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Whitephrog
Luke Skywalker
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I know it was posted a while back but I can't find it. Remind me how to test a fuel tank sending unit.
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Reminder about testing sending units
- Thread starter Whitephrog
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dklawson
Yoda
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The bugeye should have the early gauge system without the voltage stabilizer. Therefore, the sending unit will range from somewhere between 0-10 Ohms when empty and 70-80 Ohms when full. You can connect a multimeter between the green/black wire sending unit terminal and earth and hopefully use a coat-hanger through the tank filler neck to lift/lower the float to see that the sending unit operates in this range.
I've not worked on a bugeye so I don't know how difficult it is for you to access the wires on the sending unit. I also don't know if there are two wires or just one on the sending unit itself. Regardless, there will be a green/black wire which goes from the sending unit to the gauge. The gauge terminal where this wire connects will be marked with a "T". If the sending unit has a second wire, it will be black and it will be an earth connection.
If with the ignition switch in the run position you unplug the green/black wire either at the gauge or the sending unit, the gauge should go to "full". If you leave the green/black wire connected at the gauge but connect the sending unit end of the wire to an earth point, the gauge should read empty.
The early gauge systems without the voltage stabilizer require that the gauge case have a good earth connection. If you are experiencing gauge problems always check the wire connections and earth connections (including at the gauge itself) before spending money on new parts.
I've not worked on a bugeye so I don't know how difficult it is for you to access the wires on the sending unit. I also don't know if there are two wires or just one on the sending unit itself. Regardless, there will be a green/black wire which goes from the sending unit to the gauge. The gauge terminal where this wire connects will be marked with a "T". If the sending unit has a second wire, it will be black and it will be an earth connection.
If with the ignition switch in the run position you unplug the green/black wire either at the gauge or the sending unit, the gauge should go to "full". If you leave the green/black wire connected at the gauge but connect the sending unit end of the wire to an earth point, the gauge should read empty.
The early gauge systems without the voltage stabilizer require that the gauge case have a good earth connection. If you are experiencing gauge problems always check the wire connections and earth connections (including at the gauge itself) before spending money on new parts.
davester
Jedi Trainee
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How accurate is the gauge/sending unit on your cars anyway? I've had the Sprite since '73 and the fuel gauge has always exhibited the same behaviour...
It stays absolutely locked against the "full" mark until the tank is about 1/2 empty, then it sweeps from "full" to "empty" in no time flat (though as I'm driving and hitting turns it oscillates wildly. I always assumed that this was simply normal behaviour. Is it not supposed to do that?
It stays absolutely locked against the "full" mark until the tank is about 1/2 empty, then it sweeps from "full" to "empty" in no time flat (though as I'm driving and hitting turns it oscillates wildly. I always assumed that this was simply normal behaviour. Is it not supposed to do that?
OP
Whitephrog
Luke Skywalker
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Doug
The gauge is on the work bench with new green wiring and ready to install in a new tank. How if in any way does that change the testing procedure?
The gauge is on the work bench with new green wiring and ready to install in a new tank. How if in any way does that change the testing procedure?
dklawson
Yoda
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Many years ago I wrote an acquaintance in the U.K. about the early gauge system. I saved the message to him in PDF format and the link below is for that document. Hopefully the info in there will be helpful.
https://home.mindspring.com/~purlawson/files/OldFuelGauge.pdf
Another excellent reference that you must review for cleaning, restoration, and calibration of the early gauge system is the MGA-Guru web site operated by Barney Gaylord. Almost any question you could think of asking will be answered there.
https://www.mgaguru.com/mgtech/electric/fg_01.htm
(Be sure to follow the article forward using the arrow buttons at the bottom of the page that opens).
