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Thread: TR3A fuel gauge question

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    TR3A fuel gauge question

    I just started working on the fuel gauge on my TR3A. Previously it didn’t register at all. Now it is showing just under 1/8 tank of gas when I know by measuring with a stick that I have almost 2/3 tank of gas. Can someone advise me on what I may need to do to get the fuel gauge to read correctly?
    Here are the measurements I took today all with the ignition switch. turned on:
    1) Fuel gauge reading just under 1/8 tank.
    2) Voltage across the fuel gauge hot terminal and fuel gauge “T” terminal is 8.2 volts
    3) Voltage across “T” terminal and dash ground is 4.3 volts.
    4) Voltage across tank sender unit negative terminal and positive terminal is 4.3 volts.
    5) Voltage between fuel gauge hot terminal and dash ground is 12.5 volts.
    The tank sender unit is a new one from TRF.

    Thanks for the help!
    Regards,Bob

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    Yoda Geo Hahn's Avatar
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    Re: TR3A fuel gauge question

    Bob -

    This site: MGA Guru is for MGs but the gauge described is nearly identical to th eone in our TR3As.

    You can test the sender gauge relationship with a length of wire bent into a tall L -- thru the tank fill you can see the sender arm and push it down or pull it up and watch the gauge.

    Your 1/8 reading when the tank is 2/3 full immediately makes me wonder if the thing is working backwards -- a quick test will reveal if it is.

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    Yoda dklawson's Avatar
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    Re: TR3A fuel gauge question

    +1 for the MGA Guru web site. The values for the sender may or may not be the same but the gauge system functions the same and you can use the methods there to troubleshoot your gauges. An important point to keep in mind with the gauge system used before the voltage stabilizer was introduced is that the gauge itself must have a good ground connection.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1
    '72 Spifire Mk4

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    Re: TR3A fuel gauge question

    Thanks for the web site link. That is a really good article on fuel gauge! Regards,Bob

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    Jedi Trainee 6TTR3A's Avatar
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    Re: TR3A fuel gauge question

    I'm guessing that you have both the gauge and the sender installed, and you're happy with the wiring being correct.
    The 12.5 reading on the B side of the gauge is correct. Start with two things to check: The tank sending unit needs to be
    ground to the chassis. If the Moss unit doesn't have a dedicated ground terminal run a black wire from one of the attachment screws to ground. Next, run another black wire from the attachment stud on the gauge to chassis ground. (One of the two braces holding the dash steady will do.) Now check the reading again. Any improvement?

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    Yoda dklawson's Avatar
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    Re: TR3A fuel gauge question

    I hadn't noticed until now that you are in Raleigh. If there is any way I can be of help send me a message through the PM system on the board. I'm only a short drive away in Durham.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1
    '72 Spifire Mk4

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    Re: TR3A fuel gauge question

    6TTR3A. my tank sender unit is from TRF and it has a ground terminal and a "T" terminal, and I have checked the ground continuity of this unit a couple of times. Yesterday I made a ground jumper and installed it between the mounting brass hand nut on the back of the gauge over to some other ground connections for dash lights. The added ground did not make a difference.


    Based on the article by MGA Guru, I have taken some resistance measurements with the gauge still in the car. Here are my latest resistance measurements based on the article by MGAGuru.
    1) As Guru stated, I removed both wires from the gauge, and then measured across the T terminal and the gauge case ground. I got a reading of 101 ohms. Guru’s article said 99 ohms, so I think I’m ok on that.
    2) With wires removed, I measured across the T terminal and the B terminal and my reading was 64.5 ohms. Guru’ article said 61 ohms, so hopefully this is ok.
    3) I measured the tank sender unit several ways:
    A) With all wires attached (and with about 2/3 tank of gas) I measured across the ground tab and the T terminal and it read 25 ohms. I then lifted the
    sender unit float as high as I could and it read 32 ohms.
    B) I then removed all the wires from the tank sender unit and measured across the ground tab and the
    T terminal. This read 49.5 ohms (with about 2/3 tank of gas). I then raised the tank sender unit float as
    high as I could and it read 66.5 ohms. (MGAGuru said his measured 70 ohms at top)
    C) I reconnected the two wires to the tank sender unit and turned on the ignition and lifted the tank
    sender float as high as I could and the gauge read about full. ( I would not want to put enough gas
    in the tank to raise the float that high since I think it would be sloshing out of the top of the tank
    filler). As of now with everything hooked up and about 2/3 tank of gas, the gauge reads right at
    tank, so no progress on getting it to read correctly.


    Based on my measurements sorta matching the measurements advised by MGAGuru, I’m not sure what the problem may be. I’m wondering if maybe there is just a lot of dirt inside the gauge. There is a lot of visible dust on the inside of the glass and on the gauge face. Maybe just cleaning the parts inside the gauge would fix the problem. Has anyone had success doing that?
    Regards,
    Bob

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    Yoda martx-5's Avatar
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    Re: TR3A fuel gauge question

    The gauge is easy enough to take apart, and cleaning sure won't hurt it none. The MGA Guru also talks abouts adjusting the two movable coils to fine tune the readings on the gauge. If you look at the back of the gauge, you will see two small nuts and slots. When you loosen the nuts, you can move the coils. I did this with my gauge using resistors to calibrate as the guru mentions in the article.
    Art
    '58 TR3A TS236xxL
    '92 Mazda Miata -- Supercharged
    '07 Mazda RX-8
    '11 Mazda CX-7

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    Re: TR3A fuel gauge question

    Dirt is unlikely to affect this type of gauge. When you take one apart you will see how simply supported the needle is. If the needle still "jumps" off of empty when you turn the key to the run position the gauge is OK as far as dirt is concerned. You can further confirm this by pulling the green/black wire off your sending unit with the ignition on. The gauge should jump from wherever it is to beyond/above full. That will indicate that nothing inside the gauge case is obstructing or hindering needle movement.

    The two coil adjustments mentioned by Art and discussed on the MGA Guru web site will allow you to recalibrate your gauge to match the sender. It's an iterative process but goes fairly quickly. My only annoyance with it is that when you tighten those coil securing nuts the last little bit it tends to shift the reading. I'm not a hunter but I would equate this to "leading" a duck before you shoot. After a few attempts you will figure out where the needle needs to be pointing before you tighten the nut so it ends up in the "right" place when you are done.

    Don't be afraid to try calibrating the gauge yourself. It is easier to do this with the gauge out of the car but don't forget that means running jumper/extension wires (including a good ground). The gauge must also be oriented exactly like it would be in the dash of the car as gravity is important with these little magnetic gauges.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1
    '72 Spifire Mk4

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    Re: TR3A fuel gauge question

    Thank you! Some time this summer I will clean the gauge and then try recalibrating it.
    Regards,
    Bob

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