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Discussion Starter · #1 ·
Afternoon gurus, I’m having trouble understanding my fuel sender wiring.

I’m pulling the lpg out of the carby tb42 GQ. It has a sub tank fitted. The wiring coming out of the factory loom has three wires - green, yellow and white - which provide a signal to the gauge and this seems to read vaguely accurately. But the workshop manual shows only two wires - which are noted as G and E. Does anyone know which of these three is used to provide resistance to the gauge? And what does the other one do? Both the yellow and white are showing as 12v earthed when I apply power through a multimeter. The green shows about 1v.
I tried cutting wires to see if this change the dash gauge, but with all of them cut it still shows 1/2 tank!! I’m presuming that only two wires are required to provide a signal to the gauge, but I’m not sure which ones.

I have a diesel tank to fit. It has three wires - yellow, yellow/blue and black. The yellow connects to the float resistor, the black is earth and the yellow/blue is for a low fuel light (which I don’t believe my car can use). So clearly only two wires are needed to provide the signal.

I’m going to run the fuel hoses and wiring through a pollack valve.

Can anyone advise on which wires from the factory loom Is used for the fuel gauge?
 

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nissan
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The green, white and yellow wires don't sound like factory fuel gauge wiring, unless they are the wires on the short harness on the sender unit side. Factory fuel gauge wire colour is Yellow with a black trace. If you have cut all 3 of these wires and the gauge still reads, I think you have the wrong wires.

Where are these wires? Are they at the sub tank or the original main fuel tank at the rear of the vehicle?

Low fuel light wire is Yellow with a blue trace
 

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Discussion Starter · #4 ·
Thanks for your help @Yendor - I suspect that you are correct. It did appear that its was coming out of the factory loom, but (as you say) the colours aren't right.

I also found that the fuel gauge was being sent a signal from a separate ecu that switched between the petrol and lpg.

Do you have an idea on how the fuel gauge works? Is it simply a reading of resistance along a cable? If so, what is the second wire for? And why does it read as being grounded when 12v is applied to it vua a multimeter?


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Factory fuel gauge is simply a resistance reading. Two wires are needed for measuring resistance of the circuit, the third (if applicable) is for the idiot light. Any more than that would be for and in tank fuel pump.
 

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nissan
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Thanks for your help @Yendor - I suspect that you are correct. It did appear that its was coming out of the factory loom, but (as you say) the colours aren't right.

I also found that the fuel gauge was being sent a signal from a separate ecu that switched between the petrol and lpg.

Do you have an idea on how the fuel gauge works? Is it simply a reading of resistance along a cable? If so, what is the second wire for? And why does it read as being grounded when 12v is applied to it vua a multimeter?


Sent from my iPhone using Tapatalk Pro
The fuel sender unit is an adjustable rheostat. When the float goes up and down in the tank it changes the resistance reading. The fuel gauge is basically a voltmeter. It reads the voltage changes as the float goes up and down and displays this via the needle on the gauge.

So your LPG setup didn’t have a separate gauge for the LPG system, it used the original fuel gauge in the dash for both the LPG system and petrol?
 

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Discussion Starter · #7 ·
Factory fuel gauge is simply a resistance reading. Two wires are needed for measuring resistance of the circuit, the third (if applicable) is for the idiot light. Any more than that would be for and in tank fuel pump.
Thanks mate. So the circuit is through the float resistance gadget and back through the earthed out wire?

I'm just thinking of measuring resistance on a wire with a multimeter - obviously you'd put a probe on each end of the wire. As an analogy, the fuel gauge would be the multimeter, with one probe (wire) going to the float resistance gadget and the other probe (wire) going to the earth at the tank. Is that how it works?


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Discussion Starter · #8 ·
The fuel sender unit is an adjustable rheostat. When the float goes up and down in the tank it changes the resistance reading. The fuel gauge is basically a voltmeter. It reads the voltage changes as the float goes up and down and displays this via the needle on the gauge.

So your LPG setup didn’t have a separate gauge for the LPG system, it used the original fuel gauge in the dash for both the LPG system and petrol?
Thanks for that. Just clarifying, there's no voltage sent to the float sender is there?

Correct, the old system used the original fuel gauge for both petrol and lpg, showing whatever fuel type I was using at the time.


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Thanks mate. So the circuit is through the float resistance gadget and back through the earthed out wire?

I'm just thinking of measuring resistance on a wire with a multimeter - obviously you'd put a probe on each end of the wire. As an analogy, the fuel gauge would be the multimeter, with one probe (wire) going to the float resistance gadget and the other probe (wire) going to the earth at the tank. Is that how it works?
Pretty much. There is definitely voltage applied though -- that's how you measure resistance. Apply a known voltage, measure the current, plug the numbers into Ohm's Law and voila!
 

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Thanks for that. Just clarifying, there's no voltage sent to the float sender is there?

Correct, the old system used the original fuel gauge for both petrol and lpg, showing whatever fuel type I was using at the time.


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Yes you can measure voltage at the sender unit. Voltage reading will change as float goes up and down.

Somewhere in the fuel gauge circuit there is a change over set up. This enables the gauge in the dash to read what fuel type your are running. You need to find this and reconnect the petrol side. This could have been done at the LPG switch or a change over relay.
 
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