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TR2/3/3A fuel gauge / sender TR3A

NutmegCT

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Filled up the tank. First fill-up since I repaired the fuel gauge. Prior to fill-up, gauge and tank were between Empty and 1/4. Tank is now full to the bottom of the filler pipe.

Paid for the gas, turned the key, and the fuel gauge pegged Empty.

Using a multimeter, the sender wire reads 70 ohms, which is correct for a full tank.

Took the gauge out, used 68 ohm resistors to check the calibration, and all is well. One resistor reads full, two read half, and no resistance (sender terminal to ground) reads empty.

Put it back in the car, turned the key, gauge needle moves to the Empty peg.

My addled brain says this defies logic. If the sender is reading 70 ohms (it does), the gauge should read Full. With the 68 ohm resistor it does read full. But in the car, it reads empty.

Green wire connected tight to gauge battery terminal, green/black wire connected tight to gauge sender terminal, black wire to gauge ground (bracket thumbscrew).

What incredibly simple fact am I overlooking?

Tom
 

TR3driver

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Sounds to me like either the green/black wire is broken, or the tank sender isn't grounded. Or maybe the dash panel (where the gauge ground wire runs) isn't grounded.
 

Don Elliott

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Or the large round ground connector bolted down with the rear RHS bolt that holds the tank with the securing straps is not grounded, rusty or the paint is too thick.
 

martx-5

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I just looked at the back of my redone dash, and I see that is possible to get the bracket that holds the gauge in very close to the electrical studs. Perhaps when you assemble everything, you are somehow shorting out on the post that goes to the sender. That would cause the "empty" reading. If there wasn't a ground back by the tank, then the gauge would read full because of infinite ohms, wouldn't it??

EDIT: Just thought of something else. Try putting the gauge back in the dash as before without the wire going to the sender attached. If it still reads "empty" the the gauge must be shorted inside from when you had it apart. Actually, you can do this outside the car. Run 12v to the Bat term on the gauge, and then just ground the case. If it reads "empty", then it must be shorted inside.
 

TR3driver

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martx-5 said:
If there wasn't a ground back by the tank, then the gauge would read full because of infinite ohms, wouldn't it??
D'oh !
/bcforum/images/%%GRAEMLIN_URL%%/wall.gif
 

NickMorgan

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Tom,
You say that when you tun the key the gauge needle moves to the empty peg. Where is the needle pointing before you turn the key? My gauge stops on the empty peg when the ignition is switched off.
It sounds as though the problem may be the sender unit. The coiled wire may be damaged. The gauge may come back to life once the level in the fuel tank drops. You could always try pushing the sender unit down using a stick through the filler cap, while watching the gauge with the ignition on.
If the gauge was working before you filled up, I would doubt that you suddenly have a bad earth.
The other thing I discovered is that if you put an early TR4 gauge in a TR3 or TR2 it will read in reverse.
I speak from a lot of experience of a faulty fuel gauge - turned out to be a faulty sender, but it took three gauges and two senders and a complete re-wire (and about five years) to find that out.
Good luck!
Nick
 
OP
NutmegCT

NutmegCT

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Well this was interesting. I stuck a long wooden dowel (my "emergency fuel gauge") into the fuel tank, poked around, heard a thud, and the voila! The (real) gauge now reads Full.

Can a float get stuck? Weird. Maybe the tank couldn't take the emotional stress of being filled to the brim for the first time since I got the car.

Thanks folks. All the gauges, panel lights, and switches are looking good and functioning again.

I feel a topless drive coming on.
Tom
 

jdubois

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Hey, since you got your gauge working again (Excellent!), can I do a slight thread-jack? My gauge generally shows the correct amount of fuel, but bounces around a lot. Am I to assume correctly that it shouldn't do that? It's pretty steady when the engine isn't running, so I'm thinking it's a sender problem?
 
D

Deleted member 451

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I think I'll jack this one also. My gauge reads full until the level is just over half. After that the gauge is accurate. I figured it was a sending unit problem but when I check the resistance on the sender it raises and drops smoothly as the arm moves. Any ideas?
 

TR3driver

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Bill said:
I think I'll jack this one also. My gauge reads full until the level is just over half. After that the gauge is accurate. I figured it was a sending unit problem but when I check the resistance on the sender it raises and drops smoothly as the arm moves. Any ideas?
Sounds like one of the adjustment posts on the back of the gauge has gotten moved. See
https://mgaguru.com/mgtech/electric/fg_01.htm
for more information. Although written for a MGA, the TR3 gauge works the same way.
 

