Fuel gauge

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I’m finally getting to the dash wiring on my 30 Mod B, and the wiring diagram shows the fuel gauge is powered by the feed wire to one of the ignition coils, but it shows there is a resistor at the fuel gauge.

1. Is the resistor built into the gauge? I have a dash from a 29 and there isn’t an external resistor on it either.

2. If it’s not internal does anybody know what the resistance is supposed to be? Jim

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Hi Jim

On my ’29 Roadster, there is an external resistor. Actually i think there are two in parallel. i’ll have to try to take a photo to get the color bands on the resistor in an image.

Greg Long

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Thanks Greg. Jim

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G’day Sir James! The resistor is a 300 ohm wired into the circuit externally.. Any resistor from an electronic or radio supplies shop will do. Only a couple of bucks. I used one on my Nagel Fuel Gauge suscessfully. PASB 71-6 Page 4 has a short discourse on it.

Cheers

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Jak.

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Thanks Jak,

I got a new sending unit from Atwater Kent so I hope it work with the mentioned resistance. Jim

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When you get a 300 ohm resistor, please post the color-band code on the resistor, a photo or just name the colors if you can. I have a big selection of resistors, and want to try to use one with a spare fuel gauge and sender. I know I could just start trying various resistors with an ohm-meter, but I have what looks like hundreds of them and would like to narrow down the search.

Greg Long

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Resistor color code chart.

Using this you can decode almost any resistor you have in your pile of parts.

http://https://www.sysrecon.com/wp-content/uploads/2014/07/ResistorColorChart.png

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Here is the address that will get you to the website.

http://www.sysrecon.com/wp-content/uploads/2014/07/ResistorColorChart.png

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Greg,

The 300 Ohm resistor is banded red, brown, black, orange. The blue body color is just a brand identifier I think. It didn’t help.

While I was at the electronics store I got a package of the 300 Ohm, 1pkg of 51 Ohm and a variable resistor 0-500 Ohm. I figured I could dial in the gauge with the variable resistor. I need somebody that knows Mr Ohm better than I do.

The gauge has 107 ohms across the terminals, 106 Ohms, gauge + to ground and 213 Ohms through the – term of the gauge to ground.

The sending unit is from Atwater-Kent and the readings are 5.5 Ohms between the terminals with the float raised to the full level and 315.0 at the empty position.

I have been trying with the jumper leads connected to the VR then the 300 or 51 Ohm resistors. Does it make any difference which resistor is positioned where? I thought it was just the total resistance. Jim

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The only thing I recall about resistors in a circuit is that if two resistors are in series, the resistance of each is added together for total resistance in the line.

If the resistors are in parallel, the resistance is sort of an average of the two or more resistors in that circuit. I am not sure if the resulting ‘average’ is only if the resistors are all the same value.. Seems to me that if one of the resistors is of a lower value, it would have more current flow, and slant the ‘average’..

I guess i should look up those bits of rarely used knowledge and refresh my memory.

Greg Long

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An easy explanation of how to calculate series and parallel resistance.

http://www.wikihow.com/Calculate-Series-and-Parallel-Resistance

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