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Discussion Starter · #1 ·
Low fuel lights stays on all the time.

I searched the forums on this issue, and there were several threads a few years ago. Some people replaced the fuel sender unit, but it did not cure the problem. Seems there were a few people who attacked the problem, and could never figure out the solution.

I wonder if many have cured the problem by buying a new sender unit, or is the consensus that there is another cause. Hate to spend the $$ for a new sender unit, if something else is causing this.

What tests can I perform?
 

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What tests can I perform?
There are two small pin holes on the side of the in-tank low fuel sensor/switch that allows the fuel in and out. A first step is to clean out the holes with compressed air.

(There are a number of different fuel level sensor versions and types — so it's important that you update your on-line profile to specify your bike model and year.)
 

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Start by isolating the problem, unplug the pump/sensor and make sure the light goes out. If it does then start Ohm testing the sensor to make sure the sensor is not shorted in the harness externally. If that's no good then remove the sensor and try and clean it like strega mentioned. If none of these help then yes you will be looking for a replacement.
 

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Discussion Starter · #4 ·
Thanks for the tips to get me started.

I'll update the profile to show it is a 1997 900SS CR.
 

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Discussion Starter · #5 ·
When is the light supposed to come on? I looked in the tank and could see the sender unit, and the fuel is still above the top of the sender unit. Currently 127 on the trip meter. When the fuel level drops below the top of the fuel level sender unit does the light go on? How many miles do you get before the light goes on?
 

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1994 900SS CR, 2002 998 Trackbike
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When is the light supposed to come on? I looked in the tank and could see the sender unit, and the fuel is still above the top of the sender unit. Currently 127 on the trip meter. When the fuel level drops below the top of the fuel level sender unit does the light go on? How many miles do you get before the light goes on?

Yes the light turns on when the level is below the top of the sender unit. Assuming you have the old style (metal) fuel sender, there is a small switch in the head part of it, which I believe is thermally activated. Once that is above the fuel, it will close the circuit and the light will come on. These seem to be a common point of failure. You could try reaching in there and giving it a few taps to possibly unstick it.

At some point, Ducati changed the design of the sender unit to a float-based reed switch, in a housing made out of plastic. In these, a small float with a magnet in it floats in the fuel. Once it drops below a certain level, it will close a reed switch and turn the light on. I think your bike should have the older style metal unit, but in either case the light should not come on until the sender unit is poking out above the fuel level.
 

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With the tank capacity of a supersport I would expect a good 150 miles before it came on unless it is a track bike where you might be closer to 100 miles My 750ss carby track bike eats about 3.5-4 gallons of fuel at close to 100 miles on the track. My monster running similar capacity will cover about 200 miles on a tank and still have some fuel, each bike is different though so I would learn what your fuel consumption is and use the trip odometer as it is more reliable than the sensors.

Also know that on a functioning sender there is a delay in it setting off the light so do not expect to lower the level and see it instantly on, there will be a delay as the sensor recognizes that it is not covered in fuel.
 

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This is what I know. I still had the origin sender which is metal I've replaced with a plastic one from ducabike (which also didn't work out of the box) so I disected the old one to see how it works (mine is 2 wires). The 2 wires only purpose is to complete the ground side of the circuit to the fuel light. The small bulb at the tip of the sensor has one wire soldered to it and is isolated from the rest of the housing the other wire is attached to a small plastic piece in the swaged end of the housing as the fuel rises it enters the bulb and the liquid completes the circuit. But here's the problem the light goes out when the sensor is unplugged which would means the function of the switch is reversed to what it needs to be. Any thoughts would be greatly appreciated. And another note is this the new replacement never worked with or without fuel.
 

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The original level sensor has a thermistor at the end of the unit. 12 volts is supplied to the bulb in series with the thermistor. When it is covered by fuel it is cooled and the resistance is too high to light the bulb. When the fuel level drops and exposes the thermistor it heats up and the resistance drops which allows enough current to flow to light the bulb. The "usual" failure mode is for this to short circuit which repeatedly blows the associated fuse. As Ducvet suggested, disconnect the electrical connector to the fuel tank. The light should go out (and your fuel pump won't work). If it does not you have an abnormal path to ground in the bulb circuit that is oddly not a complete short. You will have to figure out where it is.

If the light does go out there is a problem with the level sensor. You would then measure the resistance across the two connector pins for the sensor when there is fuel covering the sensor and when not. This will tell you whether the sensor is working properly. It probably is not, but it is unusual for it to fail at a low enough resistance to light the bulb, but high enough to not blow a fuse.
 
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