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Removing Stock HD Fuel Guage Cap

  • Rocky2010
    Rocky2010
    13 years ago

    Removing Stock HD Fuel Guage Cap from 2010 Fatboy Lo.

    Recently purchased a matching set of RSD Billet Contrast Cut Fuel caps, one for the fuel tank the other is a dummy cap for where the original HD fuel guage was located.

    Removal that is not a problem, managed to remove the connectors from the plug which connects to the main wiring harness so I could pull the wires up and out from the tube which goes through and exits on the underside of the tank.

    Problem is that the low fuel light is constantly on now the fuel guage has been disconnected. Also recently I  fitted the new HD dual Speedo/Tacho with the bling fancy colours and all that stuff. The segmented fuel guage in the Speedo was working great till I removed the Fuel Guage on the tank.

    Had a look at the wiring diagram, noticed that there is a circuit between the Tank Fuel guage, Speedo Fuel guage, Low fuel light, and the fuel guage sender unit in the tank. Now that the fuel cap wiring has been disconnected this is why the low fuel light is on constantly. 

    Has anyone had this same problem removing the Tank Fuel Guage. 

    Im thinking that the 2 wires need to be bridged possibly with a resistor to restore the circuit again, would anyone know what type of ohm resistor it would be that I need to use.

  • rider
    rider
    13 years ago

     

    "Im thinking that the 2 wires need to be bridged possibly with a resistor to restore the circuit again, would anyone know what type of ohm resistor it would be that I need to use."

     

    How would you go running the multi meter across the removed fuel guage to check it's resistance, and then bridging the circuit with a resistor of the same value?

  • Rocky2010
    Rocky2010
    13 years ago

    I have been doing some searching, found this on another Forum, very interesting read, I will give it a go hopefully it will solve the problem

    Sounds good in Theory, just have to put it to test :-)

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    TECH NOTE FOR REMOVAL OF FUEL GUAGE ON HARLEY DAVIDSON FATBOY LO
     
    This is going to require a bit of an explanation because of how Harley has the Low Fuel Light Circuit wired on a stock bike. The new Instrument Cluster is using this circuit to run the new Fuel Gauge that is in the Instrument Cluster.

    The wiring at the Fuel Gauge connector is: Orange to Orange, Yellow with a White tracer to Yellow with a White tracer, and Black to Black with a Green tracer.

    What the wires do:
    The Orange wire is B+ switched from the Ignition Switch and should be battery voltage any time the Ignition Switch is "On."
    The Black wire and the Black wire with the Green tracer that it is connected to is Chassis Ground.
    The Yellow wire with the White tracer connects the Gauge to the Fuel Level Sending Unit and the Instrument Cluster.

    How the Gauge works:
    Current flows: 1.) From the Ignition Switch through the Orange wire to the Gauge, 2.) From the Gauge through the Yellow wire with the White tracer to the Fuel Level Sending Unit, 3.) Through the Fuel level Sending Unit to Chassis Ground.
    As the level of the fuel in the tank changes a float (that is a part of the Fuel Level Sending Unit) moves an Electrical Contact across Resistor. As the float moves up and down, this moves the Electrical Contact across the Resistor and the Resistance changes.

    Because the Fuel Gauge has electrical resistance and the Fuel Level Sending Unit has electrical resistance, and because they are wired in Series with each other; they split the battery voltage between them. Together they always add up to 12 volts (or whatever the charging system voltage is at the time). BUT, as the resistance of the Fuel Level Sending Unit changes the split in the voltage between these two changes.

    The stock Instrument cluster is connected to the Yellow wire with the White tracer between the Fuel Gauge and the Fuel Level sending Unit. In this way the Instrument Cluster reads the voltage of the Fuel level Sending Unit. As the split in the voltage between the Fuel Gauge and the Fuel level Sending Unit changes the Instrument Cluster can read the portion of the voltage that is being used by the Fuel level Sending Unit

    When you disconnect the Fuel Gauge you remove the Power Source for this circuit (the Orange wire) and so the Instrument Cluster reads 0 volts on the Yellow wire with the White tracer no matter what the Fuel level is.

    The trick is to find out what the fixed resistance of the Fuel Gauge is and then go to Radio Shack, buy a fixed resistor (probably less than a dollar) of that value. With the Fuel Gauge disconnected bridge the connection between the Orange wire and the Yellow wire with the White tracer with this resistor. Wrap it all up and tuck it away somewhere where it won't get into any sort of electrical trouble and you're done!

    Unfortunately figuring out the fixed resistance value of the Fuel Gauge is probably a bit more complex than just hooking up an ohmmeter to the Gauge. This is because the Fuel Gauge probably behaves differently in a live circuit than it will with just an Ohmmeter attached to it. I could be wrong, but it might be expensive to find out otherwise.

    So . . . you will need a Digital Volt Ohm Meter (DVOM) that can read DC Volts and DC Amperes. The cheapest $5 digital volt-ohmmeter at Radio Shack or similar source will be more than good enough: First, with the Ignition "On" read the voltage between the Orange wire and the Yellow wire with the White tracer with all of the wiring connected. Write this number down. Next, disconnect the Yellow wire with the White tracer and attach your Ammeter between the two ends of the Yellow wire with the White tracer. It may actually be easiest to use several jumper wires between the Fuel Gauge Connector so that you can keep the Orange wires connected and keep the Black wire connected to the Black wire with the Green tracer while you connect the Yellow wires with the White tracer using the Ammeter as the third jumper wire. Again turn the Ignition Key "On." and write down the Ammeter reading. Now divide the voltage reading by the ampere reading. The number you get is the value of the resistor you need to buy to use in place of the Fuel gauge.

    Simple!
     
  • rider
    rider
    13 years ago

     

    By the sounds of how the guage and sender work together, it would almost be a good idea to consider keeping the guage connected, and re-locating it somewhere out of the way.

    (or surely someone out there has manufactured an electronic gadget to replace the gauge and address the issue???,,, more searching !!!)

  • wayne2607
    wayne2607
    13 years ago

     

    after reading the technical notes you posted it is clear to me it is a simple valtage divider circuit.knowing that might not help you but i posted a diagram and explanation that might help clear it up a bit.

    Vin - B+ - orange wire

    Vout - yellow wire - fuel gauge

    black wire is ground

    Z1 and Z2 are resistors

    i would make these 2 resistors of the same value that way you will have approx 6 volt on Vout this will make the cluster think you have half a tank of petrol.

    you need to measure the resistance with a mutimeter between the orange wire and ground with no power connected this is the total resistance of the fuel sender. get 2 resisters of about half that value and connect as below, this will only replace the fuel sender and will still need to be connected to the instrumentation cluster

    hope this helps

    Wayne

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