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Thread: Fuel gauge readings?

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    Obi Wan
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    Fuel gauge readings?

    I've installed a TR4 fuel gauge, sending unit and stabilizer in my TR3. I'm also running an alternator and voltage gauge. Battery has 12 volts.When I turn the key on to accessory the gauge needle slowly goes to half way between 1/4 and 1/2 tank and stops. The tank is nearly Full. ( I've got one carb off on the bench so cant start the car at this time.) I was wondering if when the car is running would the extra current from the alternator cause the gauge needle to go the rest of the way up or is there another issues going on here?

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    Yoda
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    Re: Fuel gauge readings?

    Can you hook up a battery charger to see if that has any effect.

    Not sure what you mean by Stabilizer.

    The fuel gauge may need to be adjusted so that it indicates accurately.

    David
    TR3A TS75524L

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    Great Pumpkin TR3driver's Avatar
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    Re: Fuel gauge readings?

    With reading that low, I'm pretty sure you've got something else going on.

    Is the stabilizer an original type (thermal) unit, or an aftermarket solid-state unit? The original type will work correctly all the way down to 10 volts input, but the solid state ones need more voltage. How much more depends on which regulator IC they used inside, the popular LM7810 needs at least 12 volts input to produce 10 volts output.

    But even if the input at the stabilizer has dropped somewhat from battery voltage (due to resistance in other places), I would expect the gauge to read only a little bit low. 1/4 tank, maybe, but not 1/2 tank.
    Randall
    56 TR3 TS13571L once and future daily driver
    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Obi Wan
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    Re: Fuel gauge readings?

    The stabilizer I bought from Moss is the one upgraded to solid state. So your saying that it needs the full 12 volts to produce 10 volts output. I think my problem might be the battery itself. It sat indoors for most of the winter. I had a 1.5 trickle charger on overnight but that did not seem to do the job so I put a 10 charger on for a few hours. I put a tester on the battery and it did read 12 volts but when I put a load on it the dial read "weak"?
    I was wondering that if I had the car running the alternator would produce enough voltage for the gauge to read properly?

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    Re: Fuel gauge readings?

    Quote Originally Posted by DavidApp View Post
    Can you hook up a battery charger to see if that has any effect.

    Not sure what you mean by Stabilizer.

    The fuel gauge may need to be adjusted so that it indicates accurately.

    David
    I modified my fuel gauge to a the one used in a TR4 because I had one handy and the TR3 gauge had a dead short and was not functional. The TR4 gauge requires a voltage stabilizer so that only 10 volts go to the gauge.

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    Yoda
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    Re: Fuel gauge readings?

    You might be able to fix the tr3 gauge, they are kinda interesting inside with these little coils that can be moved around. Perhaps the gauge has cased grounded there.

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    Re: Fuel gauge readings?

    So, check the voltage coming out of the stabilizer, with the circuit complete (gauge and sender connected). If it is less than 10 volts, then the gauge will probably read higher when the battery voltage comes up (alternator is working).

    I still don't think that is the problem (at least not all of it); but if the VS isn't supplying 10 volts, then you can wait and see what happens after the battery issue is addressed.

    PS, it's also possible the new VS has a problem. We talked about this a long time ago; inside the VS can is a standard linear voltage regulator IC. The manufacturer of the IC specifies that it needs external capacitance for stable operation, but the VS being supplied back then did not have the capacitance. The result is a product that works sometimes, and sometimes not.

    I bumped into the same problem when rolling my own VS; my first attempt would work fine on the bench (using my bench power supply for a 12v source) but then not work when installed in the car. Output would be way low for no apparent reason. Adding the specified capacitor fixed it.

    Moss' VS supplier may well have fixed that problem by now; I simply don't know. But AFAIK Moss never publicly acknowledged there was a problem.
    Randall
    56 TR3 TS13571L once and future daily driver
    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Obi Wan
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    Re: Fuel gauge readings?

    Quote Originally Posted by sp53 View Post
    You might be able to fix the tr3 gauge, they are kinda interesting inside with these little coils that can be moved around. Perhaps the gauge has cased grounded there.
    I had a friend who is really into electronics test the TR3 gauge and he determined that there was a dead short somewhere in the windings and that it would be a long painstaking process to try try and locate the break. Since I had a good TR4 gauge I went that route.

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    Re: Fuel gauge readings?

    Quote Originally Posted by TR3driver View Post
    So, check the voltage coming out of the stabilizer, with the circuit complete (gauge and sender connected). If it is less than 10 volts, then the gauge will probably read higher when the battery voltage comes up (alternator is working).

    I still don't think that is the problem (at least not all of it); but if the VS isn't supplying 10 volts, then you can wait and see what happens after the battery issue is addressed.

    PS, it's also possible the new VS has a problem. We talked about this a long time ago; inside the VS can is a standard linear voltage regulator IC. The manufacturer of the IC specifies that it needs external capacitance for stable operation, but the VS being supplied back then did not have the capacitance. The result is a product that works sometimes, and sometimes not.

    I bumped into the same problem when rolling my own VS; my first attempt would work fine on the bench (using my bench power supply for a 12v source) but then not work when installed in the car. Output would be way low for no apparent reason. Adding the specified capacitor fixed it.

    Moss' VS supplier may well have fixed that problem by now; I simply don't know. But AFAIK Moss never publicly acknowledged there was a problem.
    I'll check the voltage coming out of the stabilizer as you suggest and let you know.

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    Re: Fuel gauge readings?

    Quote Originally Posted by karls59tr View Post
    I'll check the voltage coming out of the stabilizer as you suggest and let you know.
    Im confused as to Why you are running a voltage stabilizer with an alternator? Didnt you state that at the outset? They have built in voltage regulators, and typically, you gut or even remove the Lucas control unit. I know I did. The alternator and the VS may be working against each other.

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    Great Pumpkin TR3driver's Avatar
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    Re: Fuel gauge readings?

    Quote Originally Posted by Jim_Stevens View Post
    I’m confused as to Why you are running a voltage stabilizer with an alternator? Didn’t you state that at the outset? They have built in voltage regulators, and typically, you gut or even remove the Lucas control unit. I know I did. The alternator and the VS may be working against each other.
    The later gauges (TR4 through at least TR6) work on a 10 volt supply, not battery voltage. The "voltage stabilizer" (a very poor name IMO) is what supplies the 10 volts.
    Randall
    56 TR3 TS13571L once and future daily driver
    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Re: Fuel gauge readings?

    Quote Originally Posted by TR3driver View Post
    The later gauges (TR4 through at least TR6) work on a 10 volt supply, not battery voltage. The "voltage stabilizer" (a very poor name IMO) is what supplies the 10 volts.
    Randall, youre a god. Small g, but ​still a god!

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    Re: Fuel gauge readings?

    I put an early TR4 temp gauge in my TR3, so I've kind of been through this
    It's interesting to note that Smiths made both types of gauges, but Triumph chose the ones that need the VS. I assume that means they were less expensive overall even with the VS.

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