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Fuel Sender 101

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  • Fuel Sender 101

    My fuel gauge on my 99 Maristar 230 quit working so I decided to post my method for testing and replacement. When the key is turned on the gauge pegs and then returns to the empty position. This is with the key on and appx half a tank of fuel.

    The sending unit I have is a 2 wire unit. It is basically an adjustable resistor that is activated by the fuel level. Some have a float others has an electronic sender. The fuel position on the guage should read full when the sender is at 33 ohms and empty when sender is at 240 ohms. Half s tank will be indicated at appx 100 ohms. The factory sender had adjustable trim pots to fine tune the sender to the gauge.

    This procedure will NOT work with the 3 wire sender.

    First we will disconnect the sender from the harness. Then I got some resistors from Radio Shack 2 ea 100 ohm and a 33 ohm. I placed these in series to test the gauge. Should read 233 but this is close enough. Kind of a cheapo meter.

    Next I connected some test leads to the harness leading to the fuel gauge. This is the part that was previously disconnected. One side goes to the sender and the other to the gauge.

    At the other end of the test leads I connected the resistors and observed the gauge. With the key on I then placed the test lead across the 33 ohm resistor and the gauge reads full.

    Then moved the leads to the 100 ohm resistor. This gave me a half tank reading.

    Then the last test with the resistors is using the 2 100’s and the 33 in series for a total of 233 ohms. This gave me an empty reading.

    This verifies the gauge operation to be correct for the 33-240 ohm sender.

    I then moved to the sender to see what kind of ohm reading I was getting. It was reading 320 ohm across the black(gnd) to Pink (sender) leads. Since the sender should only read 240 ohms when empty then this means the sender is faulty.

    For this next step be sure to have your ignition off and disconnect or turn OFF you batteries. Make sure you have a fire extinguisher at the ready.

    I then removed the sender by removing the 5 screws and pulling the sender up and out of the tank. Have a rag handy to catch any fuel leaking off the sender.

    Once the sender is removed the fuel tank the tank will be exposed to the elements place a rag or something to keep things from falling into it.

    My sender looks like this it is 10 inches in length.

    My tank is a 65 gallon with a depth of 11 inches at the sender location. If your are not sure of your depth then get a wooden yardstick and insert it into the tank at the sender hole. Your sender should be 1 inch less than the tank depth.

    I ordered a replacement from SkiDim. It isn’t exactly the same but will work as electrically it has the same value of 33 ohm when full and 240 ohms when empty. The one I took out did not have an external float.


    disreguard pictures below they are the same as the ones used between text couldn't figure ho to get rid of them.
    Attached Files
    Last edited by mtajpa; 07-23-2018, 09:06 PM. Reason: Correction
    99 MariStar 230 Sport LTR 330

  • #2
    Fuel Sender 101 Continued

    I then took the new unit and tested it WITH THE OHM METER prior to installing it. With the float all the way up it was reading 33 ohms

    In the middle it was reading 100 ohms

    At the bottom it was reading 240 ohms.

    I then installed the sender into the tank to find the proper alignment before placing the gasket on. It only fits in one way.

    The sender then was removed and the gasket and new screws were installed on to the sender. Install the sender back into the tank with the same orientation as before. Be sure to use the correct screws as the kit comes with 2 different thread sizes.

    Gently tighten these screws in a criss cross pattern tighten until you see the gasket start to bulge just a little then stop. Overtightening will destroy the gasket and possibly damage the tank.

    Now that the sender is installed you will have to connect the wires into the old senders harness. Just cut the old wires from the sender and splice them on to the new one. Be sure to use a method that will not allow any moisture to get into the connections.

    Think that your done well not just yet One more thing to check before plugging in the sender.

    You want to check that you have a good ground connection at the sender harness. Using an ohm meter connect one of the leads from the black wire on the boat side of the sender harness. Connect the other side to a good ground (I used the negative side of the battery as it was close). You should get a reading of 0.0 ohms if you connection is perfect Mine read 1.0 ohms however the meter I was using had 0.5 ohms of lead resistance. 0.5 ohms is acceptable to me.

    To test the lead resistance simply connect you ohm meter leads together your meter will read 0.00 if your leads are perfect if not subtract this reading from all your readings if you want more accuracy.

    Now remove the meter and plug the harness into the sender and dress up the wiring.

    Turn on or reconnect your battery. Then turn on your key and you should see what your fuel level is.

