Fuel Gauge Design

[JohnM15]

I'm an engineer so I got to know how things work, including the fuel gauge. I understand how a simple potentiometer with float and current meter would work to measure fuel level but the Stude has two leads coming from the sensor (plus ground) and the manual does not explain how this operates. My guess is that this is some kind of differential system where the current through one lead increases and the other decreases when fuel lowers. I'm also guessing that this design is intended to counter ambient temperature influences on the gauge.

Anybody know how this system works?

[Jeff_H]

If you do a search, I have had posts on how the fuel (and electric temp gauges) work.

There are 2 coils in the gauge. Each has one end connected to the IGN terminal. One coil is grounded on the other end inside the gauge. The other coil has its other end coming out on the Sender terminal. The sender wire goes from there to the sender at the tank. The sender has a variable resistance in it and the other end of that resistor is grounded.

The needle position is determined by the relative currents through each of the coils inside the gauge. The current in the coil connected from IGN to ground is essentially constant and the current in the coil connected from the IGN to the sender varies with the sender resistance (what varies with the float level). The IGN/Gnd coil tends to pull the needle to F and the IGN/Sender coil pulls it toward E.

This setup makes the gauge not sensitive to the battery voltage changing since current in BOTH coils will go up and down the same amount as the voltage is varying. Its the difference in current between the 2 coils that affects the needle.

If you can lay hands on a old MOTORs manual from the 40s or 50s, there is a good generic description of how these work in there.

Loss of ground either in the dash or a broken gauge seems to be a common problem. Also, poor grounds of the sender. Its worth it in my opinon to run a separate ground wire from the gauge housing to the tank sender to get rid of problems due to poor connections at the sender or the tank to frame mounts, etc.

[Guest]

Is this in an M15 truck? I haven't seen a 2-wire sending unit, which seems to be what you are referring to, as the postwar cars just have the single wire going up to the gauge, which Jeff has so carefully explained above.

[StudeRich]

How it works?

/Cut/ the Stude has two leads coming from the sensor (plus ground) and the manual does not explain how this operates./Cut/

I understand your "Engineer" curiosity as to the "How it works" here, but the Manual is not supposed to explain how anything works, unless the knowledge is required to repair the part. A Fuel Gauge or a Fuel Sender is not considered "repairable" so a Tech. would have no need of that info, it's a R & R Item.
Of course Today, there ARE people who have to repair them due to the unavailability of Non-Studebaker Parts, (fortunately we buy NOS) they have to look elsewhere, or figure it out.

[JohnM15]

Geoff Fors, Yes it is a M15A-20, and yes it is a two wire unit. The manual even show the two wires and has a chart showing what the gauge should read when grounding or disconnecting either of the two wires depending on how much fuel is in the tank.

StudeRich, I understand that the manuals are not designed to explain the "design" but sometimes a new design that is introduced the first time is sometimes explained, just was hoping that someone could explain or had text to explain...

Thanks, all.

[Jeff_H]

Sorry did not know your gauge was a 2 wire sender. What I described is for the stewart warner 1 wire ones.

I've not messed with a 2 wire unit so....

I looked in my 1949 edition Motor's Auto Repair Manual and there are several pages devoted to different types of gas gauges with some diagrams. Not much tech info on how they work but there is troubleshooting info. There is a sort of schematic for the 2 wire ones. The book says those are Autolite. Here is my "take" on how it works from what I can glean studying it. Appears that the 2 sender wires go to opposite ends of the resistor in the tank sender. The float arm moves a grounded wiper/tap on that resistor. Each sender wire goes to one end of a heater coil inside the dash gauge. The other end of each heater coil is going to the IGN power. Depending on there the float arm has has grounded the "center" of the sender resistor, each of those heater coils will get different amounts of current. So, the temperature of each coil will vary as the float moves. So, a full tank would have say coil #1 hotter than coil #2 and a empty tank would have coil #1 colder than coil #2. The needle in the gauge is attached to a bi-metallic strip that those coils are wrapped on. So, as the 2 heater coils temperatures vary they heat the ends of the strip differently and cause it to bend one way or the other. The attached needle then moves.

Just from the diagrams of this 2 wire autolite compared to the 1 wire stewart warner, I can imagine the autolite is more expensive to both make and install. It sure looks more complicated and more wires in the harness will add to cost and installation labor. I don't know if it is "better" or not from a performance point of view.

The reason gauges are not the same as a D'Arsonval meter movement like you would find in a VOM or other analog meter used for electrical and electronic testing is those are too sensitive to vibration, temperature, and changes in the battery voltage. The gauges all seem to involve either thermal heater coils and bimetallic strips or ballanced magnetic coils with springs, etc. All with the aim to make them more immune from the harsh conditions. The thermal elements add damping so they don't bounce about with the sloshing gas too.