VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 Gas gauge problem after 12 volt upgrade

I’m completely rewiring my 50 Chevy with 12 volt wiring harness, and installed an alternator. Everthing works great.....except the gas gauge. I looking for some assistance. I installed a voltage reducer I purchased from Chevs of the 40’s, the type with the ground pigtail. I attached the reducer to the power side of the fuel gauge, and grounded the pigtail. Here’s the issue. When I turn on the ignition, WITHOUT grounding the sending unit wire the gauge goes to full. If I ground the sending unit wire WITH the ignition on, the gauge goes to E, or very close to empty. The fuel gauge is the only item (at this point) requiring voltage reduction. This doesn’t seem right. My experience with gas gauge troubleshooting, always required grounding the sending unit wire to achieve a full gauge reading. Is this normal with this type of voltage reducer? Anyone else experience this problem? Thanks in advance for your insight.
John

I also converted my 54 to 12 volts,I went out to the garage and checked my gas gauge hookup,no voltage reducer,the stock gauge reads amps not volts so a reducer is not needed,make sure the sending unit body is grounded to the frame and the gauge should work,mine works perfectly.

I also found that a fuel gauge reducer was unnecessary when I converted to 12V. Then a wiring short took out the voltage regulator in my alternator, which fried the fuel gauge, heater resistor and Pertronix ignition module. I rewired the car and bought a 12V fuel gauge and sender (just in case the sender was also damaged) from Ekler’s. The old sender was still ok, but the new gauge didn’t read properly with it. I added 18 ohms worth of resistors in-line with the sender and it corrected the gauge. I used a multimeter and my best guess of how much gas was it the tank to determine how much more resistance I need to register a full tank. I was close, but I think 20 ohms would have been better. In the end, I should have just bought the 6V replacement gauge. Pulling the gauge cluster is a PITA.

Thanks for the info. I’ll try removing the Runtz and see what happens. I’m not sure how much gas is in the tank, but it’s not full. I appreciate the replies. I’ll post my results. I agree about removing the gauge cluster. I’ve done it twice in the last week.

This is normal operation for a GM gas gauge prior to 1964 or so. The sender is 0-30 ohms, with 0 being empty. Ground the wire for empty.

When you ground the wire the gauge needs to go all the way to "e". What you see here is the lowest the gauge can ever go, no matter how much gas.. you run out of.

If it will not go all the way to the "e", you need to troubleshoot the wiring and make sure there is no extra resistance in the wire going from the tank to the gauge, that you are touching the wire to a good ground (ground your tank while you are at it), and that the gauge itself in the dash has a good ground. If it still won't go all the way to "e", you have a bad or miscalibrated gauge. If it does go to "e", you are ready to go.

I would not run the gauge itself on 12v. It is not made for that, and will probably get damaged sooner or later. Real Chevrolet gas gauges changed with the 6v to 12v change in 55, but the 0-30 ohm sending units remained the same electrically.



The reducer doesn't really have much to do with it. It is working more or less like it should. If the reducer feeds the gauge something like 6 or 7 volts, it is fine.

HOW MUCH GAS IS IN THE TANK ?

If the gauge reads empty with key on, you have a grounded wire between the dash gauge and the tank unit.
In everything I have ever read, in doing the 6 volt to 12 volt conversion, a resistor was needed at the dash unit fuel gauge.
If you burn out your dash unit without the resistor, I have a NOS unit you could purchase, if interested.

That’s what I found as well. And I bought the resistor because of it. However, my personal experience found that the resistor was unnecessary. In fact, it wouldn’t work with the resistor in-line. I’m not arguing that the traditional knowledge on this subject is wrong. It just wasn't right in my case.

The gauge is a balanced coil design, and that makes it not really sensitive to voltage. This is so the level will not vary if the engine is off and the supply voltage is lower.

The trouble is at 12v, the gauge will draw double the current it is designed for. This works out to a little more than double the power dissipated as heat in the gauge. It might last a while like that, but I wouldn't bet on it.