1946 Fuel gauge & sender

[Jyrki]

Now that I dropped the tank from my 1946 Roadmaster, might as well ask about the fuel sender. When I bought my car as a project in 1997, I recall the seller told me the tank that followed the car wasn't original. He didn't know what it was, other than it came from an American car, maybe Cadillac. I measured the ohms of the sender to be 2...39 ohms, that is, nominally something like 0 to 40 ohms. It has a cork float, and the tank has a drain plug, so it must be pretty old. Is this the ohm range that the Buick gauge is designed for?

I had this sender working with the original gas gauge, until the gauge broke (the needle dropped off the shaft). But even when it was working, the needle kept swinging back and forth without a steady reading. I thought it was due to the voltage regulator (from 12 to 6 volts) that a friend of mine made for me. Now, I recently met a Swedish guy at a show who told me he's running full 12 volts to the 6-volt gas gauge, and it works fine. Any thoughts or experience?

[pmuller9]

Jyrki

As you know the fuel gauge operates by using a heating element.
This means the gauge movement is a function of wattage.
If you apply 12 volts to a 6 volt gauge you double the voltage and also double the current going to the element. Since wattage is voltage time’s amperage, the wattage in this case is 4 times the original and the gauge element goes up in smoke.

What the guy at the show forgot to mention is he added a resistor in series (Between the gauge and the tank) that had the same resistance as the gauge (when it is hot at full swing) so it drops 7 volts leaving 7 volts for the gauge. I used 7 because actual battery voltages when the engine is running are 7+ for the 6 volt and 14+ for the 12 volt batteries.

To find the resistance of the gauge when it is at full swing, apply 6 volts to the gauge with an amp meter in series to find the current. Then using ohm’s law divide the actual voltage measured with your volt meter by the current you just measured.
Note: You will need to use the 10 amp scale on you multimeter because the 300ma scale has too much resistance and will reduce the current that you are trying to measure.

The wattage the new resistor will have to dissipate will be the current you measured time’s the voltage drop across the resistor (7 volts). The actual wattage resistor you use is double that value or more so it runs reasonably cool.

Fixed resistors come in specific values so just use the next value up from your calculated value.

Let me know what you find or any problems you run into and we will find a solution.

Paul

[Straight80]

Jyrki

You can use a gauge from a 1953 Buick Super or Roadmaster, take the gauge itself out of the clock and put the gauge itself in your clock. You can use your card from the 47 when you put it back together. I got a water temperature gauge from a 1953 Buick, I took my gauge apart and used my original clock. I had my wife paint the indicator needle silver to match the rest of the gauges. I say a 53 because they are 12 volts. If you don't understand what I'm saying, please let me know. I hope this helps you, talk to you again soon.
Anthony
aka
Straight 80

[Straight80]

Paul,

outstanding suggestion!

Jyrki

As you know the fuel gauge operates by using a heating element.
This means the gauge movement is a function of wattage.
If you apply 12 volts to a 6 volt gauge you double the voltage and also double the current going to the element. Since wattage is voltage time’s amperage, the wattage in this case is 4 times the original and the gauge element goes up in smoke.

What the guy at the show forgot to mention is he added a resistor in series (Between the gauge and the tank) that had the same resistance as the gauge (when it is hot at full swing) so it drops 7 volts leaving 7 volts for the gauge. I used 7 because actual battery voltages when the engine is running are 7+ for the 6 volt and 14+ for the 12 volt batteries.

To find the resistance of the gauge when it is at full swing, apply 6 volts to the gauge with an amp meter in series to find the current. Then using ohm’s law divide the actual voltage measured with your volt meter by the current you just measured.
Note: You will need to use the 10 amp scale on you multimeter because the 300ma scale has too much resistance and will reduce the current that you are trying to measure.

The wattage the new resistor will have to dissipate will be the current you measured time’s the voltage drop across the resistor (7 volts). The actual wattage resistor you use is double that value or more so it runs reasonably cool.

Fixed resistors come in specific values so just use the next value up from your calculated value.

Let me know what you find or any problems you run into and we will find a solution.

Paul

[Jyrki]

Paul, believe or not, I had never thought about how a gas gauge works! Thanks for the great info. So there's a bi-metal string that gets heated up according to current flowing thru it? Makes perfect sense.

And you need to measure the current while the element is hot, because it's different when it's cool. And you need a "power" resistor because it has to dissipate some heat.

To me it seems it doesn't matter if you put the ballast resistor in front of the 12V supply to the gauge, or between the gauge and sender. Either way, it will be in series.

Anthony, I know you mean, but it's a lot more work compared to above

[pmuller9]

Paul, believe or not, I had never thought about how a gas gauge works! Thanks for the great info. So there's a bi-metal string that gets heated up according to current flowing thru it? Makes perfect sense.

And you need to measure the current while the element is hot, because it's different when it's cool. And you need a "power" resistor because it has to dissipate some heat.

To me it seems it doesn't matter if you put the ballast resistor in front of the 12V supply to the gauge, or between the gauge and sender. Either way, it will be in series.

Anthony, I know you mean, but it's a lot more work compared to above

Jyrki

Yes you got it!
The resistance of a wire increases as its temperature increases.
The ballast resistor can indeed go anywhere in the line, before or after the gauge.
The best part of adding your own resistor is, your can adjust the value so the fuel gauge needle sits right at “F” when the tank is full.
Also if you are willing to attach a wire to the open end (The end not grounded) of the sender resistor you can bias the sender so the gauge needle sits right at “E" when the tank is empty.

Paul