The Stovebolt Site Forums The Shop Area The Electrical Bay Resistance needed ?

I've converted my electrical system to 12 volt in my 53 chevy. How many ohm resistance do I need to put in series with the original 6 volt gas gauge? Seems like I remember seeing this in a previous post, but can't find it now. Thanks in advance for your help.

Ever since I've used Runtz resistors on 6v gauges I've had zero problems. It is an electronic unit that maintains a steady 6v, unlike the old ceramic pot type that let the voltage rise and fall as the charge rate varied. For twenty bucks the correct resistance is all worked out for you.

Thanks coilover, how do I get ahold of a Runtz resistor and how will I know which one is right for me?

Funton,

1955 GMC 8 Cylinder pickups used a 30ohm wirewound resistor mounted on the back of the gauge to drop 12v to 6v for the gauge.

The information listed in the parts book is:

7.5120 Unit, Resistance
1516846 10-50 all, 65 all
incl. bracket*Fuel & Temp Gauge*12 Volt

Six cylinder GMC pickups that year were still 6 volt.

5watt 30ohm wirewound

OlBlue, thanks for responding but is the 55 GMC gauge the same as the 53 chevy in terms of its internal resistance? It seems like they would need to be the same for the 30 ohm resistor to work in the chevy. Also it seems to me that if the resistor is mounted behind the gauge it could be hooked in parallel. That would make the resistor act like a shunt. I thank you again for your info but I'm at a loss as to which way to go.

OlBlue , it seems to me that I need to know if the gas gauge is in reality a voltmeter or an ammeter. Knowing this I might be able to wire it.

The resistor, if you use one should be 30 ohms. It goes in series with the supply to the gas gauge. If you are just picking up a resistor from an electronics store then it should be 30 ohms with a 2 watt rating. In rough figures, the power used by the resistor would be V/R x V or 6/30 x 6 = 36/30 or 1.2 watts.

OlBlue in asking you these questions , I think I figured it out . The gauge is acting like an ammeter. Therefore the resistor should be hooked in parallel. Would someone thats added a resistor to a 53 chevy please respond. Please give all details, thanks.

truckernix, since I see by your bio-- your an elec eng. and the resistor should be in series does that mean that the gas gauge is a voltmeter???

The gauge works like an ohmmeter. When there is 30 ohms resistance on the sender it reads empty. When there is 0 ohms it reads full.

Ok cletis ,if it is an ammeter then I should hook the resistor in parallel right?

Cletis if the gauge is an ammeter then shouldn't I hook up the resistor in Parallel with the gauge????

I meant to say ohmmeter. I'm not sure how to hook up the resister. I'd recommend the Runtz and follow the instructions.

The resistor goes in the line (in series) with the voltage feed to the guage. It's purpose is to drop the input voltage to the guage from 12 volts down to 6 volts. The guage itself has 2 coils inside that work in opposition to one another. The "limiting" coil pulls the guage towards empty, and the "operating" coil pulls the guage towards full. 6 volts enters the guage and is connected to one end of the "limiting coil" which is connected in series with the "operating coil" and then to ground. If you applied 6 volts to this arrangement the current flows through both coils and then to ground, but the guage would read "full" since the operating coil has more windings than the limiting coil. Now the sender, which is nothing more than a 0 - 30 ohm variable resistor is connected at the juncion of the 2 coils. As the tank level goes down, the resistance coming from the sender drops (gets closer to 0 ohms. This causes more current to flow through the limiting coil and less current through the operating coil, and the needle moves towards empty.


The differential windings give the guage accurate readings despite the voltage fluctuations in the system, in fact, I've heard you can run a 6V guage on 12V with no problems at all. But I would think the resistor is a good thing just to keep the current within design specs.

Hope this helps.

From a few years ago:


Long Gas Gauge thread

The gas gauge is what's called a "balancing coil" meter. There are two electromagnets, one with a constant-strength magnet, and another with variable magnetic strength. The position of the needle depends on the strength of the magnet controlled by the tank unit. The resistor is inserted in series with the constant-strength magnet, and the voltage to the sending unit is also reduced by this resistor. Basically, you're limiting the current available to the balancing coil and the sending unit, making both of them operate on 6 volts, not 12.
Jerry

Although the resistance values aren't the same for a Stovebolt, THIS shows the basic principle.

The tank unit is a variable resistor,it allows current to flow thru the dash unit based on the resistance it is at determined by the float. Put your resistor in series AHEAD of the dash unit.

I have used this relatively inexpensive 12v to 6v fuel gauge resister for over 12 years with no problems. You attach the ring end to the "power-in" post of the fuel gauge (not to the sender post). The power-input line is attached to the other end of the reducer (the end with the nut and bolt).

Thanks to all who responded to my "is resistance needed" to convert the 6volt gas gauge to a 12volt gauge. By drawing out the circuit as described by Waldo53 it made sense. For awhile I thought I would put resistor in parallel with the gauge, this would have sent too much current to the gas tank sender. I would have sent my 53 to, you know where!!! Now I know to put my 30ohm resistor ahead of my gauge and all is well..One of these days I'll include a picture, after two years of parts everywhere its starting to look like a truck again. Again thanks for all the help.

So, that explains som of my gas gauge situation! I have a 12v system truck.

I got a new sender at the auto parts store, but it took all day to make it work even just a little bit. I got the gauge to move but it doesnt go full scale. And I had to insulate the gauge unit itself from the dash. I thought something might be not quite right.

So, if I get a resistor, will it work? Should I remove my insulation and let the gauge be grounded to the dash? How do I know which screw terminal connects to power and which one connects to the sender?

Now I'll need to get all upside down under there again....

Scot, you sender must match the gauge. If it is a stock Chevy gauge you need a 0-30 ohm sender.

is that something special? I just got a sender from the auto parts store? Or should I get a specific 0-30 sender and a resistor, too?

not "special", just that the early gauges were 30 ohms, later are 90 ohms, the later ones will never work with the earlier gauge, so yes, you need a correct 30 ohm sender for your series and the resistor, check your shop manual wiring diagram for correct connections

Bill