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Thermistor in a gas tank

I'm having problems with the "Fuel Reserve" light on my motorcycle. When the fuel level drops to 1.3 gallons the Fuel Reserve light on the dash board is supposed to light, few weeks ago it quit working.

I removed the fuel gauge sending unit from the tank which has the gauge rheostat and a separate metal capsule for the Reserve sensor. I split open the capsule and a very small thermistor...maybe a 1/16" fell out. It broke off the two wires soldered at both ends. After looking over the schematic in my shop manual, the circuit seems rather disturbing. 12 VDC is supplied to the 4 watt bulb in the dash which then leads to the negative coefficient thermistor in the gas tank. Submerged in fuel the circuit is open. Out of fuel it heats up, closes and completes the circuit to ground. I always thought that there were very low voltage value's sent into a tank in order to prevent explosions? When that thermistor broke off the base wire could have easily touched the metal casing and it's at least able to handle 4 watts @ ~14 volts? Is the fuel to air ratio so high that an explosion is not possible? Even so, being a motorcycle with a 5 gallon tank I would think there would be a risk when removing the cap to refuel?

Next problem is repairing it. I managed to solder the wires back on but that didn't last but a few weeks. This time the microscopic metal tab came off the black thermistor body. The bike is a very limited production 1983 Honda CX650 Turbo...needless to say the sending unit assembly is no longer available. After searching the net I managed to get some info from a Goldwing owner. He recommended a:

NTC Thermistor TYPE RL2004-582-97-T10 Rated at 1,000 ohms at 25 degrees Celsius. Cold resistance is between 900 and 1200 ohms.

This evening I made a mockup circuit using that thermistor. With a 12 volt power source and a 4 watt bulb I left it energized for a few hours. What has me worried is that the thermistor gets too hot to touch. Submerging it in a very small vile of gas causes the light to turn off. It's hot enough that I hear a very faint hiss as it goes in the gas. It works exactly as it should but I'm worried about the heat.

Anyone have experience with these thermistors? I would assume the same style setup is used in most cars with "Low Fuel" warning lights?

Thanks for reading,

George

Reply to
cxturbö
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I think it would be safest to look for a sensor form another bike that uses a similar circuit but...

If' I'm looking at the right data sheets ...

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the NTC material is D9.7A and the data for that suggests the resistance is on the high side..

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's/thermistors/keystoned97.pdf

I made it 107 Ohms at around 85C.

If that's correct I'm surprised the bulb lit up for your test.

I think the cold temp is about right (could be a bit lower) but the graph needs to be steeper with a lower hot temp. I think the resistance of the NTC needs to be less than the resistance of the bulb at whatver temp you feel is safe (say

Reply to
CWatters

I recently replaced the sending unit and fuel pump in my aerostar van and was shocked to find out that all of the wiring to the pump and the sending unit, which was basically a big potentiometer, were all exposed and submerged into the gasoline. I can't see why these things do not create a danger of explosion.

Reply to
Joey

Maybe you had the bulb replaced with a higher wattage type accidentally in the past? That might explain the failure and the not-so satisfying experiment.

Just a thought, Michael

--
One smooth landing is mostly luck; two in a row is all luck; three in a row 
is prevarication.

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Reply to
Michael Hofmann

I think the original thermistor just failed after 20 years. Bulbs in the dash are still factory. Actually the experiment worked exactly as it's supposed to. From ignition "on" the Fuel Reserve light typically takes 30 seconds to start glowing. The new thermistor does the exact same thing. Touch it with a piece of paper soaked in gas and the bulb goes off.

My biggest concern is the temperature of the thermistor getting hot enough to ignite the fuel.

Reply to
cxturbö

The ignition temperature of gasoline is approx. 240°C, so I wouldn't be too worried about that.

Michael

Reply to
Michael Hofmann

Correct. I stopped by the library today and went through quite a few books. Unfortunately all they have are Clymers and Haynes which don't go into detail. No shop manuals.

Actually it works great. Even the warm up time from ambient until the bulb starts to glow is about 30 seconds...exactly how the original used to work. Just touch it with a piece of paper dabbed in gasoline and the bulb goes out.

Reply to
cxturbö

You're not watching Hollywood movies? They do :-)

SCNR, Michael

Reply to
Michael Hofmann

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