The Stovebolt Site Forums The Shop Area The Electrical Bay gas guage

i just added a voltage reducer leading to my gas guage. as with the one i added to the heater circuit (an earlier post), this one also registers 12 volts on both sides of the reducer. i thought it should read 6 volts on the gas guage side. also the guage continues reading full, with or without the reducer, even though there's only a couple gallons of gas. any guidance on these two issues would be appreciated. thanks so much.

The resistor that drops the voltage to the heater will act like a dead short on the way to the gas gauge. You'll need a totally different resistor to handle the tiny current draw of the gauge, with a lot more resistance that the one used for the heater motor. If you don't have a pretty extensive knowledge of electronics, you're going to be spinning your wheels trying to make the gas gauge work. I'd suggest a voltage divider circuit using a potentiometer that applies just enough voltage to make the gauge read full with the float at the top of its travel, and empty as it reaches the bottom.
Jerry

Did you add a resistor to the voltage feed to the gauge or a true voltage regulator/reducer aka transistor. If you added a resistor and didn't complete the circuit then both ends of the resistor will read source voltage as the circuit is not complete as the gauge acts like a series resistor to the one you added. Have you checked the gauge with an ohmmeter to make sure it's not open?

Jerry, does voltage really affect a gas gauge? My '36 Chevy pickup is restored now. But when I was in college 40 years ago I ran it as a Chevy V8 powered jalopy with a 12 volt system. The gas gauge worked equally well on 6 or 12 volts. An old friend told me that the gauge has 2 coils that pull the needle in opposite directions and that system works independently of voltage. I never looked into his explanation but the voltage seemed to make no difference.

Your old friend and your own experience pretty much tells the story. The guage will work equally well regardless of the voltage. I know of at least one fellow bolter that has been running his '49 on 12V since the 70's with no problems.

To troubleshoot your guage misread problem, take the sender wire off at the tank and let it dangle in the air, turn the key on, the guage should read full. Now with the key still on, ground that sender wire to a known good ground, the guage should swing to empty. If this test proves the guage to be good, the problem with your guage reading full all the time is either an open sending unit or a bad ground at the sender tank flange. Yoou may have to run a seperate ground lead from the sender flange to a good ground.

Waldo53, you have made a good point. I think I'll test it in the next several days as I have the same kind of gas gauge and lots of electronic test equipment. I also have detailed schmatics/descriptions of the various instruments placed in vehicles from the 30's on. I'm a little curious about the linearity. Learning something new is a good thing.

It depends on the design of the gauge. The 2-coil system your friend is talking about is called a "balancing coil" gauge, and it will probably work OK on a wide range of voltage. One coil has a constant current flow, and the other one is variable according to the position of the tank float. The differential magnetism on the needle moves it from empty to full based on the variable magnetic signal. Late 50's and newer Ford systems used a 12-volt to 5-volt regulator to power the gas and temp gauges, and used an idiot light for oil pressure. The gauges could be made more sensitive by wiring them to work on the reduced voltage. Not sure which type of gauge system the stovebolt trucks used.
Jerry

Good point. The Stovebolts used a "balancing coil" type fuel guage (a "limiting" coil and an "operating" coil).