Changing the ohms of a fuel sender

stuwindsurf · Nov 9, 2023

Hi all,

After some help please!

I have a motorcycle dash that has a built in fuel gauge. According to the manual the fuel gauge needs a sender of either 100/250/510 ohms To give a correct reading. The dash shows full at 0 ohms and empty at either 100/250/510 ohms (selectable).

I have found a suitable sender, fitment wise, for the custom tank I am making but it is only available in either 0 - 190 ohms or 240 - 33 ohms. Is there a certain value resister or other component I could use to make this work with the dash?....

Thanks in advance for any help!

Stu

alec_t · Nov 9, 2023

Welcome to ETO!
How does the dash gauge select either 100/250/510 ohms?
Are you able to build a small circuit if I provide one?

stuwindsurf · Nov 9, 2023

After a bit more research it seems the 0 - 190 ohms is the European standard. So I need to either reduce the max ohms of 190 to 100 or increase it to 250 or 510 ohms. I would have thought reducing it to 100 ohms would be the best option but I have no idea how to calculate what resister value I would need to do so?

Also I have learnt that placing a resister in parallel will lower the ohms?

Thanks

stuwindsurf · Nov 9, 2023

alec_t said:
Welcome to ETO!
How does the dash gauge select either 100/250/510 ohms?

Thanks
In the dash menu you can select either 100/250 or 510 ohms

stuwindsurf · Nov 9, 2023

alec_t said:
Welcome to ETO!
How does the dash gauge select either 100/250/510 ohms?
Are you able to build a small circuit if I provide one?

..yes I should be able to, thanks

crutschow · Nov 9, 2023

To reduce the 190Ω to 100Ω equivalent, add a 210Ω resistor in parallel with the fuel sender.
The main negative to that, is it will make the gauge reading somewhat nonlinear.

The LTspice simulation (below) of the two resistors in parallel shows the parallel resistance (vertical scale) versus the sensor resistance (horizontal scale).
As you can see, a tank half-full sensor reading (90Ω) gives a parallel value of 63Ω to the gauge, whereas 50Ω is gauge half scale.
The half-full gauge reading will occur at a sensor value of 65.8Ω or about 35% full.
So the gauge will read some high when the tank is between full and empty.
The full and empty readings will be okay, of course.

ZipZapOuch · Nov 9, 2023

crutschow said:
To reduce the 190Ω to 100Ω equivalent, add a 210Ω resistor in parallel with the fuel sender.
The main negative to that, is it will make the gauge reading somewhat nonlinear.

The LTspice simulation (below) of the two resistors in parallel shows the parallel resistance (vertical scale) versus the sensor resistance (horizontal scale).
As you can see, a tank half-full sensor reading (90Ω) gives a parallel value of 63Ω to the gauge, whereas 50Ω is gauge half scale.
The half-full gauge reading will occur at a sensor value of 65.8Ω or about 35% full.
So the gauge will read some high when the tank is between full and empty.
The full and empty readings will be okay, of course.

View attachment 143275

All tanks are non-linear because the cross sectional area is not the same at each level of fullness. Your solution is more than accurate enough for a fuel tank. Every car I've owned, I've gotten fewer miles in the second half of the full gauge reading than in the first half.

stuwindsurf · Nov 10, 2023

crutschow said:
To reduce the 190Ω to 100Ω equivalent, add a 210Ω resistor in parallel with the fuel sender.
The main negative to that, is it will make the gauge reading somewhat nonlinear.

The LTspice simulation (below) of the two resistors in parallel shows the parallel resistance (vertical scale) versus the sensor resistance (horizontal scale).
As you can see, a tank half-full sensor reading (90Ω) gives a parallel value of 63Ω to the gauge, whereas 50Ω is gauge half scale.
The half-full gauge reading will occur at a sensor value of 65.8Ω or about 35% full.
So the gauge will read some high when the tank is between full and empty.
The full and empty readings will be okay, of course.

View attachment 143275

Thanks, that's really helpful. I have found a 210k resistor that is rated at 3w, I'm presuming that will be ok?...
Thanks again

Les Jones · Nov 10, 2023

210 ohm was suggested so 210 K is FAR TOO HIGH a value.

Les.

stuwindsurf · Nov 10, 2023

Les Jones said:
210 ohm was suggested so 210 K is FAR TOO HIGH a value.

Les.

