Repairing Low fuel warning light not working

[Old Rider]

On my bike the low fuel light does not work when the fuel get low.It lights up when pushing the starter switch so its not the bulb.
I first tried to replace the wiring and connectors from the fuel level sender.That did not help.You can read abut that in this tread:
https://www.fjowners.com/index.php?topic=20119.0
Anyway found the problem and it is a little strange thing called thermistor that is the bug.After a little research if found out that a thermistor works by
resistance not letting current flow thru it when it is at a specific temperature hot or cold. There is 2 types NTC is for negative temperature control and
PTC is for positive temperature control. The fj uses a NTC type .It sits inside the gastank and is constantly powered with 12 Volt when cooled by the fuel it
stops current flow so the fuel light not lights up.When fuel level gets low it is no longer immersed into the fuel and then it is able to heat up and the
current flows to the low fuel bulb.So here is a little writeup on how i replaced the thermistor in my bike.I have to place a WARNING if you plan to do this
job That is that I'm not a professional and did this work on my own risk.So do it on your own risk i think this may be dangerous if using wrong thermistor i
don't  know if i have used the right thermistor and it does not look the same as the oem one that was in the gastank.
Here is a link to a youtube video that explain better how a thermistor works https://www.youtube.com/watch?v=UEmbRJKlSeI

[Motofun]

I applaud your insistence on having a working low fuel light!  My lazy ass would just use the tripometer.

[fj1289]

Awesome job and write up!  These type repairs will probably become more and more common as the components on these bike show their age more than the bike itself! 

Like motofun said above - "little" things like this will be the difference of keeping an old FJ "running" vs one that is still a fully functioning example like it came new ... not that I'll ever be in danger of that myself!!

[red]

Old rider,

Inventive work, but it may not be safe over time.  I strongly suggest that you replace the fuel sender with correct parts, even a used fuel sender.

The risks here are not worth the rewards.

Soldering can be easy and fun, but not with dirty tips, wrong Wattage soldering irons, poor solder, and bad fluxes.

Make a hole in a common sponge.  Cut the sponge to fit in a flat jar.  Wet the sponge with water.  Cap the jar, when finished work.
Rosin flux is lame.  Acid flux destroys wires.  Use LA-CO paste flux (non-acid, washes away with water).  Do not leave any flux on finished work, not ever.
Clean the soldering tip, bright and shiny, in the sponge.  Add fresh solder to the tip.  Clean the tip again, after every minute working.
Use temperature-controlled soldering irons. 50 to 100 Watt soldering irons will do most small-sized work.
A soldering iron running too hot gets too dirty, too fast.  The wet sponge can clean and cool the too-hot soldering iron, but you will need to clean the tip very often.
A too-small soldering iron will not get the work hot enough to take solder well.
When finished, the solder should look smooth and shiny, like silver.

[Old Rider]

I applaud your insistence on having a working low fuel light!  My lazy ass would just use the tripometer.

I use the tripmeter all time and know that my bike goes around 170 km on 10 litres .But i like that all bulbs are working on my bike 

[Old Rider]

Awesome job and write up!  These type repairs will probably become more and more common as the components on these bike show their age more than the bike itself! 

Like motofun said above - "little" things like this will be the difference of keeping an old FJ "running" vs one that is still a fully functioning example like it came new ... not that I'll ever be in danger of that myself!!

Thanks  that is true about the old components i will be the testpilot to se how it goes but im now confident that this repair is safe after some more testing 

[Old Rider]

Old rider,

Inventive work, but it may not be safe over time.  I strongly suggest that you replace the fuel sender with correct parts, even a used fuel sender.

The risks here are not worth the rewards.

Soldering can be easy and fun, but not with dirty tips, wrong Wattage soldering irons, poor solder, and bad fluxes.

Make a hole in a common sponge.  Cut the sponge to fit in a flat jar.  Wet the sponge with water.  Cap the jar, when finished work.
Rosin flux is lame.  Acid flux destroys wires.  Use LA-CO paste flux (non-acid, washes away with water).  Do not leave any flux on finished work, not ever.
Clean the soldering tip, bright and shiny, in the sponge.  Add fresh solder to the tip.  Clean the tip again, after every minute working.
Use temperature-controlled soldering irons. 50 to 100 Watt soldering irons will do most small-sized work.
A soldering iron running too hot gets too dirty, too fast.  The wet sponge can clean and cool the too-hot soldering iron, but you will need to clean the tip very often.
A too-small soldering iron will not get the work hot enough to take solder well.
When finished, the solder should look smooth and shiny, like silver.

