Fuel gauge

[sargez900]

Hi. My fuel gauge is not working. Now I tracked the initial issue down to a broken sender wire. While replacing that I think I have shorted out the dash unit. Now can't get it to move at all. Question. With the gauge out of the dash. Should the needle move freelie when I tilt the gauge?
Sarge

[59wagon]

Hi Sarge, I have one here that goes to full when you tip it upside down.
A better test is to put 12V across the gauge. Connect positive wire from your battery to the positive of your gauge (left hand side terminal looking from the front) and negative to the chassis of your gauge or dash cluster, NOT the sender wire terminal. The gauge should go to full.

Cheers,

John

[sargez900]

Thanks John.
Yeah before I started mucking around with it the gauge was reading above full. I replaced the sender wire and it came down to about 3/4 which I believe to be correct. Unfortunatly I neglected to turn off the ignition before refitting everything and think I may have fried the gauge. therewas some sparking and now gauge will not read at all. May have to get a replacement.
Sarge

[59wagon]

Bugger. You may have looked already, but if you need a replacement, you can get a whole dash cluster on eBay or gumtree for around $100 if you can’t find anything else.

[Craig Allardyce]

Hi Sarge, I have one here that goes to full when you tip it upside down.
A better test is to put 12V across the gauge. Connect positive wire from your battery to the positive of your gauge (left hand side terminal looking from the front) and negative to the chassis of your gauge or dash cluster, NOT the sender wire terminal. The gauge should go to above full.

Cheers,

John

Gauge should go to empty with it powered up and earthed, not full. Powered up with high resistance or open to earth will always read full. power it up with it not earthed and give it a few taps. Sometimes they will stick in the empty position.

[59wagon]

Hi Sarge, I have one here that goes to full when you tip it upside down.
A better test is to put 12V across the gauge. Connect positive wire from your battery to the positive of your gauge (left hand side terminal looking from the front) and negative to the chassis of your gauge or dash cluster, NOT the sender wire terminal. The gauge should go to above full.

Cheers,

John

Gauge should go to empty with it powered up and earthed, not full. Powered up with high resistance or open to earth will always read full. power it up with it not earthed and give it a few taps. Sometimes they will stick in the empty position.

Hi Craig, I agree with your statements above if the sender unit is fully installed and the tank is empty, or the sender unit is removed with the sender wire still connected and the body of the sender unit is earthed.
Sarge said his gauge was out of the dash, so I assumed the sender wasn’t connected. With the sender disconnected, if you put +12v on the +ve terminal of the gauge and negative to the body/chassis of the gauge as I said initially (NOT the sender terminal), it’ll read full if everything’s ok. If you put 12v across the +ve terminal and sender terminal, it’ll read empty.

For anyone interested, here’s a brief bit on how the fuel gauge works. First, a CAD schematic



Inside the gauge there are two resistance coils. In the CAD above, the top coil between “P” and “T” measures about 54 ohms. The lower one between “T” and the gauge body (which is to earth when installed in the dash cluster which is screwed to the car body), measures about 30 ohms, which is the same as the sender unit when in the full position.

Here’s a pic of the gauge coils



There’s a moving magnet between the coils that’s attached to the gauge needle. As the current running through the coils changes, the magnetic field changes and affects the movement of the magnet/needle unit.

So we have two circuits - one from P to earth via the two gauge coils, and one from P to earth via the top gauge coil and the sender unit resistor. Electricity takes the easiest path, or path of least resistance. The resistance across the gauge’s two coils is about 84 ohms. When the sender is in the full position, the resistance through P, T, B to earth in the CAD above is also about 84 ohms and the gauge will show full. In this instance, the current is the same through both circuits. Anything less than a full signal from the sender will have less resistance and that’s the main flow path of the current, so the gauge will show less than full. There’s still some flow (current) through the lower coil, but less than through the sender resistor. When the tank is empty, the circuit resistance through P, T, B to earth is at 54 ohms, which is when the gauge shows empty.

Here’s a couple of pic’s of the sender unit guts, showing the resistor and moving arm that changes the resistance as it moves.





With power to the gauge, if you disconnect the sender wire from the gauge (“T”) or the sender unit itself (“A”), or remove the sender unit from the vehicle so that it’s not earthed (“B”), the current flow can only go through the 84 ohms of the two coils in the gauge, therefore reading full on the gauge.

Another type of gauge used in cars uses a resistance wire wrapped around a bimetallic strip connected to the gauge needle. As resistance decreases at the sender unit, the gauge’s resistance wire heats up and the bimetallic strip starts to bend and move the needle.

Sorry if I’ve hijacked your thread Sarge, but thought this relevant and someone might find it interesting.

