Low fuel light behavior?

[SugrueNSX]

Hey Guys,

As you know when you turn on the ignition the warning lights come on so you can see if all the bulbs are good. My low fuel light does not come on when I do this so I figured that the bulb was bad but today I got really low on fuel and it came on. Is this expected behavior?

 

[NSXTech]

Yes it is. The low fuel light has its own sensor, not at all like the simple float and potentiometer setup used for the guage. The light uses a thermistor, which is a very temp sensitive transistor. It uses a temp variation to switch on and off. You know how your finger would feel cold even in a warm room if you dipped it in, then removed it from, a bottle of rubbing alcohol? The thermistor hangs from the float assembly, but is fixed. When the thermistor is suddenly exposed to the air in your gas tank, the cooling surface excites the thermistor and bingo- orange light.

MB

 

[MarkB]

Thanks for the explanation Mark.

 

[Griffen]

Yes it is. The low fuel light has its own sensor, not at all like the simple float and potentiometer setup used for the guage. The light uses a thermistor, which is a very temp sensitive transistor. It uses a temp variation to switch on and off. You know how your finger would feel cold even in a warm room if you dipped it in, then removed it from, a bottle of rubbing alcohol? The thermistor hangs from the float assembly, but is fixed. When the thermistor is suddenly exposed to the air in your gas tank, the cooling surface excites the thermistor and bingo- orange light.

MB

So how do you know if the bulb is bad other than running the car out of fuel?

 

[NSXTech]

There is a test you can do at the connector behind the panel behind the drivers seat. If you do it incorrectly you can let ALL the smoke out of the wire. If you want to do it, (the test, that is) PM me for details.
MB

 

[SugrueNSX]

Yes it is. The low fuel light has its own sensor, not at all like the simple float and potentiometer setup used for the guage. The light uses a thermistor, which is a very temp sensitive transistor. It uses a temp variation to switch on and off. You know how your finger would feel cold even in a warm room if you dipped it in, then removed it from, a bottle of rubbing alcohol? The thermistor hangs from the float assembly, but is fixed. When the thermistor is suddenly exposed to the air in your gas tank, the cooling surface excites the thermistor and bingo- orange light.

MB

Interesting. So it’s inherently self damping. That’s clever. But I still don’t understand why there is no bulb check at start up.

 

[Briank]

A Thermistor is a resistor that changes it's resistance in relationship to its temp. When used in this application a current is applied which causes the device try to warm up but since it in the fuel the heat is dissipated and its resistance stays high. When the fuel gets low enough to expose it, it warms up and its resistance goes down which turns on the light.