Thanks for you advice Red ,but i have seen others done the same repair.They have used 1k thermistor  as i did.I have now done some more testing how
hot it gets and what happens if shorting the wires on it, but that is very unlikely.I now feel confident that nothing bad can happend

about the soldering
Thanks for the soldering advices i have the solderingstation on the pic it is almost first time soldering and did much of what you have sayd ,but
my soldering iron now looks like something from first world war and it is impossible to get the solder to stick to it i think i got the wrong flux too
it is called "lodenwasser" solder water

[Old Rider]

Since i did not manage to measure the temps on the thermistors with a ir thermometer last time.I got a better thermometer and re-measured.
It is more accurate i think and it goes up to 300 degree celsius.I think the thermistors gets hotter than this thermometer reads but not much
mayby 10-20 degree more than measured value.I wanted to be sure that the temp stay under 250-280 degre that is gasoline ignition temp.
The hottest temp that was measured was 89 degree but it is probably a little more so i feel that the repair is safe.Nothing happened on  the
2 days about 100km of riding with low fuel.I dont know what resistance the oem thermistor in a fj has ,but it is normal on other bikes to have
a 1K NTC installed same as i have done.

[Old Rider]

Maybe someone with a working fuel light can measure with multimeter to see what resistance the oem thermistor is .measure on the wires at the connectorplug it is 3 wires there.unplug the connector and measure on the black and on the grenbrown
wires. set multimeter at ohm

[T Legg]

 Thermistors change resistance with temperature. To compare them you must measure the resistance of each thermistor at the same temperature. There is also a temperature curve which is how much the resistance changes as the temperature changes so the resistance should be measured and compared at different temperatures to see if the curves are similar.Devices I work on that use thermistors usually provide a temperature vs resistance curve chart. Since you are only turning a light on and off it probably has some leeway.

[red]

about the soldering
Thanks for the soldering advices i have the solderingstation on the pic it is almost first time soldering and did much of what you have sayd ,but
my soldering iron now looks like something from first world war and it is impossible to get the solder to stick to it i think i got the wrong flux too
it is called "lodenwasser" solder water

Old Rider,

Soldering irons are really pure copper, most often.  Some soldering tools have replaceable tips.  Contact the company who makes the tool, if possible, for new tips.  Otherwise, you can refurbish the end of the iron.  First, use a soft fine-wire brush to clean away the crusty flakes.  Use a flat file to shape the copper into a blunt common screwdriver shape, or shape it like a large common nail, with three flat surfaces at the tip /\.  With only bright shiny copper showing, wrap the tip copper tightly with solder-wire, so the solder-wire looks like a tight coil spring on the tip.  Add flux, and turn on the iron at a low setting, hot enough to melt the solder on the tip, but not extremely hot.  When the solder melts, wipe the tip clean in the damp sponge.  That process should give you a good working tool.

New solder tips may be coated (plated) with soft iron.  I like to preserve the iron plating which you may get on some new tips, so a soft wire brush starts the tip cleaning.  That brush may be enough to get the iron clean, and ready to be tinned with solder again.  Once the plating gets hopelessly corroded, however, you will have to file down the end of the tip (when cold) to see the clean copper.  All soldering tools were bare copper a few decades ago, so do not be concerned about any iron plating lost to the file.  The tool will work well, with or without the iron plating.

A local plumbing supply house should have several good fluxes available; rosin is a minimally capable flux, and acid flux is not for electrical work.  I suggest LA-CO flux (or similar), if possible.

https://markal.com/collections/soldering-fluxes/products/regular-flux-paste?variant=9204786921519
-

[Old Rider]

Thermistors change resistance with temperature. To compare them you must measure the resistance of each thermistor at the same temperature. There is also a temperature curve which is how much the resistance changes as the temperature changes so the resistance should be measured and compared at different temperatures to see if the curves are similar.Devices I work on that use thermistors usually provide a temperature vs resistance curve chart. Since you are only turning a light on and off it probably has some leeway.

Thanks for that info .The thermistors i have has a range from 25 to 150 degree it was 17 in the room and i noticed that when the thermometer raised to 25 the bulb started glowing.
When i did the testrides it was only about 4-8 degree outside i then had filled 4 liter before the ride and  the bulb started to glow anyway ,but that is maybe because it was heated from the current .When at gas station filled 4 liter again the bulb shut of and started to light up faint again after riding 50 kilometer but by then the heat from engine must have heated the fuel in tank
wonder how it is in hot summer temps cant check that before next summer as the first snow fall came tonight

[Old Rider]

Red : Thanks for many good hints to save the solderiron i will try them out .The label on the flux i got says it is acid so now i know that was no go  

[Pat Conlon]

Here in the US there is a goofy TV commercial for a mortgage company selling home equity loans.
The commercial shows a guy driving a crane with a wreaking ball, towards his house. He said to his neighbors who are watching, that he is going to find the "hidden money" in his home.

The neighbor wife said to her husband, "Should we tell him about home equity loans?"

The husband replies, "No, let's just see where this goes..."

I'm with Red on this one....but still.....I can't seem to look away.

Please be Uber careful...I really appreciate your documentation. Really.

[Old Rider]

He he i could always put in a 3w bulb or less that will keep the thermistor cooler ,but i think the temp on it is safe.
I will report how it goes but so far no problems.