John Grady set up a test bench to determine what a magnetic gauge needs in order to read E when empty and to read F when full. He made a tester out of a 300 ohm 25 watt potentiometer and marked the ceramic periphery at 10, 30, 70 and 240 ohms. He found that at the 240 ohm position, the gauge needle is right on the E mark. At 200 ohms it is maybe 1/16 up from E. At 30 ohms the needle is almost but not quite on F.

Now that we have the numbers the gauge wants, let's see what numbers the sending unit gives us. With the float arm sitting on the bottom stop, the sender is 200 ohms and the needle is slightly above E. Here, on the bottom stop, the sender is at maximum resistance. Relocating the float or bending the sending unit arm won't improve accuracy, but why would you need to? Having a little reserve in the tank is a great thing to avoid running out of gas. While a perfect sending unit would have 240 ohms at E, a sending unit with 200 is very much OK.

John continued his investigation. He set up a sender installed in the tank. He found that at the bottom stop, the sender float is 1/2" off of the tank floor, providing a reserve of 1 or 2 gallons. This is OK.
At a full tank, John found the float is below the level of the fuel. When the sender arm travel is at full up, the float does not reach the top of the tank. This means the car runs maybe 50 to 100 miles before the needle actually starts dropping. This is not necessarily a bad thing, just something to be aware of so you don't think the gauge is not working.

How It Works
Magnetic Gauges

There are two coils in the gauge. Each coil creates a magnetic field when current is passed through them. The magnetic fields from the two coils fight each other to position the needle.

The coils are wired so they each get the same generator output (11 to 14 volts). If generator output increases, both coils get equally stronger and no matter where the needle is on the dial, it does not change position. For this reason, no voltage regulator is needed.

One coil is wired for full strength all the time. The second coil is weaker. It actually has a little less volts due to the drop at its ground end in the tank resistor. To compensate, the gauge is designed such that the needle rests on the F mark at float full up. If you were to ground the tank lead, the needle goes beyond F. Going beyond the E or the F is not a good thing as the needle can stick off scale.

As the float drops more ohms are added to the second coil. Current decreases, magnetic strength decreases. The full strength coil begins to win the tug of war and the needle moves toward E. At 200 ohms, float full down, the needle is just above E. Should your needle go off scale below E, look for a broken wire or intermittent contact sender. John had a gauge where the needle stuck below E, you can read about his repair here.

For those that like numbers and test results, John measured coil volts. With the input held at 13.6 volts, at F the coil is seeing 10.5 volts (a drop of 3.1 volts due to 30 ohm resistance from the sender). At E the coil sees 4.5 volts due to the 200 ohm resistance.

How It Works
Thermal Gauges

Chrysler started using thermal gauges in 1960. They are given a 5 volt supply from a voltage limiter. The movement of the needle is caused by a bimetallic strip expanding and contracting as the current changes. The current is controlled by a resistor in the circuit. For the gas gauge, the resistor is in the fuel tank sender. For the temperature gauge, the resistor is in the sender in the engine block water passage. High resistance means little current (heat) to the bimetallc strip and there is little movement of the needle. Low resistance means more heat and more needle movement. A broken wire or broken sender is infinite resistance (no current) and your gas gauge is on E or your temperature gauge is on COLD.

Just my opinion, thermal gauges seem barbaric when compared to magnetic. Thermal gauge needles move because metal expands and moves a lever whereas magnetic is elegant with its invisible and mysterious electromagnetic force. Thermal gauges will smoke and fail if supplied with 12 volts. Magnetic are made for 12. Thermal gauges respond slowly as the bimetal groans and expands. Magnetic gauges move quickly, especially the gas gauge which will dance as the fuel sloshes in the tank.

John Grady writes: On my gauge, a previous owner had broken the very fine wire off of the S coil. He had been holding the gauge with a finger over the back sheet metal — (easy to do). These gauges are extremely fragile things. When out of their gauge pod, there is no case around it for protection. Carry it wrong and you break it. Carry it by grabbing the front and back faces with no fingers in the space in between. Another thing to be careful of; any bumping of the needle can get it misaligned to magnet bar. There is a tiny rivet in there that gets loose.
A previous owner had painted the needle — nice job but he broke the wire and misaligned needle.
Excessive pulling on the S terminal or wiggling to get the harness wire off will break the same tiny wire inside. Work the harness off straight back with pliers / long nose.

John Grady's math for magnetic gauges



Those fine wires inside the gauges