The Stovebolt Site Forums The Shop Area The Electrical Bay 6-12 volt fuel guage/sender theory ??

RE: using a dropping resistor for 6 volt fuel sender on a 12 volt system......

I kinda thought that the fuel guage was a current operated device. If I put a resistor in the circuit then the current is increased and the guage reading goes out in the weeds. Yes/No? It seems like the traditional solution for the fuel sender in a 12 volt upgrade is to simply put a resistor in the line, but the theory kind 'scapes me. Soooooo.....does using a resistor actually work or just save the fuel sender/guage from being cooked? Thanks for all your thoughts

47 chev,

The 6 volt gauge and sending unit are calibrated to operate together. The sending unit is a variable path to ground that changes with fuel level and influences where the needle sits. This combination of electrical values needs to stay constant for the system to read correctly.

The only easy way to accomplish this is to reduce the voltage to the fuel gauge with a small voltage reducer (resistor!). This provides the gauge and sender with a voltage that won't burn them up and one that makes the gauge read properly at any fuel level.

I think you have already figured this out!

Stuart

Thanks for the call Stuart. So....the guage must be a voltage sensitive device and not current sensitive? I guess if a resistor works then I don't have to understand it. Any ideas about resistor values?

cheers

47 chev,

If it were me, which it isn't, I would skip the higher math and buy a nice little volt-a-drop from one of the friendly suppliers...like the outfit at the top of this page, American Classic.

Stuart

Your sending unit does vary the amount of current sent to the gauge. Zero ohms or there abouts is empty & 30 ohms is full. There are many here who advise that you MUST put a resistor in line to the gas gauge. I am not an expert when it comes to electricity but when I converted my 55.1 to 12 volts I didn't put a resistor in the circuit & never had a problem. I may have just been lucky but I put over 100K on the truck after the conversion and the gas gauge worked OK the whole time. I can't recommend not putting a resistor in the circuit. I'm just passing on my personal experience.

12 volts probably won't toast a 6 volt device as the 6V circuits are heavier. 12 volts will push higher current through a given resistor (the float assembly)which means more currant at the gauge and higher fuel levels indicated at any given tank level.
A constant volt regulator, such as a Runtz, will keep your gauge as accurate as it was on 6V.

The fuel gauges in our old trucks use two coils (electromagnets) to position the needle.

One coil (called the limiting coil) pulls the needle to the left or empty position. The other coil (called the operating coil) is positioned at 90 degrees to the limiting coil and pulls the needle towards the full side.

Current from the ignition switch, enters the limiting coil when the switch is in the 'on' position. The other end of the limiting coil is connected to both the operating coil and the tank sending unit. The other end of the operating coil is connected to ground.

If the gas tank is empty, then the resistance of the sending unit is near zero ohms and all of the current from the limiting coil goes to ground through the sending unit. With all of the current flowing through the limiting (empty side) coil then needle is pulled to that side.

If the gas tank is full, then the sending unit has about 30 ohms of resistance and most of the current from the limiting coil will pass through the control coil to ground. This current will cause the needle to be pulled away from the limiting coil (empty side) and towards the operating coil (full side). The operating coil has more windings than the limiting coil so it takes less current to generate enough magnetic force to overcome the limiting coil and pull the needle to the full side.

Since the needle is controlled by the currents and resulting magnetism, the fuel gauge operates on current and is independent of the voltage supplied to it. If it wasn't, then the fuel level would 'change' every time the ammeter indicated a discharge or the system voltage changed depending on the speed of the generator and the charge in the battery.

I also have run the fuel gauge in my '49 on 12 volts since the mid '70's without problems.

Hey 'OlBlue....thanks for the insight into the gas guage. The limiting coil and operating coil description was really great.
If the voltage is doubled 6V to 12V then the currents must be doubled. Is this extra current what fries the senders of those with fried senders?
cheers

OlBlue,

If the gauge isn't sensitive to voltage would it be possible to connect it to 100 volts DC and have it function properly? I wonder if the wire that is wound around the coils has dielectric limitations.

Stuart

I have a couple of fuel gauges for the '55-'59 series GMC trucks. One is early '55.2 and is 6v, the other is later and is 12v. I measured the the resistance of the limiting and control coils in each and did a few calculations.