The short answer to your question is that if you want to bench test the gauge and sender you must duplicate earth grounds in your wiring to take the place of the car body. Specifically, you must connect your power supply's 0V (earth) wire to BOTH the sending unit and the gauge. Your 12V supply will connect to the gauge's "B" (b for battery) terminal. The green/black wire goes between the sending unit's isolated terminal and the gauge's "T" (t for tank) terminal. That's about it. The MGA-Guru site will walk you through other checks and steps for calibrating the gauge.
Regularman (a.k.a. Kim Webb) is the Spriget gauge guy. My answers above are very generic. Kim knows all the points that are unique/specific to the Bugeye.
https://home.mindspring.com/~purlawson/files/OldFuelGauge.pdf
Another excellent reference that you must review for cleaning, restoration, and calibration of the early gauge system is the MGA-Guru web site operated by Barney Gaylord. Almost any question you could think of asking will be answered there.
https://www.mgaguru.com/mgtech/electric/fg_01.htm
(Be sure to follow the article forward using the arrow buttons at the bottom of the page that opens).
The short answer to your question is that if you want to bench test the gauge and sender you must duplicate earth grounds in your wiring to take the place of the car body. Specifically, you must connect your power supply's 0V (earth) wire to BOTH the sending unit and the gauge. Your 12V supply will connect to the gauge's "B" (b for battery) terminal. The green/black wire goes between the sending unit's isolated terminal and the gauge's "T" (t for tank) terminal. That's about it. The MGA-Guru site will walk you through other checks and steps for calibrating the gauge.
Regularman (a.k.a. Kim Webb) is the Spriget gauge guy. My answers above are very generic. Kim knows all the points that are unique/specific to the Bugeye.
OP
Whitephrog
Luke Skywalker
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Meant to say the sending unit is on the work bench. The gauge is NOS. Just want to test the sending unit for now.
dklawson
Yoda
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Even though the gauge is NOS, it isn't likely to "exactly" match the sending unit in your tank. Per your other thread on fitting the fuel tank, you may want to bench calibrate the gauge and sender together before you mount that tank. Regularman (and the article on the MGA-Guru site) should be able to walk you through how he does it.
As for testing the sending unit by itself off the car, all you can really do is clip your multimeter leads to the sending unit and move the float arm up and down. As mentioned above, with the meter clipped between the bare metal of the sender and the insulated lug for the green/black wire you should measure about 0-10 Ohms with the arm "down" on empty. You should measure somewhere between 70-80 Ohms with the arm "up" on full. Make these measurements without any of the cars wiring attached to the sending unit.
As for testing the sending unit by itself off the car, all you can really do is clip your multimeter leads to the sending unit and move the float arm up and down. As mentioned above, with the meter clipped between the bare metal of the sender and the insulated lug for the green/black wire you should measure about 0-10 Ohms with the arm "down" on empty. You should measure somewhere between 70-80 Ohms with the arm "up" on full. Make these measurements without any of the cars wiring attached to the sending unit.
OP
Whitephrog
Luke Skywalker
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Good tips. Thanks.
dklawson
Yoda
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davester said:
Dave, your profile says your Sprite is a '65. That's about the time the gauge system transitioned from the system without the voltage stabilizer to the one with the stabilizer. You say the gauges can oscillate wildly going over bumps. That is "normal" for the magnetic gauge systems without the voltage stabilizer. The later gauges are bimetallic, respond slowly, and don't typically have wild needle bounces.
I would start by confirming that the gauge has the correct sending unit attached and that its internal windings are not broken. With the wires disconnected from the sending unit, check its resistance range as described above. In particular, look to see if the resistance goes to "infinite" when the float arm goes above or below a certain spot in its range of motion. It's worth mentioning that you cannot use the early sending units with the later gauges and vise-versa. Changing or reversing the wire connections won't allow you to mix components from the two gauge systems. If you find a different resistance range, please post back with your findings.
If your sender operates within the range mentioned above and has no "breaks", visit the MGA-Guru site mentioned above and read up on how to calibrate your gauge to match the sender. I suspect that's where your particular gauge problem lies.