Fuel gauge "reading" inaccurate - can I do this?

[CountryBumkin]

My fuel gauge is not reading empty accurately. I will run out of fuel when the gauge is still showing about 1/8 to 3/16 above the "E".

I've tried several times to get the sending unit float arm adjusted. I removed the tank, adjusted float arm and reinstalled. Would show empty before tank was actually empty. I lived with this for a while - I just made sure to fill up when/before the gauge read empty.

But now - I had to take the tank out to weld two bung fittings to it for my fuel injection install. Before reinstalling the tank,I bought a new sending unit. I set the tank on some saw horses and leveled it, and add about 2 gallons of water. Then I adjusted the float arm so I was reading 73 ohms on the sender terminals. I reinstalled the tank added fuel and the gauge is about 1/8 off (above "E" as stated in first line of this post).

It's a lot of work getting the tank out (plus I have a skid plate) - but it's not hard to get to the sender unit wires at the top of the tank.

So my idea is that, I could add a resistor (1 or 2 or 5 ohm, whatever it takes. I have an assortment). I would add enough resistance so the tank reads empty now (as I have just enough fuel in it now to be considered empty). I don't care if the "full reading" is off. The empty reading is what's important.

The sending unit/gauge specs say when the tank is empty the sender should be at 73 ohms and when full it should read 10 ohms. So I need to add some resistance to move the gauge reading "down" to show empty.

What do you think? Any considerations I may be overlooking doing this?

[Danny G]

Is the tank level when its mounted to the truck?

I wish someone would make a capacitance based sending unit. No floats needed, it measures capacitance in picofarad's to give you a reading on volume. The truck could be upside down and still read.

[Gary Lewis]

That will work. However, you need a fairly high-wattage resistor as the current approaches one amp. I used 2 watt resistors in my calibration work and they got pretty warm.

So a 5 ohm resistor would need to be a 5 watt unit as at Full the equation is: Watts = Current x Current x Resistance. So 1 x 1 x 5 = 5.

Anyway, stick a resistor in and see what you think. Won’t hurt anything.

[Gary Lewis]

Is the tank level when its mounted to the truck?

I wish someone would make a capacitance based sending unit. No floats needed, it measures capacitance in picofarad's to give you a reading on volume. The truck could be upside down and still read.

My Arduino plans could easily be altered to be used on a Bullnose sender as well as a later sender. And with the algorithm it would be easy to adjust it to make any situation work. Guess I need to make it happen sooner rather than later.

[CountryBumkin]

My Arduino plans could easily be altered to be used on a Bullnose sender as well as a later sender. And with the algorithm it would be easy to adjust it to make any situation work. Guess I need to make it happen sooner rather than later.

Gave it a try - but the results are not what I expected.

I tried several resistors (1, 10, 12 ohm) until I got the fuel gauge needle to sit right at the "E" mark (actually just slightly above it) where I wanted it. It ended up being a 20 ohm resistor.

Then I added some fuel from my can, and went down to gas station and filled truck up. But it is only reading less than 1/2 full with a full tank now.

I was hoping I could move the needle up a fraction on the low side (empty reading) and the high side (full reading) with move an "equal amount".

I guess I need to go back and pull the tank and try to readjust the float.

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The tank does not sit completely level (mid ship tank). The back of truck sits higher - but not by much.

What are the "Arduino" plans? Something to do with fuel sender/gauge accuracy?

[Danny G]

My Arduino plans could easily be altered to be used on a Bullnose sender as well as a later sender. And with the algorithm it would be easy to adjust it to make any situation work. Guess I need to make it happen sooner rather than later.

In a capacitance set up could modify an existing sending unit with two full length probes for send and sense so it would be easy just to pop and swap.

For those who may be wonder what I am on about basically you are creating a system that is sensing electrical capacity of the fuel cell in the picofarad range. So we're talking 1 trillionth of a unit of capacity, very safe. So it doesn't matter where the fuel is in the tank, it will read the same so long as both probes have contact.

I have seen some retrofit probes for those wanting an in tank pump, maybe that's an option. Maybe be cheaper in the long run.

[ArdWrknTrk]

In a capacitance set up could modify an existing sending unit with two full length probes for send and sense so it would be easy just to pop and swap.

For those who may be wonder what I am on about basically you are creating a system that is sensing electrical capacity of the fuel cell in the picofarad range. So we're talking 1 trillionth of a unit of capacity, very safe. So it doesn't matter where the fuel is in the tank, it will read the same so long as both probes have contact.

I have seen some retrofit probes for those wanting an in tank pump, maybe that's an option. Maybe be cheaper in the long run.

Capacitance is an interesting idea!

Would it change with different blends of fuel?

Not sure why anyone would have to weld in TWO new bungs for an FI setup.

Just use the later tank, sender and pump.

Then Gary's backwards reading Arduino would make the gauge work.

My truck (being a one year wonder) has the opposite problem.

[Gary Lewis]

Capacitance is an interesting idea!

Would it change with different blends of fuel?

Not sure why anyone would have to weld in TWO new bungs for an FI setup.

Just use the later tank, sender and pump.

Then Gary's backwards reading Arduino would make the gauge work.

My truck (being a one year wonder) has the opposite problem.

Yes, capacitance is a interesting approach!

As for the Arduino, it is explained in this thread: http://forum.garysgaragemahal.com/Bricknose-Sender-to-Bullnose-Gauge-Interface-td7174.html. And I plan to implement it as part of the EFI system for Big Blue as I’m going to use the later fuel delivery module that Jim described. Their beauty is in the fact that there’s no external pump or fuel tank selector valve as all of that is integrated with the FDM.

The downside is that their level senders work on a different resistance range than ours, hence the need for the Arduino to translate. But once the output, or Bullnose, side of the Arduino is figured out we can adapt the input side to about anything we want.

[CountryBumkin]

Not sure why anyone would have to weld in TWO new bungs for an FI setup.

Just use the later tank, sender and pump.

Your suggesting buying a new fuel tank with a sender unit and in-tank fuel pump, rather than just weld in a bung at the bottom for the external fuel pump (that comes with the FI kit) and bung on the top for the fuel return? I suppose if I needed a new fuel tank, that would have been a good way to go.

Is the tank mounting/positioning and straps the same for the 19-gallon mid-ship (with carb engine) and the EFI setup? with the only difference being the size of the opening for the sender/pump unit?

Anyways, I went the re-using existing tank route. No problem with that - I'm just having a problem getting the sending unit float arm bent to the correct angle.

[Gary Lewis]

Not sure why anyone would have to weld in TWO new bungs for an FI setup.

Just use the later tank, sender and pump.

Your suggesting buying a new fuel tank with a sender unit and in-tank fuel pump, rather than just weld in a bung at the bottom for the external fuel pump (that comes with the FI kit) and bung on the top for the fuel return? I suppose if I needed a new fuel tank, that would have been a good way to go.

Is the tank mounting/positioning and straps the same for the 19-gallon mid-ship (with carb engine) and the EFI setup? with the only difference being the size of the opening for the sender/pump unit?

Anyways, I went the re-using existing tank route. No problem with that - I'm just having a problem getting the sending unit float arm bent to the correct angle.

I'm not Jim, but I think I know the answer. From 1985 on the tanks had the larger opening that allowed the fuel pump to go in. I'm going to use a 38 gallon Bronco tank with the later fuel delivery module and it appears to fit just fine - although I will have to lengthen the pickup and the float arm.

And the tanks up well into the 90's mount the same way, using the same strap arrangement.