# Thermistor help please

#### ceefna

Joined Feb 21, 2012
37
Hi all I need a bit of help understanding correct thermistor for a faulty motorcycle low fuel light. The thermistor sits in the fuel tank and when the fuel drops below it heats up and the resistance drops and triggers the low fuel light.
After a fair amount of searcing on google I found other people with the similar issue had fitted a NTC 1KΩ. This did not cure the problem for me so I did more testing. I can turn the light on with a 56Ω resistor in place of the thermistor but the light will not come on with a 68Ω resistor so the trigger must be between these two values.
What I cant get my head around is for instance a 100Ω thermistor is 100Ω at 25°C but how hot would it need to be to trigger the light at about 60Ω?
Also not sure how hot the fuel in a motorcycle tank will get and affect the thermistor reading?
thermistors are available in 68Ω and 100Ω or do I need a higher value to compensate for fuel temperature?
The bike is a 2004 GSXR 1000cc and the genuine part is £178........ thats why I would prefer using a £1 thermistor.

Thanks in advance for any help
ceefna

#### crutschow

Joined Mar 14, 2008
25,133
The curve for a typical 100Ω NTC thermistor shows it reaching 60Ω at about 40°C.

#### LesJones

Joined Jan 8, 2017
2,676
It may take a few minutes for the temperature to rise enough tor the resistance to drop low enough to turn the light on.

Les.

#### ceefna

Joined Feb 21, 2012
37
Thanks for the replies. With the 1K thermistor fitted after maybe 15 mins powered up its resistance didnt drop much below 500Ω so I think 1K is way too high. I am still confused how this system can work when fuel temperature must effect the resistance of the thermistor. I cant think of a way to nail the correct thermistor. I am sure fuel temp will be above 40° when hot so this would switch the light on even if a tank was full?

With a 1K thermistor bridged with a 68Ω resistor dipped in some fuel the light stays off when removed from fuel after about 20 seconds the light comes on as it should but the resistor is getting quite hot to the touch. How hot will the thermistor get?

sorry im rambling its confusing me!

ceefna

#### crutschow

Joined Mar 14, 2008
25,133
How hot will the thermistor get?
Depends upon the voltage across it.

Might be better to use a 100Ω thermistor and add a small series resistance so that it doesn't turn the lamp on when the fuel is hot.

#### ceefna

Joined Feb 21, 2012
37
The thermistor has battery voltage accross it. I will try the 100Ω in different temp water to pich a suitable resistor.

Thanks for your time and patience

ceefna

#### BobaMosfet

Joined Jul 1, 2009
1,124
Your problem is because it's not a thermistor. You don't heat something up inside a tank, exposing it to fumes- it could detonate.

It's a circuit that is designed to behave electrically *like* a thermistor.

Here's a GM example:

Note that the video discusses a thermistor but the instructor clearly states that a fuel tank does not use one, that he is merely using a thermistor's operation as an example for understanding.

In actuality, it's a PCM signal through a variable resistor whose whisker is tied to a float.

Last edited:

Joined Jan 15, 2015
5,513
The ones I have seen were 1K NTC thermistors with a 4 watt lamp in series. The idea is when the thermistor is in fuel the fuel sinks the heat. When the fuel level drops below the thermistor the thermistor begins self heating and being NTC the resistance drops, when it gets low enough the in series 4 watt lamp illuminates. Those are the only ones I have seen or read about. They are popular on motorcycles and are only a low fuel indicator. On a 5 gallon tank they are set around the 1.0 to 1.5 gallon level.

Ron

#### crutschow

Joined Mar 14, 2008
25,133
Your problem is because it's not a thermistor. You don't heat something up inside a tank, exposing it to fumes- it could detonate.
Why do you assume that just because GM doesn't use one.
The thermistor is used only for a low fuel warning and warms up only slightly when the fuel level drops below its location, way below the temperature where it can ignite gasoline fumes.

#### ceefna

Joined Feb 21, 2012
37
Hi again
I have found an online manual for the bike and it shows how to test the thermistor. Is there a way to work back from this to pinpoint the correct thermistor?

Joined Jan 15, 2015
5,513
That is also how I would go about it. The 3.5 watt lamp runs pretty close to the 4.0 watt lamp I have seen used. The suggested 68 Ohm thermistor should work also. I guess someone could eliminate the thermistor for the time being and try a few resistors in series with the bulb (a decade resistance box would be ideal) and see how the lamp actually behaves. That will only be a rough but should give some idea. What I am not sure about is with self heating just how warm the thermistor gets when not submerged in fuel. Free air ambient verse submerged.

Ron

#### BobaMosfet

Joined Jul 1, 2009
1,124
Why do you assume that just because GM doesn't use one.
The thermistor is used only for a low fuel warning and warms up only slightly when the fuel level drops below its location, way below the temperature where it can ignite gasoline fumes.
True- it just seems counter-intuitive.

#### MrChips

Joined Oct 2, 2009
21,399
The idea is to reduce the voltage across the lamp to about half its normal operating voltage, i.e. 12V reduced to 6V by putting an equivalent resistance in series.

Hence the thermistor will see about 6V. Power dissipated is about 0.7W.
Temperature rise is approx about 20C above 25C ambient, i.e. to about 45C.

Joined Jan 15, 2015
5,513
True- it just seems counter-intuitive.
It may seem peculiar but motorcycles have used it for decades. I had bikes in the early 90s which used the thermistor based low fuel warning light. They are simple and actually work well enough. Decades ago I had trucks which had a fuel level sensor above the tank with a mechanical linkage arm connected to a float in the tank, normally a cork float. My fuel pump was mechanical and on the side of the engine block. There were no wires actually inside my fuel tank.

Today my wife and I have two trucks sitting in the driveway. Each has a wiring connector taking a half dozen wires into my fuel tank where we feed an electric fuel pump, fuel level sensor, fuel tank pressure sensor (least my fuel fumes not escape) and God knows what else is stuffed in there? Yet, fuel tanks are not exploding around me.

Looking at all of this I really don't have much resistive instinct or inclination that bad things will or can happen.

Ron

#### ceefna

Joined Feb 21, 2012
37
Hi and thanks for all the replies. I have scoured Google to try to find any info on motorcycle fuel tank temperatures. I would have thought that fuel stored in a non insulated metal tank above a 1000cc winging would get quite hot indeed! Looking at the 68Ω thermistor it would only need a 5° temp rise to drop below the 56Ω light trigger.
I have ordered a 68Ω / 100Ω and 150Ω thermistor to do some tests.
The 150Ω looks like it will trigger the light between 45 and 50° which may give me a better tolerance with hot fuel.
Ceefna

#### ceefna

Joined Feb 21, 2012
37
Should read 1000cc engine!!!!

#### crutschow

Joined Mar 14, 2008
25,133
The 150Ω looks like it will trigger the light between 45 and 50° which may give me a better tolerance with hot fuel.
A small resistor (a few ohms to tens of ohms) in series with the lower value thermistors will tweak the point at which they trigger the light.

#### ElectricSpidey

Joined Dec 2, 2017
1,177
What happens if the light bulb is shorted?

3 watt glow plug?

#### crutschow

Joined Mar 14, 2008
25,133
What happens if the light bulb is shorted?
I've never seen a light bulb short failure.