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13-12-2018, 08:41 AM | #1 |
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Join Date: Mar 2018
Location: Blakeney
Bike: M900
Posts: 134
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'95 M900, testing the fuel sensor...
In my head this is a stupid question, as this seems like it should be obvious but I've tested three of them and I dont get the fuel light on.
I noticed while working on my bike that there was no fuel light when the tank was almost empty. If I unplug the connector and short the terminals then the warning light comes straight on so the circuit is good. I have a spare tank with a sensor, so in my head if I plug in the spare sensor, without it being in the tank, then there is no fuel and the light should activate, but it didn't. So I grabbed a used, tested working, sensor off of eBay and plugged it in but nothing. From testing the wires, it should be plenty of fuel breaks the circuit, low fuel makes it and so no fuel should be well and truly made. I even tried blowing in the little holes as the top in case something was stuck. Am I on the right track and have another broken sensor (3 of them) or is it that I don''t really get how they work and one of the gurus on here can explain a better way to test them? |
13-12-2018, 09:20 AM | #2 |
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Join Date: Jun 2018
Location: Clevedon
Bike: M1200s
Posts: 560
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Have you tried a multimeter on the sensor to see if you get a change of resistance when you move the float and a short when in the lowest position?
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13-12-2018, 09:58 AM | #3 |
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Join Date: Mar 2018
Location: Blakeney
Bike: M900
Posts: 134
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It's one of the older sealed metal ones with no visible moving parts. Maybe I could try to submerge it while measuring the outputs to see if they change but as it's not a fuel level sensor, just a warning light i would expect it to be on/off.
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13-12-2018, 10:32 AM | #4 |
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Join Date: Aug 2011
Location: Southampton
Bike: M1100evo
Posts: 2,465
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I think that's a thermistor type sensor where the resistance of the unit changes depending on the temperature difference between full and empty. When the sensor is covered with fuel, the temperature is lower than when uncovered and the resistance is high. When empty, the sensor is exposed to the air and the temperature will be lower, reducing the resistance.
I don't know what the readings should be, but if you attach a multimeter switched to kOhms and then immerse the sensor in fuel, you should see an increase in resistance if the sensor is working. |
13-12-2018, 12:50 PM | #5 |
Lord of the Rings
Join Date: May 2013
Location: Norwich
Bike: M900sie
Posts: 5,845
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Well I didn't know they worked on temperature, but a quick think leads me to believe that a dunking test won't work.
It can't work on a nominal temperature threshold, because the fuel temp will vary by several degrees depending on whether it's a frosty Scottish morning or standing in the Mediterranean Sun, with a similar range for the air within the tank. It must work on the differential between the top and bottom of the sensor? (I'm guessing here.) So dunking the whole thing in and out will not produce a differential. Probably better to dip the tip into (say) cold water whilst holding the bottom with a warm hand (Oo er missus!) Warning.. Do not take the test suggestion literally. I knew someone who badly scolded their feet whilst preparing a can of soup. The instructions said .."Stand in boiling water for 5 minutes".
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13-12-2018, 01:25 PM | #6 |
You Are What You Is
Join Date: May 2005
Location: A Foward Location
Bike: S4r
Posts: 1,948
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There is a float, it has nothing to do with temperature
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13-12-2018, 02:38 PM | #7 |
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Join Date: Aug 2011
Location: Southampton
Bike: M1100evo
Posts: 2,465
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We need an auto-electrician here as I'm relying on my old college physics lessons here!
I think the thermistor is driven by a the battery voltage, which generates a self-heating effect. When the thermistor is immersed in petrol, its temperature, and therefore its resistance, will remain relatively constant; the fuel acting as a heat sink. If the fuel level drops, however, the thermistor is exposed, the heat sink effect of the liquid disappears, the temperature rises, and the resistance increases. That being the case, you would probably need to energize the thermistor before trying the immersion test if you want to get accurate readings. If you try that, be careful connecting the voltage source - electrical sparks and petrol vapour are not a good combination! |
13-12-2018, 03:17 PM | #8 |
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Bike: M1100evo
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13-12-2018, 05:38 PM | #9 |
You Are What You Is
Join Date: May 2005
Location: A Foward Location
Bike: S4r
Posts: 1,948
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The float does not have any switches, it uses some form of capacitance/ induction
I tore one to bits and have photos |
13-12-2018, 06:30 PM | #10 |
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Location: Southampton
Bike: M1100evo
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13-12-2018, 08:34 PM | #11 |
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Join Date: Mar 2018
Location: Blakeney
Bike: M900
Posts: 134
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Looks like this one turned into a bit of a can of worms!! I'm glad I'm not the only one scratching my head on this one!!
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14-12-2018, 09:01 AM | #12 |
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Join Date: Jun 2018
Location: Clevedon
Bike: M1200s
Posts: 560
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If you are going to test it in liquid just use water not petrol, it's safer
You are just 30 odd miles from me if you fancy a ride out bring one with you and we can have a look at it together, I have a multimeter and plenty of water, even got coffee.
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14-12-2018, 01:43 PM | #13 |
Transmaniacon MOC
Join Date: Sep 2013
Location: Sutton In Ashfield
Bike: Multiple Monsters
Posts: 6,029
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Yea, it should work in coffee. That's probably what they use to test them anyway.
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14-12-2018, 02:02 PM | #14 |
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Join Date: Mar 2010
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Bike: M900
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Illy or Lavazza?
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14-12-2018, 02:40 PM | #15 |
Transmaniacon MOC
Join Date: Sep 2013
Location: Sutton In Ashfield
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Posts: 6,029
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Got to be Lavazza
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Roast Beef Monster! Termignoni and Bucci - Italian for pipe and slippers! S4 Fogarty, S4R 07T, 748, Series 1 Mirage |
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