Perplexing "FUEL" light malfunction

[dmorales-bello]

I am afraid I am at a loss for any other help. Let me know if you want to try the rheostat, but I honestly wonder if that will help. The whole system in predicated on a variable resistance. Try used a different light while monitoring the voltage at the lamp connectors. You want to drop that mV reading below what you are seeing now.

I will try the bulbs when I get them at the end of this week and hope at least one of them works. If not I'll hit you up for a rheostat as a final attempt. I'm very thankful for all your help Dave, plus all the insight from @captainobvoius and the others.
I'm still holding on to hope that this "little light problem" won't lick the crowd knowledge base in this amazing forum.

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[Mark Maras]

 I noticed in the OE one there's a hole in the top and side but in the new one the holes are in the ends. Is it possible there's an air bubble in the top of the unit that could be eliminated with a hole in the top?

[Dave WM]

in order for that light to come on you have to get much lower resistance, at 12/943 ohms is .013 amps or 130mA seems low. CO needs to chime in...

just guessing really as to the normal operating parameters. maybe it self heat up to a much lower resistance. Must be for the light to come which means the thermistor is sitting in a hot hot pool of gasoline to turn on. maybe 124f is still not that hot, try boiling hot. that has to turn it on with a much lower resistance.

feel free to ignore that last bit about boiling, I am just guessing now, but would like to see that resistance get down to about 100ohms (.1k)

Edited July 28, 2020 by Dave WM

[dmorales-bello]

in order for that light to come on you have to get much lower resistance, at 12/943 ohms is .013 amps or 130mA seems low. CO needs to chime in...

just guessing really as to the normal operating parameters. maybe it self heat up to a much lower resistance. Must be for the light to come which means the thermistor is sitting in a hot hot pool of gasoline to turn on. maybe 124f is still not that hot, try boiling hot. that has to turn it on with a much lower resistance.

feel free to ignore that last bit about boiling, I am just guessing now, but would like to see that resistance get down to about 100ohms (.1k)

Wouldn't you have to have the thermistor receiving power in order for it to heat up enough to drop the resistance even lower?

 

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[Dave WM]

heating from within (current passing thru a resistor)or outside (hot water heating up the resistor) should have the same effect, drop in resistance. I don't know the specs of the thermistor so really just guessing as to what temps things happen. The only hope is a resistor in series with the circuit will keep the already  high resistance (3k) a bit higher to keep the thermal runaway from happening (more heat generated than is dissipated by the fluid surrounding it). adding resistance will how ever limit the overall max current allowed (a fixed resistance in series) so as to limit the brightness the lamp could ever achieve. Sounds like a balancing act that Nissan got right but is hard to reproduce from you experience. Has the vendor commented on any of this? would be nice to hear that they have fielded working units.  

[dmorales-bello]

I've not contacted the vendor waiting for us to come up with a viable solution. I did ask him for a couple of weeks time to try out the second unit he sent but that was before I realized it was having the same issue as the first unit I got from him. He seems very amicable so I don't expect anything negative but... You never know.

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[Captain Obvious]

Well I'm no help on the sensor resistance front. I pulled my HVAC bezel off and measured the resistance. Open circuit. Thinking that maybe I had a bad wire or connector somewhere, I pulled my little access hatch off in the rear and measured it right at the tank. Open circuit.

I know the rest of the system works because if I ground the bulb wire back at the tank, the lamp comes on, but without a working sender, it'll never come on. I don't usually run the tank down so far as to really care, but my warning sender is fubar.

[Captain Obvious]

8 hours ago, Dave WM said:

in order for that light to come on you have to get much lower resistance, at 12/943 ohms is .013 amps or 130mA seems low. CO needs to chime in...

I can help with that tomorrow. I will push current through one of those little bulbs until it starts to "noticeably" glow. The current measured at that point will enable me to extrapolate how low the sender resistance needs to go in order to light the bulb.

I had previously measured the bulb current (directly connected to a power supply) at about 230 mA. So that's "full bright". In order to reach that level, the sender unit would have to drop to 0.0 Ohms. Clearly the current at which the bulb is "noticeable" will be less than that.

[Dave WM]

I think I did my math wrong, current = 12/943*1000 (milliamp) 13mA no where near enough.

[dmorales-bello]

I think I did my math wrong, current = 12/943*1000 (milliamp) 13mA no where near enough.

Could you explain this a little further?

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[Dave WM]

milli often used in electronic terms as 1/1000th

so 1000mA = 1 amp

the math I used was ohm law current in amps=volts / ohms

so you had .943k ohms or 943 ohms (in resistance K is 1000, confusing eh?)

12 volts/943 ohms=

.0127 AMPs

.0127 AMPS * 1000 to convert to milliamps

12.73 mA rounded to 13mA

Lots of times values are stated as milliamps or mA due to the smallish current and its easier to say 130mA that .013Amps

 

Edited July 29, 2020 by Dave WM

[Dave WM]

be glad we are not talking about capacitors and Farads...

Capt, I remember using uuf vs pf...

Edited July 29, 2020 by Dave WM