will not go up as far.

Again I dont care were it reads when full but I dont want to be running out of gas when showing 1/8 tank. Had a Toyota LC that happened all the time on after the 4th time, each heading to a station, I bent the arm to read empty with a few gallons in the tank. Dont remember what it read when full as I cant remember ever filling it, kid and no money LOL

Dave ----

Hey Dave, not to be a dunce but where is this stop you speak of an how does one adjust it? Not sure what brand I have but here is one, I’m not seeing any obvious stops that one could adjust to correct a “full” reading.

[taskswap]

Hey Dave, not to be a dunce but where is this stop you speak of an how does one adjust it? Not sure what brand I have but here is one, I’m not seeing any obvious stops that one could adjust to correct a “full” reading.

I can't speak for any stops but it was common in Subaru Imprezas I used to own to just bend the arm a bit. They were notorious for reading full-E with a few gallons still in the tank, and reading 1/8th past F when full.

[FuzzFace2]

https://forum.garysgaragemahal.com/file/n113981/463614B3-408D-4282-94FF-D696FD8414DD.png

Hey Dave, not to be a dunce but where is this stop you speak of an how does one adjust it? Not sure what brand I have but here is one, I’m not seeing any obvious stops that one could adjust to correct a “full” reading.

I cant really see from that picture but some senders have a metal tab the metal arm hits that might be adjustable?

I was talking to a member to day about this and he said to get the E / F to read right, with a lot of testing, he had to bend the arm it 2 places.

One to read right for E and then one for F.

If you only did the 1 bend of the arm it would only read right for E or F.

All this testing is best done with bed off truck.

Dave ----