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  1. #1

    Default 1984 mustang fuel gauge issues

    I have an 1984 Mustang lx convertible. I am in the middle of a restoration and I need some help. Fuel gauge does not work and always reads empty. I pulled the sender out to check it, and it OHMs out properly so that rules out the sender. I checked for voltage going in to the sender and I am getting a pulse ranging from 0-10 volts. When I temporarily connect to ground, the gauge does not move. I tried to use a 80 amp resistor between to simulate the sender and the gauge still won’t move. I pulled the instrument cluster to verify voltage at the gauge. I have 12 volts going into the voltage regulator, the pulse coming out of the gauge, and the pulse coming to the sender. All is well but the gauge does not budge. Any suggestions?

  2. #2

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    Can you expand on the comment that the sender "ohms out properly" please? Just trying to see how you tested it. I'm guessing you moved the float up and down while testing and the readings changed accordingly when all the way up and all the way down and in the middle?

  3. #3

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    Quote Originally Posted by homer302 View Post
    Can you expand on the comment that the sender "ohms out properly" please? Just trying to see how you tested it. I'm guessing you moved the float up and down while testing and the readings changed accordingly when all the way up and all the way down and in the middle?
    I hooked up a multi meter and read 20 ohms at the empty position and 160 in the full position. I moved the arm up and down to verify the reading in between those ranges and the ohms readings correlated to the position that the float was in.

  4. #4
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    I don't have a schematic, but if you have the proper current through the gauge and the needle doesn't move, it seems that the gauge is bad. Have you measured the current? I don't know how many amps it takes to move the needle, but it shouldn't be very much. I would guess well below one amp. Maybe the needle is stuck or corroded. You might remove the gauge from the cluster and tap it gently against a table on the "full" side to see if the needle moves.

  5. #5

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    So what happens if you reconnect the sender to the harness and operate the float by hand? Does the gauge work in that instance?

    Not sure about the pulsed 10V you're reading, could very well have something to do with your problem.

    My gauge would always read empty, also. I finally decided to drop the tank and check out the sending unit. I checked it out on the bench and it ohmed out within spec. Connected the sending unit back to the harness under the car and operating it by hand made the gauge needle move. The floats are made of a type of foam and I think after a period of time they will degrade, absorb fuel and sink. I made a replacement from brass sheet and installed it in place of the original foam float. Been working fine ever since.

    Something to check if the gauge checks out good.

    I ended up making a new float from brass shim stock. My gauge works great now and no worries of future foat failure.
    Thomas

    1985 Mustang GT - Build Thread
    347 (10.5CR, AFR 185's, PP Crosswind, Comp Custom Cam, Holley SA "670"), T-5, 8.8 w/3.55's, MM SFC's, T/A, PHB, LCA's, STB, KMB, Bilsteins, MM/H&R Springs, SN95 5-Lug, Cobra Brakes, '04 Mach 1 Steering Rack

  6. #6
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    Boss102372, I owe you an apology for asking about current flow through the fuel gauge. As I should have remembered, trying to read pulsed voltage or current is almost impossible without an oscilloscope, especially when using a digital VOM. You might be able to use a steady 5v DC source to approximate the IVR supply.

  7. #7

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    Quote Originally Posted by Boss102372 View Post
    I hooked up a multi meter and read 20 ohms at the empty position and 160 in the full position. I moved the arm up and down to verify the reading in between those ranges and the ohms readings correlated to the position that the float was in.
    If those are truly the resistance readings you got, your sender is not for an '84. The '84 sender should read around
    73Ω empty, and around 10Ω full. You didn't mention, but I'm gonna presume here that your car is, or was originally,
    a CFI/Automatic, 'cause the 22Ω empty/145Ω full version only comes in the style that fits CFI/SEFI fuel tanks.

    It's a common mistake for someone to replace the sender using the wrong one, because they look the same, and many
    parts vendors don't know that the resistance ranges changed in '87. And, used to be, the '83-'86 senders were not
    available, because the parts vendors didn't know...

    Today, they are available from most of the usual Mustang repro parts sources.
    https://www.npdlink.com/product/send.../195670/150056

    While you're waiting for parts, you can test the rest of the system, by grounding the yellow/white stripe wire at the
    sender, through a 10Ω resistor. If everything is as it should be, the gauge should read within the fat line at the "F".
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ... '68 Vert, Mexican block 307 4-speed...
    '71 Datsun 510 ... And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

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