Thermistor Low Fuel Warning Light Sender

This sender is a direct replacement for the thermistor low fuel senders found on both Motorcycles and Cars.

It has a room temperature resistance of approximately 1 - 2K Ohms.

Works with conventional incandescent bulb warning lamp, or our LED warning lamp module.
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In response to requests for information of whether the thermistor will work for this vehicle or that vehicle, and the occasional customer who lacks the information to test the replacement, a description of how they work follows.....

The thermistor senders work in a rather unusual way, so to test them it is important to understand how they work.
Usually, they are connected with the case to ground, and the lead connected to a 3 watt bulb, the other terminal of the bulb goes to +12V

When the thermistor is submerged in fuel, the fuel keeps it cool, and its resistance stays high, 1000, to 2000, Ohms.
a very small amount of current will flow through the thermistor from the bulb, but because the resistance of the thermistor is high, it is a tiny amount and will not light the bulb.

When the thermistor is removed from the fuel and in air, things start to change very slowly, that small current heats the thermistor itself very slightly, the increase in temperature causes the thermistor to reduce its resistance.
Because the resistance is reduced, the current flowing through the bulb, through the thermistor to ground increases a little.
As the current increases, the heating effect increases and the thermistor reduces resistance even more, so the temperature of the thermistor and the current flowing increases more... and so on !

This current and temperature keep increasing until there is sufficient current to light the bulb.

At this stage, the thermistor is hot but still has a few Ohms resitance, The bulb is on, but not quite as bright as if it was connected directly to ground.

Then fuel is added , the thermistor cools and the bulb goes off !

There is a thermal shock when you add fuel, so it is best to have the ignition off when filling ( you should do that anyway)

Turning on a reserve tap allows the cold fuel to hit a hot thermistor and as some 70's bike owners will tell you, reduces the life of the thermistor due to repeated thermal shock.

Hopefully, describing how they work will help you in deciding if your thermistor sender is faulty, as well as help you fault find if the new one does not immediately solve your low fuel light problem.