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Thread: TR3A fuel gauge question

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    Jedi Warrior
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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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    Jedi Warrior
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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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    Yoda dklawson's Avatar
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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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    Jedi Warrior
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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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    Yoda dklawson's Avatar
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    Re: TR3A fuel gauge question

    If when working with the calibration you run into problems, send me a P.M. As I mentioned, I am only over in Durham and am happy to help any way I can.
    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 Doug!
    Regards,
    Bob

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    Yoda CJD's Avatar
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    Re: TR3A fuel gauge question

    These Gage's are really tricky to calibrate. Moving the two coils is easy, but the needle response to the movement is far from linear. Also, the coil wires are as fine as hair, so very easy to break. I finally got mine wired, but had to resolder the wires twice before I was done. You have to move each coil a little and then test it at full, empty, and half....then try it again...and again...and

    Well, you get the picture!
    John

    Most of a 1955 TR2

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

    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)


    The Smiths-Jaeger fuel gauge (FG2630/20 or X.49422/220) is designed to register 3 ohms at Empty and 86 ohms at Full. It was built to match the resistance of Smiths tank sender unit FT3331/02. I'm not familiar with TRF's replacement sender but 66.5 ohms is way shy of what the stock gauge can be adjusted to for Full. The MGA gauge and sending unit have a much narrower
    resistance range between Empty and Full (as MGA Guru mentioned). It is my opinion that your gauge is OK and the fault lies with the sending unit. I wasn't aware that TRF offered a tank sending unit for a TR3. I have a feeling that you have installed TRF part # 203610 which (I believe) is a sending unit for a TR4. This part looks the same and fits in a TR3 but works on a different principle. For some reason TRF doesn't make the distinction between TR2-3s and later TR4-6 models. I seem to remember a simple test to determine that your tank unit is correct for a TR3. Hook everything up, turn on the key and read the gauge. Then pull the float up towards full. If, as you do so, the pointer on the gauge moves in the opposite direction (that is, towards Empty) you have a TR4 tank sending unit. If so, I'm sure TRF will refund your money. NOS Locators sells the correct sender for a TR3. I think Moss does as well, but I've had good luck with the NOS unit.

    Frank

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    Yoda dklawson's Avatar
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    Re: TR3A fuel gauge question

    Frank, the behavior you are describing of reverse acting gauge needles is what happens when you use an early gauge with the later sending unit designed for systems that include the voltage stabilizer. Almost all of the voltage stabilizer system Smiths sending units (regardless of make model and year) operate from about 30 Ohms = full to "about" 270 Ohms = empty.

    The 66.5 Ohms Bob mentioned was not for the sending unit resistance. He was relaying his measurements across the gauge's coils. I agree with you that the gauge appears to be OK based on the resistance measurements.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1
    '72 Spifire Mk4

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

    Doug,
    I hope we're not confusing Bob with all this discussion. I agree with you on all points...I guess I misread Bob's measurements... All my TR3 gauges read 61-66 ohms across the coils... But I'm still convinced that he has the stabilizer type tank unit.
    You live close and can help him on a one to one better than I can via the forum.
    Keep us all posted on how the problem was solved (as I know it will be)
    FRank

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    Re: TR3A fuel gauge question

    Frank. I did order and received TRF part number 203610, which TRF lists as a TR2/3 tank unit. They show p/n 209195 as their TR4/TR4A tank unit. Here is the reading I got when I had removed all the wires from the sending unit and measured the resistance across the terminals of sending unit with the float raised as high as I could get it, the resistance measured 66.5 ohms. This seems a bit too low.

    As another test, I reconnected all the wires and turned on the ignition and raised the float as high as I could and the fuel gauge needle moved toward the Full mark, although it only went to 3/4 full when I had it raised as high as i could get it.

    When I measured the resistance across the "B" and "T" terminals of the gauge with no wires attached to the gauge, I got a reading of 64.5 ohms, which is consistent with the 61 to 65 ohms readings that Frank gets on his TR3 gauges.

    Regards,
    Bob

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

    Quote Originally Posted by TuffTR250 View Post
    ... Here is the reading I got when I had removed all the wires from the sending unit and measured the resistance across the terminals of sending unit with the float raised as high as I could get it, the resistance measured 66.5 ohms. This seems a bit too low.

    It is too low. In the raised position (full tank), it should read 90 or so ohms. I just measured one I have in the shop, and it measured 98 ohms. It should be at, or close to zero when down (empty tank). Mine measured 4 ohms.

    Looks like the sending unit is not right. Was it out of the tank when you did the test??
    Art
    '58 TR3A TS236xxL
    '92 Mazda Miata -- Supercharged
    '07 Mazda RX-8
    '11 Mazda CX-7

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    Yoda dklawson's Avatar
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    Re: TR3A fuel gauge question

    Agreed.

    I misunderstood which readings you posted earlier.

    The "arm up" reading for full should be closer to what Frank said earlier. Are there "hard stops" limiting the travel of the sending unit float arm? On the later type Smiths senders there are typically stops that can be tweaked to adjust the arm travel (and thus the resistance range).
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1
    '72 Spifire Mk4

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    Jedi Warrior
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    Re: TR3A fuel gauge question

    The gauge is in the tank and that is where I tested it. I think I can get it out of the tank without removing the tank. I did not test the sender before putting it in the car several months ago. I think it would be a good idea to take the sender out of the tank and dry bench test it, and see if there is any way to adjust it. I'd rather adjust the sender than the gauge if that is possible with this TRF sender.
    Regards,
    Bob

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