Fuel Gauge Challenge!
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Club Member
Joined: Jun 2007
Posts: 45,429
Likes: 2,897
From: Bedford UK
Fuel Gauge Challenge!
As some of you know, I have a secondhand 6 gallon gas tank and am working on fitting it to my carb 1990 FLHS. I have been sitting on it for a while now, but have just gone back to it. I have just one small challenge to overcome! 
I have bought a modern fuel level sender, hoping it will work with the fuel gauge on my bike. It has the part # 75281-08 moulded into it. The good news is it does work (outside the tank, I haven't tried it fitted inside yet), but only lifts the needle just above the red zone on my fuel gauge, when the sender float is in the 'full' position. The needle responds to moving the float arm up and down, but not across the whole width of the gauge.
So do any of our experts have any ideas? I suspect there is some electrical difference between my Evo gauge and later bikes, but can anyone suggest how I might persuade the needle to register 'full' on my gauge? If I can crack this my cup of joy will overflow!
There will be cake and coffee for the winning suggestion!
I have bought a modern fuel level sender, hoping it will work with the fuel gauge on my bike. It has the part # 75281-08 moulded into it. The good news is it does work (outside the tank, I haven't tried it fitted inside yet), but only lifts the needle just above the red zone on my fuel gauge, when the sender float is in the 'full' position. The needle responds to moving the float arm up and down, but not across the whole width of the gauge.
So do any of our experts have any ideas? I suspect there is some electrical difference between my Evo gauge and later bikes, but can anyone suggest how I might persuade the needle to register 'full' on my gauge? If I can crack this my cup of joy will overflow!
There will be cake and coffee for the winning suggestion!
Grand HDF Member
Joined: Mar 2007
Posts: 4,920
Likes: 264
From: Deep in the heart of New Jersey
I don't consider myself an expert, but I do have a service manual for '93-'94 Flh's. It shows on page 2-87, a diagram of the gauge/sender unit. It says a reading of 40 ohms should give you a full reading, and 250 ohms should give you an empty reading. The question now is does your new unit give those readings? Those are for use with a stock factory gauge. If they don't match right out of the box, I would think you might have to adjust the resister somehow to give you the required resistance. Hopefully the resister is external to the gas tank. Hope that sheds some light .,,
Seasoned HDF Member
Joined: Aug 2007
Posts: 8,195
Likes: 3,085
From: NW AR
Humm, Tup. I'm trying to recall how it all works without digging my manual out. But, if BC's manual is accurate for your bike, then I'm guessing that the variable resistor on your new sender isn't the same as the one on your bike. As a wild guess, would your old sender work on the new tank? The senders on our bikes are pretty generic in how they mount. I think my Esprit sender even mounts the same. After that, get the resistance of your new sender at empty and at full with an ohm meter and maybe we can figure out a bridge circuit to make it all work.
HDF Community Team
Joined: Dec 2005
Posts: 5,377
Likes: 2,574
From: NC USA
Hess' idea of a bridge is the only way to make it work. I'm not the electrical genius (mathematically) so you could try my "toothless hillbilly" trick I use to calibrate/correct guage...
Get a variable resistor (pot) from wherever, even a speaker fader switch will work... It will need to be a value in the range of the gauge, i.e. 0-250 (or more) ohms. Tap the center lead of the pot and one of the outer leads. Place that across ground (earth) and the post of the sender. Keep in mind that if you're doing all this "on the bench" the sender must be grounded to the same source as the gauge. If it's already on the bike, then across any earth source and to the post of the sender...
Adjust the variable resistor until the readings on the gauge coincide with the float level position. At this point you can remove the pot and using an ohm meter, take a reading off the same two leads you used. That reading will be value of resistor you can permanently install as a bridge, as Hess suggested and it can be installed anywhere along the circuit of the fuel gauge sender wire as to make a hidden or neat job.
Get a variable resistor (pot) from wherever, even a speaker fader switch will work... It will need to be a value in the range of the gauge, i.e. 0-250 (or more) ohms. Tap the center lead of the pot and one of the outer leads. Place that across ground (earth) and the post of the sender. Keep in mind that if you're doing all this "on the bench" the sender must be grounded to the same source as the gauge. If it's already on the bike, then across any earth source and to the post of the sender...
Adjust the variable resistor until the readings on the gauge coincide with the float level position. At this point you can remove the pot and using an ohm meter, take a reading off the same two leads you used. That reading will be value of resistor you can permanently install as a bridge, as Hess suggested and it can be installed anywhere along the circuit of the fuel gauge sender wire as to make a hidden or neat job.
Elite HDF Member
Joined: Sep 2007
Posts: 4,468
Likes: 12
From: Bluffton, South Carolina
Tj, if 40 ohms shows full and 250 ohms shows empty it would seem that there is too much resistance to begin with. Wouldn't adding resistance make it worse?? Perhaps Gordon needs to check the resistance of his original and compare it to the new one. Maybe the easiest way would be to make an adapter plate and use the old one???
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HDF Community Team
Joined: Dec 2005
Posts: 5,377
Likes: 2,574
From: NC USA
Tj, if 40 ohms shows full and 250 ohms shows empty it would seem that there is too much resistance to begin with. Wouldn't adding resistance make it worse?? Perhaps Gordon needs to check the resistance of his original and compare it to the new one. Maybe the easiest way would be to make an adapter plate and use the old one???
Adding resistance in-line would be backward to what he needs to get a closer to correct reading. My suggestion was to add a resistor across the line (wire from gauge) to ground. This has the effect of sending a signal of less resistance to the gauge. Obviously if you ground the signal wire coming from the gauge, it'd peg-out. So adding "some" extra grounding via a resistor would make it read higher.
Here's an example of a mod I've done before - if you add a 50 ohm resistor, connected to ground (that's earth to you Graham)
with the other end connected to the wire at the oil temp sender on 80's - early 90's era Evos with VDO gauges, it will raise the reading on the temp gauge by about 40-60 degrees. Now, that mod won't work for oil pressure, because that VDO gauge/sender works backward of the fuel or oil temp gauges. With oil pressure, you'd have to add resistance in-line to make it read higher.
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Seasoned HDF Member
Joined: Aug 2007
Posts: 8,195
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From: NW AR
Depends on where you add it. You put it in parallel to the other resistor (the sender), thus, in a parallel circuit, the resistance is less than either (any) of the individual resistors.
1/Rtotal=1/R1+1/R2+1/R3...
1/Rtotal=1/R1+1/R2+1/R3...
Seasoned HDF Member
Joined: Nov 2009
Posts: 15,705
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From: Farmington ,MS
I work on rental equipment and have to repair fuel gauges a couple times a week. 20-40 ohms is pretty standard for full on most stuff...so before you start reinventing the wheel with resistors and bridges...I would make sure you have a really good ground on the sender....number one cause I find of a gauge not reading full.