XV750 (and probably others) low fuel warning sender

I wanted to post this on the Virago tech forum, but it looks kind of dead... no new activity this year, and MySQL errors when you try to do anything. Hopefully someone will find it here and it will be useful.

My '89 Virago 750 doesn't have a fuel gauge. It just has a Fuel warning light in the tach. The light would light up when I was pressing the start button (a check to make sure the bulb is OK), but wouldn't light when fuel was low. I have Haynes and Clymer manuals for this bike... both are wrong about the fuel sensor... instead they talk about regular fuel level senders with float and such. The Clymer manual is particularly bad... also making such incorrect claims as, "All models have a fuel tank under the seat." The '89 XV750 has a very normal tank that looks like a tank, and a battery under the seat.

Anyway, the way this system actually works (when it's working) is that the fuel level sensor is a post with a little can on it, and in the can is a thermistor (a type of resistor that changes resistance with temperature). There are two wires that run to the sensor, and they feed 12V through the thermistor and into the warning lamp. When the thermistor is covered with fuel, it is cooled by the fuel and its resistance stays high... so, little current flows and the lamp doesn't light. When the fuel level drops below the thermistor, it's no longer being cooled, and it will heat up. When it heats up, it's resistance drops, and more current flows, so, after a couple of minutes, the fuel warning lamp turns on.

Testing this sensor is pretty simple: when cold (like room temperature or colder) it should read about 1K Ohms between the two pins of the connector. Mine read about 7.5KOhms. Mine would still lose resistance when I heated it up, but it was starting too high, and even when very hot was still about 5K Ohms. The replacement sender assembly is NLA from Yamaha. I found an NTE part, which is a 1K Ohm and NTC (loses resistance as temperature goes up). Its part number is 02-N102. I see now that they're available individually from eBay. I bought mine from NTE, and had to buy 25 of them plus shipping... but still a lot cheaper than a used working sensor.

I opened the little cylinder, removed the old, bad, thermistor, soldered in one of the NTEs, and good to go... my low fuel light works again.

I suspect that any bikes that use a low fuel light like this probably have a similar setup.

 

Photos from the process

 

To my address:
Ebay $5.99 + $0 shipping + tax Qty 1 ($6.47 for 1)
Digi-Key $2.70 + $10.74 shipping + tax Qty 25 ($14.52 / 25 = $.58 ea.)

 

Ha! Now you tell me! I bought a new one last year!

The old one was pretty crusty, so it's probably a good thing I got a new one anyway... but it's nice to know they are serviceable.