Rl = Resistance of the limiting coil (empty side)
Rc = Resistance of the control coil (full side)
Rs = Resistance of the sender = 0 - 30 ohms


6 volt gauge: Rl = 53 ohms, Rc = 30 ohms
12 volt gauge: Rl = 109 ohms, Rc = 30 ohms

The equivalent resistance of the sender and the control coil is the same for 12v and 6v. At the empty position the equivalent resistance is 0 ohms because the the resistance of the sender is 0 ohms. At the full position, we have two equal 30 ohm resistances (Rc and Rs) in parallel and the equivalent resistance is 15 ohms. We'll use Re for the equivalent resistance.


Maximum Current (empty gauge) (Volts/Ohms):
Imax = Voltage * (Rl + Re)

6 volt Imax = 6/(53 + 0) = 0.113 amps
12 volt Imax = 12/(109 + 0) = 0.110 amps

6 volt gauge on 12V = 12/(53 + 0) = .226 amps

Power in watts: P = I*I*R

6 volt Pmax = (0.113)*(0.113)*(53) = .667 watt
12 volt Pmax = (0.110)*(0.110)*(109) = 1.3189 watt

6 volt gauge on 12v
Pmax = (0.226)*(0.226)*(53) = 2.71 watts

I made this calculation years ago and based on the small amount of power (read heat) being generated decided that it would be safe to run the 6 volt gauge on 12 volts. I have run the gauge in my '49 truck many years on 12 volts without problems.

I suspect the leading cause of gauge failure is getting the source voltage onto either the sending unit wire or the control coil terminal on the back of the gauge. Most gauges have a red tag on the sending unit terminal warning against this.

Sheesh, can't argue with Mr. Ohm. It would be interesting to dissect a after market gauge that is marketed as being a 12 volt replacement for the 6 volt model. I wonder what the construction differences would be? :confused:

Stuart

On the more practical side, GMC used all 6 volt fuel gauges in the '55.2 trucks 6 volt or 12 volt. They put an external resistor of 30 ohms between the ignition switch and the fuel gauge on the 12 volt models.

The statement about the fuel gauge being voltage independent is true ONLY for a limited range. The power (heat) created in the gauge goes up with the square of the voltage. i.e. double the voltage to the gauge and it generates 4 times as much heat.

If you increase the voltage enough, the gauge will likely fail by a conductor or connection burning open. In other words, you'll let all the smoke out and then it doesn't work any more. (Thanks Stuart)

this was a good tread. i learned a lot. thanks everyone.

I learned, don't let the smoke out. It's clear to me that smoke, makes electrical stuff work.

Hey Atomarc......let me know when you're gonna put 100 volts on your gas guage ! I'll buy tickets for the show.

cheers and thanks all for all the great info.

Does the temp gauge work the same way?

MJVAUGHANS,

If it is a electric temperature gauge it works the same general way. The sending unit that is plugged into the water jacket supplies a variable ground to the gauge dependent on coolant temperature. Not ALL temp gauges are electric though.

Stuart

Kudos to OlBlue for his very good explanation of the workings of the fuel gauge. One question comes to mind that I have not seen addressed. Lineraity? Electromagnets produce their flux as a product of the current passing through them, right? The gauge limiting coil has, at any fuel level, all the current passing through it It (current total). The operating coil is in parallel with the sending unit. As the sending unit is a variable resistor the amount of current passing through the operating coil will vary. With twice as much current in the limiting coil but not twice as much current in the operating coil at all times does linearity not suffer?

beltfed,

I'm guessing that there is enough iron in the coil cores to keep them operating in the linear portion of the (close your eyes non-techies) hysteresis curve.

The magnetic flux (read magnetic force) in the coil is dependent on the current and the number of wire turns. I'm presuming that the coils have differing numbers of turns to deal with the differing currents flowing in them.

The most accurate thing in my '49 was the tank sending unit. When I could hear it start to rattle, I was down to the last gallon or so!

Interesting! I would test the linearity of my six volt gas gauge on 12 volts but alas, it has be torn apart and now has a 5.5 volt regulator sitting inside the housing. Maybe someone like Atomarc has a 6 volt fuel gauge sittting around that he can test. It would give the guys a handle on whats in the tank versus what the gauge reads.

beltfed,

I wouldn't know linearity if it came up and hit me smack dab in the gib's!

Stuart

I'll see if I can borrow one. If I can I'll bench test it and report back with the results. Bob