The Fuel Gauge Thread... readings?

[SolarPoweredPickles]

I've poked around at the countless fuel gauge threads but didn't see this in particular, maybe someone knows what's up. I think I've got the original fuel gauge in my 65' Deluxe, a brand new stock fuel tank from WW, and a new fuel sender unit also from WW. This one:

http://www.wolfsburgwest.com/cart/DetailsList.cfm?ID=271919051b

My gauge works, and looks like it's hooked up right. When the tank is fuel the gauge reads just a tad over 1/2 on the gauge. When it nears a half or 1/4 tank, the needle gets pretty jumpy. Grounds look ok... wiring seems ok?

[Kharon8]

Fuel gauge is really a voltmeter: It measures the voltage left after the variable resistor in fuel tank.

So items to check, about in order:
1) 6/12V to the gauge & fuel sender
2) wiring resistance between these two pieces (ohmmeter or volt meter for signal from fuel level sender, should be almost nominal voltage if fuel tank is full. )
3) internal resistance of the gauge itself. Needs a disassembly of the cover to fix, but is easy to measure resistance (ohms) by comparing to a working gauge.

Unfortunately I couldn't find the measurements I made from a friends '66 bus, having also fuel gauge issues. His were in the sender unit: The sliding wires were so dirty they made almost no contact.
(Doesn't make sense? See the pic: http://kharon.suomiforum.com/www/ghia3/IMG_0788.jpg)

I had the case 3) in bay window fuel gauge: Wire under nut and nut was rusted, therefore additional resistance -> never full reading on gauge.

Cleaning and brushing and work and it got much better, showing almost full when tank was full, but more important, it wasn't showing empty until I had about 10 liters left.

Instead of showing empty at half tank, essentially being useless.
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Kharon -- '62 typ1, '63 typ14, '61 typ21, '65 typ34, '74 412, '75 typ26
FVWA.fi support group.

[busdaddy]

^Yeah, what he said^
To add to that new senders suck, even the supposedly good ones. The floats often sink and the contacts oxidise.
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[olliehank47]

The gas gauge is a balance coil type where the needle is deflected between two electromagnets. Take a look at the simplified diagram below:

The magnet on the left pulls the needle to the E position, and the magnet on the right pulls it to the Full position.

Current takes the path of least resistance from battery to ground. The right magnet is connected to the fuel tank sending unit which has variable ground resistance--low resistance when tank is full and high resistance when tank is empty. On a full tank (with low resistance), current flows through "T" connection in the wire to the right magnet because that is the path of least resistance to ground. This increases the strength of the right side magnet (and also decreases the strength of the left side magnet as less current flows through it) and pulls the needle to the Full position.

As the fuel level drops, resistance increases in the ground connection to the right magnet and current begins to takes an "easier" path to ground through the left side magnet, pulling the needle to the E position. When the tank is empty (or the sender wire is disconnected or broken), the resistance is high enough that nearly all current flows through the left magnet and the needle is pinned to the E.

In your case, I suspect the sender is not dropping to a low enough resistance reading to fully move the needle to Full. Further, the jumpiness of the needle once the tank is less than half full, may indicate poor connections within the sender itself or a broken connection inside the fuel gauge.

I have had lots of problems with the repro senders. They will work well outside the tank where you can turn them upside down while connected to the gauge and the gauge will move smoothly through its arc from Empty to Full. Once installed, however, they tend to not read completely Full when the tank is filled, and they eventually get stuck or otherwise fail to register at all after several months. At this point, I would suspect your tank sender. If you can get your hands on a rheostat (potentiometer) that ranges from 0-100 ohms, you can connect it to your gauge's sender connection and test the movement of the needle as you increase resistance from 0 to the max. If there is jumpiness, then the gauge has an issue.

[SolarPoweredPickles]

Thanks for all the detailed tips and instruction. I'll start by taking the gauge off and apart and clean it thoroughly. Hopefully that fixes it. Too bad about the aftermarket senders, it's not easy to get behind there and replace them.

[Split 66']

This thread seems more like it should be titled: "The Definitive Fuel Gauge Thread". Great information in here, I really appreciate the explanation olliehank47, that makes more sense than any other I've heard.

I've been driving around with a fluttering gauge for decades, I had memorized all the various fill level flutter-patterns. Stoked to be able to finally understand the mystery well enough to fix the problem. Thanks guys.
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[Help Help]

Ollie or anyone else for that matter, I need to replace my sender because my 18 month old sensor gave up the ghost and read -0- when the tank was full. Which sender to buy from whom to avoid the same issue another 18 months from now??

Or, how to rework my 18 month old sender to make it work right? I did open it up and it began working as designed but I may have "adjusted" it a little too vigorously to be useful going forward.

[olliehank47]