# Dual fuel sensors, one gauge

#### Grinchy

Joined Mar 23, 2021
17
You could use a bicolor LED and make it green for one and red for the other. That way you have a positive indication that it is working.
yes, of course. great idea. i've seen some circuits using bicolor LED to show fuse state etc.

I'd be happy to add that in once we can settle on a design for the switching

#### crutschow

Joined Mar 14, 2008
26,963
Really need to know the voltage at the gauge to suggest a good solution.
Can you measure it at the tanks sensor?

#### Grinchy

Joined Mar 23, 2021
17
Really need to know the voltage at the gauge to suggest a good solution.
Can you measure it at the tanks sensor?
Here's the factory diagram for how this would work with the instrument cluster that automatically adjusts between tanks (I do not have this instrument cluster). I'll try to get the tank 1 (main tank) voltage after work today.

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#### Grinchy

Joined Mar 23, 2021
17
boostbuck
Joined Oct 5, 2017 27
Today at 3:11 AM
It seems the TS is regarding both tanks as effectively a single reservoir of fuel, and the gauge could reflect this. Assuming for example that tank 1 is one third the volume of tank 2, then by paralleling the Tank 2 sender with 750 ohms and the tank 1 sender with 200 ohms and connecting them in series, the gauge would read 'full' when both tanks full, down to '1/3' when tank 2 is empty, and on down to 'Empty' when both are empty. Correct divider values would be calculated on the actual ratio of tank volumes. The linearity of the gauge would of course would be poor with a simple divider, but it would provide a simple indicator of the combined volumes and remaining fuel.
I am absolutely willing to explore some parallel resistances and not switching the inputs and going single sweep on the gauge!
Tank 2 (depleted first) has roughly 1/3 of the total volume (full 410 ohm empty 15 ohm)
Tank 1 (depleted second) has roughly 2/3 of the total volume (full 410 ohm empty 15 ohm)

I attached a screenshot earlier of the series values for some other gauge points . . .
I am liking this idea more and more.

The identified values of 200 ohm for the parallel resistor for Tank 2 and 750 ohm for Tank 1 look entirely appropriate.

Is there a way to adjust the final resistance when tank 2 is empty (14 ohm in the parallel network) and tank 1 is empty (15 ohm in the parallel network) = 29 ohm down to 15-20 ohm? This would trigger the built in 'low fuel' light and dash warnings.

So add in an additional 30 ohm or resistance in parallel when the sender network hits 29 ohm, or switch to a 15 ohm resistance at 29 ohm.

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#### crutschow

Joined Mar 14, 2008
26,963
The identified values of 200 ohm for the parallel resistor for Tank 2 and 750 ohm for Tank 1 look entirely appropriate.
It should be 200 ohm for Tank 1 and 750 ohm for Tank 2.
Is there a way to adjust the final resistance when tank 2 is empty (14 ohm in the parallel network) and tank 1 is empty (15 ohm in the parallel network) = 29 ohm down to 15-20 ohm? This would trigger the built in 'low fuel' light and dash warnings.
I would think the low fuel warning light goes on before both tanks are completely empty (!).
So add in an additional 30 ohm or resistance in parallel when the sender network hits 29 ohm, or switch to a 15 ohm resistance at 29 ohm.
Either approach could be done with a comparator controlling a relay, but we need to know the voltage when the resistance hits the desired value.

#### Yaakov

Joined Jan 27, 2019
2,861
This is very different but...

If you used an MCU you could use the two sending units as inputs and produce any arbitrary voltage that you want to send to the gauge. It wouldn't have to be a very big MCU, just something with a couple of analog inputs and an analog output.

Then you could make it do whatever you want.

#### Grinchy

Joined Mar 23, 2021
17
Looks like I misremembered the ohm values for Full and Empty.
Full is 15 ohm, Empty is 410 ohm

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#### Grinchy

Joined Mar 23, 2021
17
It should be 200 ohm for Tank 1 and 750 ohm for Tank 2. <- thanks
I would think the low fuel warning light goes on before both tanks are completely empty (!).

Either approach could be done with a comparator controlling a relay, but we need to know the voltage when the resistance hits the desired value.
Yes, the low fuel light comes on for Tank 1 with about 4 gallons remaining.
That is about 1/6 it's capacity or roughly 345 ohms.
So now that I have the direction of resistance figured there isn't an issue with the low resistance to Empty, and I can solve the problem with a parallel resistance network.

#### Grinchy

Joined Mar 23, 2021
17
This is very different but...

If you used an MCU you could use the two sending units as inputs and produce any arbitrary voltage that you want to send to the gauge. It wouldn't have to be a very big MCU, just something with a couple of analog inputs and an analog output.

Then you could make it do whatever you want.
Yes, of course, but I think that might be more than is required. I'm happy with approximations, like paralleling the resistance or using a transistor to switch between the two gauges based on a voltage / resistance and sweeping the gauge twice.

#### Grinchy

Joined Mar 23, 2021
17
<Solution>
Use the senders in serial, with a parallel resistor added to each sender to approximate:
Tank 2 - 1/3 volume (1/3 of 395 ohms sweep) -> 200 ohm
Tank 1 - 2/3 volume (2/3 of 395 ohms sweep) -> 850 ohm

Full value - 29 ohm (as compared to 15 ohm factory)
Empty value - 410 ohm (as compared to 410 ohm factory)

I think this will work great!