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Ok I have been working diligently on my fuel gauge. It only travels to a little over 1/2 full even if the tank is full.

I had to rebuild the gauge and physically ground the sender and the gauge to get that much out of it.

Then I turned my attention to the sender. I removed it to see if it need to be adjusted....not. In the mean time I broke it. So I took it apart and repaired it.

Still no improvement. I have 2 questions.

1. The gauge does move up slightly when I rev the engine. Why is this? I have a new sender and new wiring harness. The gauge moves to extremes if I close or open the circuit. But in the truck it only moves a small amount. I suppose I should be happy it works at all.

2. I was amazed when I took the sender apart that there is an open connection in there. The rheostat (I think that is what it is) is in a small unsealed case. Just a piece of copper moving a long a coil. I am sure gas gets in there. How does it not ignite? I realize this is a very low voltage and there is theoretically no spark under the fluid level and when there is constant contact but what about when the tank is almost empty. This just puzzles me. Any of you engineers or chemists out there care to enlighten me as to how these don't ignite?

Sounds like the sending unit might be bad. You stated that the gauge moves to the extremes if you open or close the circuit. That tells you that the gauge is working as expected. And if you're closing or opening the circuit by grounding and ungrounding the wire that connects to the sending unit, then you've verified the entire wiring of the circuit.

The rheostat on the sending unit is a variable resistor (the amount of resistance is controlled by the location of the wiper arm on the rheostat), and the total resistance of the circuit determines the location of the gauge needle. If the needle never goes above 1/2, there are several possibilities.

To troubleshoot, remove the sending unit from the gas tank, connect its flange to ground (either clamp it to ground or wire it to ground) and then operate the sending unit manually. If the gauge goes to full when the float is all the way up and goes to empty when the float is all the way down, then the electrical part of the sending unit is working properly. If it doesn't, replace it.

If the electrical part is working correctly, there's two possibilities that I can think of. There's extra resistance in the circuit somewhere, or the float is bad and never rises to the top. The former is possible. The latter happens frequently. Floats get soaked with fuel (if they're made of absorbent material like cork) or they get a pinhole opening that takes in fuel and makes the float heavier so that it floats below the surface instead of on it.

Too much resistance can be caused by bad wires or bad connections. Sometimes fuel gauges don't get a good ground through the tank and need to have a ground wire connected directly to a flange mounting screw.

BaldEagle,

I did just what you describe. Removed the sender and worked it manually to see if the gauge would follow. I got the same gauge movement as in the tank. I tried to move it past the stop to see if that was the issue. That's when I derailed the resistor, I took it apart and everything inside looked right. the point moves along the entire coil.

Right now I get about 2/3 of the movement I would expect. From just above E to about 5/8 a tank. The tank is very close to full.

It seems like it should work the sender is new. Why would the gauge move when the engine is reved up? That doesn't make sense to me.

I did add a ground wire from the sender flange to the cab (which is grounded to the frame). I also cleaned off the gauge contact to the gauge body and the body to the dash.

I just cna't figure it. Like I can't figure how the resistor doesn't ignite the fuel.

Rich

Sounds like the sender has the wrong resistance for the gauge. The gauge should read empty when it's grounded and full when the total resistance to ground meets the specs for the gauge. It sounds like there's not enough resistance to drive the gauge to the full mark.

Here's a wiring diagram for a 1938 Chevy that might help explain how the system works. This Army manual has a good explanation of the way the tank unit works.

Regarding the movement, the gauge could move from vibration, especially when you're revving the engine. I guess it depends on what you mean by movement.

The movement of the wiper on the resistor doesn't ignite the fuel because it's so close to ground that there's very little current or voltage inside the tank.

Thanks Paul
I believe the sender is 30 ohm. The gauge is original.
I think I will live with it for now.
Rich

As to why the fuel doesn't ignite, it is quite simple. There is not an adequate air/fuel ratio to support combustion. I once replaced the fuel pump/sender unit in an 02 Chevy pickup in which the sending unit had shorted and melted, but without enough oxygen in the tank, no ignition of the fuel was possible. Kinda like your truck won't start if it's flooded.

I too was amazed that they don't cause an explosion. The explanation of not enough oxygen is one thing but there are occasions where trucks sit with little gas and people try to start them. Obviously it works or there would have been many incidents over the years.

The Gauge: If you can't get full movement while it is mounted in the truck with open and short circuits on the sender connection, then you have a connection problem. The gauge needs to be well grounded in the dash.

Is your setup still 6V? or have you changed to 12v. I haven't had good experiences with the new 12v guages.

I have a repro 12V guage in my truck, in fact I'm on my second one as the first one didn't read true either. I have a 30 Ohm sender which works correctly, but neither guage ever read to full. I cured this problem by bending the needle on the guage, it now reads full correctly, and also empty correctly. The bend in the needle is right down by the spindle so can't be seen when the face is fitted to the guage cluster.

Stuart.