Low fuel light quit

[BigRed]

Hi, All.

My low fuel light quit working. So far, that's as far as I've gotten with my trouble shooting.

I did have the tank off, and I checked the connector when the light first didn't work. I figure I can use that connector to check both the level sensor and the bulb. I plan to put 12V to the bike side to check the bulb (inless someone tells me that there's not 12V on that circuit).

My question is: how does the fuel level sensor work? Note that this is for a low fuel light, not a fuel level gauge. I can check for resistance, but that's no good if I don't know what I should be reading when the tank is empty or full. I expect that it should read infinite resistance with gas and low/no resistance when it's empty, but wanted to check in case there's soemthing more complicated that goes on.

Thanks,
Brian

[CatatonicBug]

The light turns on when the sensor in the tank gets exposed to air, and is allowed to heat up. I'm not sure of the actual name for the component that does that though. If it were me, I would just use a 9v battery to see if you can get the bulb to light up by touching it to the ends of the leads.

[Lamric]

Woah.

First lets understand that circuit...the sender is a special type of resistor called a Thermistor.

It sits in fuel so normally it's cold and has a high resistance, so there's low current and no red light.

When fuel is low the little current that is flowing warms up the thermistor, the resistance drops, the current increases, red light comes on brighter.



The sender is pretty robust so check the bike side of the circuit first. You should have 12v at the green wire on the bike side of the sender connector block (by the rear of the tank). Short the green & black wires together and the red light should come on. If no red light then check bulb and bulb holder.

If you get a red light then the bike side is OK, check the sender resistance, across the other side of that connector block, should be around 1500 Ohms at room temp. (Note that for this test it doesn't matter if there's fuel in the tank or not, because once you disconnect the connector block to stick your multimeter in there won't be a current flow so the thermistor won't be at low resistance.)

I broke my sender last week and I don't fancy paying Yamaha prices for a new one so I'm going to solder in a used thermistor from a Honda VFR Well, they look the same to me.

[BigRed]

Thanks! That's exactly the info I needed. I always loved the low fuel light on this bike... I hate finding out that I need to find reserve when I'm in the middle of trying to pass someone.

[Ivan]

Well, if you are passing some one the right way, even if you have to hit reserves, you shouldn't coast down to much less than 100 before the fuel gets back to the carbs.

[BigRed]

Yeah, but once I coast down to 100, it might take me 0.6 seconds to finish getting past my string of drag-asses when I've only got 0.55 seconds before opposing traffic arrives

[Lamric]

Update:

I've now replaced the broken thermistor in my fuel sender with one from a 1999 Honda VTR1000F Firestorm. It works fine.

I wrote the procedure up with pics - here