TR3driver

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jdubois said:
My gauge generally shows the correct amount of fuel, but bounces around a lot. Am I to assume correctly that it shouldn't do that?
Guess it depends on your definition of "a lot". The TR3 fuel gauge is kind of jumpy compared to later Triumphs, since it responds instantly to what the sender tells it. And the fuel tank has no internal baffles, so it's easy to get the fuel sloshing from side to side (even enough to spill fuel out the original vent on the tank). A quick lane change on the freeway with a quarter tank of gas, for example, will cause mine to jump between 3/4 and empty several times.

One thing to check for, though, is the ground wire on the fuel gauge. People seem to frequently forget it, and the clamp doesn't make a very positive ground. There should be a short black wire, with a ring terminal under the lockwasher & thumb nut on the gauge, running over to the ground tie point on the back of the panel.
 

jdubois

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<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Another thing that happens is instant reaction of the gauge needle to changes in sender unit resistance, so that when the fuel in the tank sloshes the gauge wavers accordingly.[/QUOTE]

I guess that link answers my question, as does your post above. Thanks! I think mine bounces more that what's being described here. I like the description of testing the gauge in that link so much I think I'll pull and test the gauge just for the nerd factor, and while I'm doing that I can check the ground on the center post as well.
 

jdubois

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I took my fuel gauge out of the car last night, took it apart and cleaned it all up, calibrated the magnets with a benchtop 12VDC power supply and a decade resistance box, and it's working perfectly and the pointer is rock solid. Easy job to do, and fun to get the thing apart and see what makes it tick. I calibrated the gauge slightly differently than the write-up linked above suggested, but the write-up is dead on in it's description of everything and their way would work fine too.

I put the gauge back in the car and tightened down the connections good, and the reading is still fluttery. So I know it's something further down the line that's causing the issue. I guess I'll work on the sender next. But I'm also now wondering how stable my 12VDC line is when the car is running, as any instability in that would make the gauge quiver.
 

Adrio

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If you have a decade box you must be in the "know". Take a look at the schematic for the gauge and sender and you will see that the whole design of that system is such that it does not fluctuate with a change in rail voltage. Also, that is the reason you can't simply integrate the output of the sender.

If you wanted a "project" you could design an interface to put between the sender and the gauge that would do this integration for you. All that would be fun for the nerd factor (as someone said before), but I never seem to find the time.
 

jdubois

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Adrio said:
If you have a decade box you must be in the "know". Take a look at the schematic for the gauge and sender and you will see that the whole design of that system is such that it does not fluctuate with a change in rail voltage. Also, that is the reason you can't simply integrate the output of the sender.

Heh, no, not all that in the know, just very dangerous with other people's equipment here at work /bcforum/images/%%GRAEMLIN_URL%%/grin.gif . Thanks for the tip, I won't bother to follow up on the voltage then.

Adrio said:
If you wanted a "project" you could design an interface to put between the sender and the gauge that would do this integration for you. All that would be fun for the nerd factor (as someone said before), but I never seem to find the time.

Yes, that immediately occured to me too. Think it's just a little beyond my capabilities to do it though. I've a rough idea what I'm doing because I program embedded systems here at work, but I have to lean heavily on the EEs for any analog stuff in the circuit.
 

NickMorgan

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jdubois said:
My gauge generally shows the correct amount of fuel, but bounces around a lot. Am I to assume correctly that it shouldn't do that? It's pretty steady when the engine isn't running, so I'm thinking it's a sender problem?
My gauge used to do exactly this. It was OK when the tank was full, but flicked about violently when the tank was between a quarter and three quarters. It turned out to be the sender unit.
 

Twosheds

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My gauge reads F when I fill the tank, but as I drive, it slowly moves to E. Weird.
 

jdubois

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Twosheds said:
My gauge reads F when I fill the tank, but as I drive, it slowly moves to E. Weird.

Mine used to do that. What a pain! Here's the specialty tool I found to fix the problem. Luckily it's cheap, and fixes a lot of other things on the car too!
 

martx-5

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jdubois said:
Twosheds said:
My gauge reads F when I fill the tank, but as I drive, it slowly moves to E. Weird.

Mine used to do that. What a pain! Here's the specialty tool I found to fix the problem. Luckily it's cheap, and fixes a lot of other things on the car too!

I clicked on the link, but just knew before it came up what it would be. /bcforum/images/%%GRAEMLIN_URL%%/hammer.gif
 

TR3driver

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Twosheds said:
My gauge reads F when I fill the tank, but as I drive, it slowly moves to E. Weird.
Either the gauge or the sender is from a TR4. They work backwards to the TR3.
 
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