Cheers,

John


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[sargez900]

No worries John. Very interesting. I've sourced a replacement gauge. I'll let you know how that goes. I will then open up the old one and see what has happened inside. Any tips on disassembly?
Thanks for all the replies. Sarge

[FireKraka]

I love your CAD drawings John can you teach me how to use CAD

Neil

[59wagon]

No worries John. Very interesting. I've sourced a replacement gauge. I'll let you know how that goes. I will then open up the old one and see what has happened inside. Any tips on disassembly?
Thanks for all the replies. Sarge

Sure Sarge. Before you start, I find it a good idea to take photos before each step so you can put it back together correctly.
To remove the gauge from the cluster, remove the two outside screws that attach a couple of small hold down “L” brackets and the whole gauge unit can be popped out.
If you look around the edge of the front bezel, there are five tabs that need to be prised open - I found a tiny flat, sharp, screwdriver good for doing this. Take your time and be gentle so you don’t brake the tabs off.
With the front bezel and glass face removed, remove the nuts from the two terminals on the back of the gauge, which enables you to remove the gauge light wire from the +ve terminal and an insulator that goes across both terminals. Be sure you hold the gauge internals in somehow so that it doesn’t fall out and get damaged when you remove the final nut - it’s a pretty sensitive and fragile unit.
Now you can carefully push the internals out.
A common symptom with these gauges when they don’t work, is that the tiny diameter coil wires burn through and separate, much like a fuse wire. Often though, and if you’re lucky, they burn through at the ends between where the wire leaves the coil and where it joins the soldered joint. If you can’t see a break, you can test the continuity of each coil with a resistance (ohm) meter across the soldered joints. If it’s not continuous and you can’t see the break, you’re probably pushing shit uphill, though if you had the patience of a Buddhist monk and nothing else to do , you could try unwinding the coil to find it.
I just fixed one a couple of weeks ago on my brother’s ‘57 Chev with a ‘56 dash that was broken at the ends of both coils. If you want to have a go at repairing it, and you’ve got old failing eyes like me, you’ll need some good glasses or a magnifying glass as the wire is tiny.
You’ll need to unwind a turn or
two from the coil to get some length back after it was shortened when it burnt through. The wire is insulated with a varnish type substance which you’ll need to remove to allow soldering - a sharp fingernail or blade can do this.
Depending on where it’s broken, you can either twist the two ends together and solder, or solder directly to the already soldered joint at the back of the unit. Once you’re done, check for continuity again to make sure there are no more breaks.

All the best,

John


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[59wagon]

I love your CAD drawings John can you teach me how to use CAD

Neil

Absolutely Neil. To do the course, you’ll need a set of crayons, a six pack, and the cardboard carton that the six pack came in. Some people need more than a six pack but you’ll be right, you’re a clever fella and fast learner.


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[Harv]

Love the reverse-engineering John - well done. I really appreciate it when people take the time to understand how things work, rather than just replacing them.

Cheers,
Harv

[sargez900]

Thanks John I appreciate you taking the time to explain things. I got a replacement gauge this morning and I now have a working fuel gauge Next day I get some spare time I will operate on the old one. If only for learning where the failure was. Great CAD by the way. Sarge

[Craig Allardyce]

Hi Sarge, I have one here that goes to full when you tip it upside down.
A better test is to put 12V across the gauge. Connect positive wire from your battery to the positive of your gauge (left hand side terminal looking from the front) and negative to the chassis of your gauge or dash cluster, NOT the sender wire terminal. The gauge should go to above full.

Cheers,

John

Gauge should go to empty with it powered up and earthed, not full. Powered up with high resistance or open to earth will always read full. power it up with it not earthed and give it a few taps. Sometimes they will stick in the empty position.

Hi Craig, I agree with your statements above if the sender unit is fully installed and the tank is empty, or the sender unit is removed with the sender wire still connected and the body of the sender unit is earthed.
Sarge said his gauge was out of the dash, so I assumed the sender wasn’t connected. With the sender disconnected, if you put +12v on the +ve terminal of the gauge and negative to the body/chassis of the gauge as I said initially (NOT the sender terminal), it’ll read full if everything’s ok. If you put 12v across the +ve terminal and sender terminal, it’ll read empty.

Sorry John, I meant without the sender in the circuit. I've misread what your wrote. All the gauges I have tested and repaired will read full if powered is supplied to the positive and case earthed to the cluster only. If the earth terminal on the gauge is then earthed to chassis whilst the unit is powered up it will pull down to read empty. I think this is what your saying above?

[59wagon]

Sorry John, I meant without the sender in the circuit. I've misread what your wrote. All the gauges I have tested and repaired will read full if powered is supplied to the positive and case earthed to the cluster only. If the earth terminal on the gauge is then earthed to chassis whilst the unit is powered up it will pull down to read empty. I think this is what your saying above?

No drama Craig, it’s just your last sentence I’m not sure about here. When you say “the earth terminal on the gauge” are you referring to the brown sender wire terminal (“T” in the CAD)? If so, then yes, it’ll show empty as you’re applying the voltage across 54 ohms rather than 84 ohms. To clarify, all this is with the sender disconnected.

Have you had to repair coils before? Or the needle mechanism? I haven’t actually pulled the guts apart before, just repaired broken wires. Got any tips?

Cheers,

John


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[Errol62]

Revisiting John's very thorough and I formative post here. I know my gauge is good as it reads empty when I earth sender terminal to body and full when sender terminal isolated.

My problem appears to be in the sender. At full resistance is 33ohms. At empty though it reads 7ohms. I've tried bending the arm but the rheostat appears to be at the full extent of its travel. Next idea is to clean the insides but this will mean drilling out and re riveting. Anyone tried this?

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