Thanks Les, my bad. They list a 210 ohms too in a 1/4w value

Nigel Goodwin · Nov 10, 2023

ZipZapOuch said:
All tanks are non-linear because the cross sectional area is not the same at each level of fullness. Your solution is more than accurate enough for a fuel tank. Every car I've owned, I've gotten fewer miles in the second half of the full gauge reading than in the first half.

Yes, all fuel gauges are crap

Every car I've owned has always been completely non-linear, adding a resistor 'might' even make it more linear in this case, or it might make it worse - it depends entirely on the sensor and tank shape.

I'd like to think that fuel gauges are specifically designed to be non-linear, in order to match the tank shape they are fitted in - but I'm pretty sure that's not the case.

My current car has ten bars for the fuel gauge, has a 30L tank, and a range of around 300 miles per tank.

I zero the odometer every time I fill up, and it's still on six bars when you've done 160-170 miles, and then the bars seem to drop off more rapidly as you get above 250 miles. It's nothing like linear.

alec_t · Nov 10, 2023

Here's a circuit which uses an actual 0-190 Ohm sender and simulates a 0-250 Ohm sender.

stuwindsurf · Nov 10, 2023

alec_t said:
Here's a circuit which uses an actual 0-190 Ohm sender and simulates a 0-250 Ohm sender.
View attachment 143291

Wow! thank you for going to the trouble of doing that for me, it's much appreciated

ZipZapOuch · Nov 10, 2023

My current car uses the you could use the sender as-is and set your gauge to 250ohms. Your gauge will show 3/4 when the tank is full. Otherwise it should work normally.

crutschow · Nov 10, 2023

stuwindsurf said:
Thanks Les, my bad. They list a 210 ohms too in a 1/4w value

You'd have to measure the voltage across it when connected to the gauge with the tank empty to see if 1/4w is sufficient.

crutschow · Nov 10, 2023

Nigel Goodwin said:
Every car I've owned has always been completely non-linear, adding a resistor 'might' even make it more linear in this case, or it might make it worse - it depends entirely on the sensor and tank shape.

Since this is a motorcycle, the tank shape is probably fairly symmetrical, so the sensor resistance with gas level should be reasonably linear.

stuwindsurf · Nov 10, 2023

So quick update, the supplier of the sender have told me they are able to supply it so the ohms match any my gauge requirements. Brilliant! no resistors or extra circuitry required! They can also switch the sender from 0 ohms full/ max ohms empty to max ohms full 0 ohms empty, which is ideal for my application as I want to mount the sender in the top of the tank. I've gone for the 100 ohms- 0 ohms

Thanks everyone for your help with this

ZipZapOuch · Nov 10, 2023

crutschow said:
Since this is a motorcycle, the tank shape is probably fairly symmetrical, so the sensor resistance with gas level should be reasonably linear.

They are symmetrically shaped (left-right) but most sport and touring bikes are narrow at the top, wide in the middle and funnel shaped at the bottom so the vertically mounted sender is note consistent. A cruiser bike usually looks like a teardrop mounted in its side. So, not so consistent and fuel volume per verticals mm is inconsistent.

Nigel Goodwin · Nov 10, 2023

Yes, all kinds of shapes for motorbike fuel tanks.

Back in my day (and probably still?), you had main and reserve settings on the fuel tap - with main being fed from a small pipe sticking part way up the tank.

I used to run on main, and as soon as I had to switch to reserve I went to fill up (as I'm sure you're supposed to) - but my friend Alan used to run permanently on reserve - needless to say, he ran out of fuel a LOT!!.

I've never owned, or ridden, a motorbike with a fuel gauge

ZipZapOuch · Nov 10, 2023

Nigel Goodwin said:
Yes, all kinds of shapes for motorbike fuel tanks.

Back in my day (and probably still?), you had main and reserve settings on the fuel tap - with main being fed from a small pipe sticking part way up the tank.

I used to run on main, and as soon as I had to switch to reserve I went to fill up (as I'm sure you're supposed to) - but my friend Alan used to run permanently on reserve - needless to say, he ran out of fuel a LOT!!.

I've never owned, or ridden, a motorbike with a fuel gauge

Valves are more expensive than a fuel gauge so, the pet clocks are mostly gone these days. I have one modern bike and because it's a Honda, everything, including the fuel gauge, works perfectly. It's the worst bike I've ever owned. I could never justify spending an afternoon of "me time" tinkering with the bike in the garage because nothing ever broke or needed tuning. Just a perfect piece of machinery - VF800F with 82k miles.

Changing the ohms of a fuel sender

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