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Fuel sensor - over my head

2211 Views 6 Replies 4 Participants Last post by  olddude
Ok..... Ive had 2 vrods one -'13DX and present '15F
Both have suffered the same malady. Fuel gauge. Empty one second, to slowly creep to proper level OR read proper level and suddenly drop to E. Yeah Yeah Yeah...... They ALL do that, go by trip meter...... whatever...... Its a MECHANICAL THING built by MAN.... therefore can be fixed by same. MY question would be to the ppl that ACCEPT going by the mileage is WHY?????? Are you truly that accepent of things that are supposed to work that dont that your willing to accept it? OR "just buy a new sender" MINE WORKS albeit sometimes. But they arent gonna make new forever, right? Theyve already discontinued countless things.
Ok THAT rant outta the way.............
Im a mechanical guy, bored to death, due to unforseeable circumstances in my life, and I cant ride for afew more weeks, so I tackeled what SHOULD be a relatively EASY cure. Fix my intermittently working fuel sending unit.
Took unit from tank. Disassembled. Theres a long white tube, roughly 3/4" ID with a "float" inside that floats on top of fuel level, obviously. Ive read that the top of this float becomes damaged from hitting what I call the sending unit at top of tank, thus making the float no longer flat on top. I found, on mine, that the top was not flat. but more "bubbly" in appearance. So, I Filed it flat with a fine mill file, then polished with a plastic buffer to a dUll mirror-like finish. I then connected sensor back up to bike, not in tank, but rather horizontally along side, so I could slide the float inside the tube hoping to see the fuel level change. This did not work, I got a reading on fuel gauge of roughly 5/8 full, no matter where the float was positioned inside the tube.
So, being the curious type, and a slight knowledge of electrical things, I looked at the wiring diagram. I determined the sensor has a 12VDC hot, that is hot whenever the keyswitch is on. The ground is tied in with afew other grounds. SOOOOOOO that leaves 1 other wire, that goes to the IM module (yellow / white wire). I then made myself a "test tube" that mocks the fuel cell on the bike out of 2"PVC pipe, with a cap on one end. Now, I cut all 3 wires from their connector so I can mess with them. Hooked the "hot" to a small 12V battery, and the ground on the sensor to the negative on the battery, and filled the tube with fuel. Now THIS is what puzzles me. Anywhere I want to position the fuel level, does not matter where, using my digital miltimeter, with black from meter connected to ground on battery, and red on meter going to yellow / white wire, I get 1.65 volts. No matter what. when it should read full, 1.65 volts, 1//2 full, 1.65 volts, or empty, 1.65 volts.
I could add pics, but dont see a point. I need an electrical guy, or instrument guy. This shouldnt be this difficult. Im missing something............
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Hello Johnr.
Well I have replaced about four of thoes things and have the old ones just in case I can figure something out. Well to half ass answer your question the sender is a circuit board of some type. And I would venture to guess that there is a capacitor of some type or something that increases the current at different positions of the float. In other words it would be hard to repair the sender to work w/that gage. It probably uses micro mili amps to operate the gauge. You would have to know what voltages the gage could take. It looks like it would take more than just regular wiring to get it to work. It also has that function that tells how many miles you can go etc. And I bet it all works within that circuit board.These boards seldom work on large current or voltages. There could be current limiting devices and it may be designed to work on less than 1.65v. I think its pretty complicated w/o some type of diagram that tells us exactly what is doing what. If not u can risk damaging that high dollar gauge. But I am w/u on trying to figure something out. I think the gauge and the sender act as a complete circuit.They compliment each other mated some how. I have not truly proven this theory, but my last sender failed because it came apart. But I think when filling the bike, do not top it off. Just fill it up and stop. I use to cram the fuel as much as I could. This may not be good for this type of sensor. Just a thought, because I seem to have got longer life from the sender that came apart. Last about 2yrs or 15000 miles of daily riding. I am still watching this new one replaced.
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Well, small update. A friend gave me a copy of the electrical service manual. This helped, somewhat. It gives a test procedure, but just says if it does not pass "relpace sender". HOWEVER it DID give SOME insight. On the sensor icon in their diagram it gives the symbol for a sonic unit. I know this from working with the control guys from work. It also says the test "must be completed with a full tank. Allow bike to idle for 10 minutes to purge all air from tank, then take reading." So, emvironment is important. It also does give a voltage on yellow/white for empty tank, and for full. I dont have access to the book at this momont, but recall it being a fairly significant range. 4V maybe. When i get a chance to look it up again ill update post. So, someone pointed out that the sensor may not work in the PVC tube due to shape, so my next experament will be to leave snap ring off and wires cut, and try within tank itself. Possible................ Also, the book did say the IM module is the thing that does the mileage remaining calc,
Ill either figure it out, or hafta go back tonwork.... whichever comes first. My next experament may be to JB weld a small piece of shim stock to top of float (better reflectivity and not subject to alcohol in fuea as the plastic float seems to be)
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another update

Book says .9V (full) and 3.8V (empty). And thats what the output is from sender to IM (gauge). Through the y/w wire.
I would love it if you guys can figure this out. I’ve replaced my sender twice and I can tell I’ll be due another one soon. This isn’t a problem that cannot be resolved, we just need to find the “right” knowledge and motivation. That combination is clearly missing at H-D as they continue to sell pieces of junk for $100 each.
Would appreciate anyone giving me failed units, so i can disect them. In return, IF POSSIBLE ill return them, fixed, no charge. But i cannot guarantee either a fix, or return. Some may need to be consumed to figure it out. I am willing to pay shipping to me.
junk fuel sender

sender on my 2014 F crapped out on way back from Sturgis. 8K on bike. The ususal symtoms. Just thinkin about that sender a lot. How does a sonic sender work and is it possible the float is sticking in the
Darth Vader light sword because the dimensions change with temp of fuel? The tube and or the float itself. Maybe reduce the outside dimension of the float with some very fine sandpaper. In the meantime its the trip odometer for a fuel gauge just like my Sporty. Still, if it has a fuel gauge the damn thing should work properly. When I get my new sender i am going to take a closer look at that sender and try some modification to tube or float. Just thinkin.
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