VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 12 volt gas gauge

Since I didn't make the mod, I am not too embarrassed to ask. The wire for the dash gauge goes from the ignition (12 volts)to a rather large ceramic type item. Then another wire comes out of that ceramic and goes to the dash gauge. Both sides of the ceramic item have 11 to 12 volts. Then of course the other wire from the gauge goes to the sending unit. Now, when you turn the ignition on, the gauge pegs on the full side. The ground goes from the sending unit to the frame. The ground was taken off and everything cleaned and it made no difference. My question is: Is the ceramic a voltage reducer? And does anyone know what I can replace it with to get the gauge working?

are you saying that the gauge is a 6 volt ? If so it may be toast. Always thought the ceramic was like a wire cooler to prevent overheating... could be wrong. to get 12 down to 6 you must use an in-line reducer.

The www.fillingstation.com sell 12 to 6 volt reducers for the fuel gauge.
Part number FS-94

Many folks swear you "must" put a voltage drop in line to keep from burning out your gauges if you convert to 12 volts. My experience has been a bit different. Years ago when I owned a 55 first series panel truck I converted it to 12 volts. All I did was change the coil & bulbs. It never occurred to me that I might need to do something with the gauges. I put over 100,000 miles on that truck after the conversion and never had a problem. Am I saying one never needs to put a drop in line to their gauges? No, I may have just been lucky but my point is that the admonition "one MUST put a drop on the line" isn't always true. Having said that the default position for a gas gauge with an open circuit is the "full" position. I don't know what vehicle you're referring to but on my '38 there is no ground wire. I have a hot wire coming from the ignition switch to the inside terminal of the fuel gauge. The wire from the outside terminal of the gauge goes to the sending unit in the fuel tank. The ground is at the tank with the sending unit grounding to the fuel tank.

Tiny,
The gauge case is also grounded through the dash. Since gas gauges operate by "comparing" the resistance of the sending unit and "compensating" with the needle in the dash unit the voltage may not be a factor. If the wiring/insulation can take the higher voltage then the gauge may work just fine. If not then the gauge will be destroyed or not operate properly.

If you have 11 volts on the gauge when it is turned on, the resistor is incorrect. A large resistor with a resistance of around 1 ohm is used to run a 6 volt blower motor on 12 a volt supply. For a gas gauge, the resistance would be quite different. On a 1951 truck you would use a 30 ohm resistor. However, your reading problem is from another cause. If you ground the sender lead at the gauge, it should read empty. If this is the case either your sender is faulty, the sender ground is faulty or the wire to the sender is broken.

I don't know how well a resistor will drop your 12 volts to 6 volts for the gas gauge. There is very little current consumption in a gas gauge (high resistance) so there will be not much voltage drop across the resistor. You will need a voltage regulator to be sure of getting the correct voltage at the gas gauge. I suppose that you could rig up a low ohmage heatsink from the input to ground and it will soak up quite a bit of voltage, also cause a quite a bit of heat. You would have to experiment by adding and removeing resistors.

I don't know what year of car you have but I just measured the resistance on my 38. From the supply pin to ground with no sender lead on it is 58 ohms. When I put my Master together, I will use somewhere between a 30 ohm and a 60 ohm resistor to drop the voltage from 12 volts. An adequate wattage rating will be 1 watt. The voltage doesn't have to be exact because it is an apposing coil scheme. If you do want an exact voltage then you will need to use a three terminal device that actually gives you a regulated voltage. There is one available for these applications that is made by Runtz.

I agree!
Just a resistor will create funny results because the resistor will not give out a stabile voltage! The follow up is a not linear working gauge. The gauge takes more when the tank is full, that lets the current go down. If the tank goes empty the voltage will go up and show a half tank just moments before it is empty. The best solution is a small electronic device like a transistor, sorry, I know only the german word for it. I try to translate. Maybe it is called a stabilized voltage regulator. pin 1 is 12V input, pin 2 is 6V output, pin3 is ground. Go to a shop for electronic parts and ask for it. It should take 1 ampere or a bit more.

I am an electronic hobbyist since I was 14, began with pre war radios, so that is not complicate for me to understand the circumstances why just a resistor will not work satisfying...

Stefangermany

germanchevy,
I am curious about something. Did you have to convert to 12 volts because of headlight laws or are you not allowed to run on 6 volts at all?