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Discussion Starter · #1 ·
If you just replaced your old fuel sending unit with a new one or just upgraded to an Autometer gas gauge with your stock sending unit here's why you don't get the correct reading:


The problem is that the new sending unit is a Linear device, whereas the OEM sending unit and gauge are a non-linear device. Ever wondered why the gas gauge was somewhat cocked to one side and that half was not really at the normal half way point? Simply put the two can't communicate correctly and will be right only twice - at empty and full. Everything else will be off by a few gallons. I've tried for years to get it to work correctly, and it won't. Either live with it or try and find an OEM sending unit.


Top gauge is a nonlinear gauge, and bottom is a liner gauge. The 1/2 mark is closer to the Full on the nonlinear gauge. It has to do with the electrical winding of the fuel sending unit. The nonlinear has the windings start out compact and then as they get to the 1/2 mark the windings start to get larger in the loop.


On a linear sending unit the electrical windings are all even across the entire range.


Anyway I hope this will help clear up why your gas gauge is now registering about 1/4 tank off.
 

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Well, I put all electric autometer gauges in my gauge cluster about 15 years or so ago. The fuel gauge has always been off. Now I know why! Thanks. I had changed the sending unit several times over those years, even adjusted the thing on the bench with a meter to the proper ohms etc. Still would not read correctly. It would always "sort of" give me an idea of how much was in the tank...lol :surprise:
 

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Thanks for the info Peter! Mine was never accurate either and got worse after I replaced the tank and sender the first time. It would stay at full for a long time and then drops to half real quick.

After changing to the Tanks Inc tank & sender early this year, it was still not accurate. I figured I know when it's full and when it's empty, I need to know when it's half full and I can guestimate in between full and empty so I lowered the center section of the sender twice. Lowered it 1"-1.5" total and now the halfway mark is fairly accurate. Might be more difficult or not possible to do with a stock fuel tank sender, I don't know.
 

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Discussion Starter · #4 ·
Well, I put all electric autometer gauges in my gauge cluster about 15 years or so ago. The fuel gauge has always been off. Now I know why! Thanks. I had changed the sending unit several times over those years, even adjusted the thing on the bench with a meter to the proper ohms etc. Still would not read correctly. It would always "sort of" give me an idea of how much was in the tank...lol :surprise:
I recently found out that Autometer is making 73-10 ohm linear gas gauges for the early Fords. If you replaced your sending unit all you need to do now is buy a new linear Autometer gauge and you would be set.
 

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Discussion Starter · #5 ·
Thanks for the info Peter! Mine was never accurate either and got worse after I replaced the tank and sender the first time. It would stay at full for a long time and then drops to half real quick.

After changing to the Tanks Inc tank & sender early this year, it was still not accurate. I figured I know when it's full and when it's empty, I need to know when it's half full and I can guestimate in between full and empty so I lowered the center section of the sender twice. Lowered it 1"-1.5" total and now the halfway mark is fairly accurate. Might be more difficult or not possible to do with a stock fuel tank sender, I don't know.

As long as the two devices are different electrically it would be next to impossible to get them to read right. The Tanks Inc. sending unit is linear, and if you have the stock fuel gauge it will not read right as it will still be 2-3 gallons off. If can you live with that, and know that say at 1/4 tank you still have about 1/2 tank you will be OK. It's when you get to empty and the FI is starting to starve then it becomes an issue or if you run out of gas.
 

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I recently found out that Autometer is making 73-10 ohm linear gas gauges for the early Fords. If you replaced your sending unit all you need to do now is buy a new linear Autometer gauge and you would be set.
I just finished (today) putting an efi tank with the in tank fuel pump and the sender that drops in from the top. I dumped a little less than 5 gallons in and it is reading a little below a 1/4 tank. So I think this one is fairly close. I've been running efi for quite a while but got tired of the exterior type pump getting hot and stop pumping. Not fun to have an angry red head woman that's getting very hot because I have to turn the a/c off to attempt to get the car to run. Things can get real ugly real fast with a red head. :surprise:
 

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"The Tanks Inc.". That's the efi tank/drop in pump/fuel sender, I have. The fuel sender has to have a rod cut off until it is in the proper position for the float to operated. Different dept for different cars. I'll fill it up tomorrow and see how it reads.
 

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Discussion Starter · #8 ·
I just finished (today) putting an efi tank with the in tank fuel pump and the sender that drops in from the top. I dumped a little less than 5 gallons in and it is reading a little below a 1/4 tank. So I think this one is fairly close. I've been running efi for quite a while but got tired of the exterior type pump getting hot and stop pumping. Not fun to have an angry red head woman that's getting very hot because I have to turn the a/c off to attempt to get the car to run. Things can get real ugly real fast with a red head. :surprise:
Been there, done that, about the red head getting angry along time ago.
 

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Discussion Starter · #9 ·
"The Tanks Inc.". That's the efi tank/drop in pump/fuel sender, I have. The fuel sender has to have a rod cut off until it is in the proper position for the float to operated. Different dept for different cars. I'll fill it up tomorrow and see how it reads.

Good luck. If you have the stock fuel gauge it won't read right though.
 

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I've been married to her for 39 years....I spend more time in the barn or shop or on the tractor than inside...lol Got to have a "safe" place to go when she riled up about something. "I can't hear you dear the tractor is running"...works every time >:)
 

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Problem can be solved with an Arduino board and program to convert the linear signal from the sender to a non-linear output. You simply need to know the correlation between the input voltage provided by the sender and the output voltage sent to the gauge. Once programmed you're done.

 

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Discussion Starter · #13 ·
I'll check on getting a new gauge. But, I am not doing any more work on any more mustangs while it is this freakin' hot and humid. Dang, I just about passed out the last couple of days and I usually quit by noon. Alabama in the summer time=he** :frown2:

I hear you as it's been really hot in Phoenix this last Summer too. :crying:
 

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Discussion Starter · #14 ·
Good find especially if you have the OEM gas gauge. For those who have aftermarket gauges it still might be easier just putting in a new gauge.


Problem can be solved with an Arduino board and program to convert the linear signal from the sender to a non-linear output. You simply need to know the correlation between the input voltage provided by the sender and the output voltage sent to the gauge. Once programmed you're done.

 

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I remember an aftermarket digital fuel gauge installation you could get for small aircraft. You would put a gallon of gas in the tank and mark it on some computer laptop software. You'd add another gallon and mark it until you had the full 13 gallons. The gauge became very accurate from this calibration.
 

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I'd think it would be cheaper and easier to just purchase the correct sending unit for your specific gauge. I know Stewart Warner also has specific sending units, including for fuel level, for each of their gauges. The fuel level sending units are available in a variety of resistances (64-66 Mustang requires resistances of about 75-83 ohms at empty and 12 ohms at full).
 

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I'd think it would be cheaper and easier to just purchase the correct sending unit for your specific gauge.
That's the problem...I've had 3 different senders for my OEM fuel gauge, and neither one has been accurate. It gets old after awhile having to figure out fuel levels based on odometer readings instead of the gauge.
 

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I tested for full deflection on my OEM fuel gauge by grounding it out. Since the gauge was operating then I knew that the intank sender must be bad so I replaced with this modern, digital sender:

http://www.virginiaclassicmustang.c...-Modern-Technology-38-inch-inlet-P482349.aspx

That fixed my issue of my fuel gauge not going completely to full.

However, the back of a '66 fuel gauge has small holes to adjust the calibration of the full and empty locations that can be tuned with a small flathead screwdriver.

http://www.mustangandfords.com/how-...ord-mustang-instrument-panel-troubleshooting/
 
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