    Looks like I better get me some fuel. Its below the ½ level don’t want to have to read the FUEL PUMP 101 thread.

    Hope this helps.

    Pictures below are same ones used between text. disreguard
    Attached Files
    Last edited by mtajpa; 06-27-2012, 07:53 PM.
    99 MariStar 230 Sport LTR 330


    • #3
      Very detailed and very good information. Good post, wish I could see the pictures you included. For some reason they're not showing up for me.


      • #4
        Ditto, cannot see pics. Awesome data.


        • #5
          No pics here either.
          Being on the water is best, but when I'm not on the water I'm on TT.

          2002 X Star
          Sold - 76 S&S


          • #6
            Finally the pictures are up.

            Sorry had the pictures linked to my computer so I could see them but nobody else. I logged off and they are now appearing. Hope this works. Also for some reason the pictures are all together at the end of the thread. OH well at least there pictures now. Enjoy
            99 MariStar 230 Sport LTR 330


            • #7
              nice post - glad you're back up and running. I had a problem with fuel gauge reading erratically - turned out I drilled into the sender wires when replacing the rub rail. oops.


              • #8
                That's the way to do it! Great post!


                • #9

                  This should be a sticky.

                  Nice work and thanks for taking the time do do this!
                  1991 ProStar / 351W with 1:1 Velvet Drive


                  • #10
                    Nice! Need to do some testing as our gauge only reads 5-7 8ths full when it is actually full. Makes for lots of fun at the gas station.


                    • #11
                      VERY NICE, Thank You.
                      Seeing something in real time, makes all difference.
                      Camera Views were sharp & well lit, Star for your fridge.


                      • #12
                        Has anyone ever been successful converting a 3 wire floatless sending unit to a 2 wire unit with a float? What do each of the 3 wires do?
                        2004 X-Star


                        • #13
                          why isn't this a sticky. this should be a sticky in the electrical forum.


                          • #14
                            I have a 2002 X10 and its reading full all the time. I am using a calibrated Fluke 702 to send a signal to the gauge.

                            This is what I'm seeing:
                            800ohms is displaying Empty
                            708ohms is displaying 1/2 a tank
                            610ohms is displaying full
                            My sending unit is reading 264.8ohms with a full tank of gas.

                            Does anyone know if the gauge in my boat has a different range? So far all I'm seeing is the 33 to 240 range and it seems to be universal.

                            This is an awesome thread....helped allot!!
                            Last edited by Smogg; 08-08-2017, 03:36 PM.


                            • #15
                              New Fuel Gauge Reads ¼ Tank When Empty

                              I would like to add a little extra to this in hopes of saving someone some money. I had a fuel gauge that was reading full all the time on my 2002 X10. I purchased a new OEM sending unit ($109.00) and was still having problems. With the sending unit removed from the tank, I was getting a reading of about 3/8 on the gauge and pegged beyond full when inserted about 1/2 way into a full tank of gas. I tried the calibration procedure that is available on the Centroid Website to no avail. So I guessed that I must have a bad fuel gauge. I ordered a new gauge ($160.00) and installed it only to find it still does the same thing. At this point I was speculating that I was sent either a wrong or defective sending unit. On a whim, I decided to call Centroid Products to inquire about one of their sending units as a replacement. Mari was very helpful and she quickly established that they were the manufacturer of my newly purchased sending unit. I was put in touch with engineer Joel who was able to get me pointed in the right direction.

                              The first step was to check the voltage between the Black and Pink wires on the 3 wire connector that goes to the sending unit which read 7.22vdc. The problem is that the voltage between “display Send” (pink) and “display ground” (Black) should be exactly 7.00vdc. That means “display ground” is at (7.22vdc - 7.0vdc = ) 0.22 volts rather than 0 volts. When “display ground” is 0.22v, it makes the 2.81v empty signal from the sender look like (2.81 - 0.22 =) 2.59vdc to the display, which as you can see from the black table below is a 1/4 tank reading. Joel included some drawings and of a typical location (under the helm) of an MDC and pictures of the MDC plug.

                              The fix was to jumper the ground wire from the MDC to a nearby ground bus located under the helm. The Ground wire on my MDC is pin #24 on my 24 pin plug. It took all of 10 min to connect in a wire splice and BAM! fixed. If you have an 18 pin connector its pin 2C.

                              Last edited by Smogg; 08-31-2017, 12:03 PM.