Gas gauge calibration unit

I have read many posts on this forum about fuel gauge problems after replacing the tank fuel sender in our A-Bodies. Some problems are related to mechanical issues of float travel, the instrument voltage regulator, and sender ground. However many have found even after fixing those, the gauge readings for empty and full still persist as problems.

The original system is simple electrical circuit, consisting of a primitive instrument voltage regulator, a gauge and the sender unit in series. If all components are in specification it works. Like most circuits, it is a good example of Ohms's law. If the instrument supply voltage is increased, both the gauge indications for "empty" and "full" will increase. Adjusting the voltage may work in some situations, but not others.

I think it is possible to make a circuit to separate the reading the sender value from driving the gauge. The separate drive of the gauge, enables a means for adjustment. So the basic fix will involve supplying the sender with a constant current. The sender voltage will vary, directly related to the sender resistance. The sender voltage is then measured by a micro-controller on a continuous basis. The gauge is driven by a transistor, not the sender, using PWM (pulse width modulation). The PWM is a convenient means to vary the duty-cycle, to vary the gauge reading. There will be a minimum PWM that results in "empty" reading, and a maximum that results in "full". The program in the micro-controller, will align the sender minimum reading with "empty" PWM and the maximum sender value with "full" PWM. Switch 1 and 2 provide the means to indicate when at empty and full. This is a bit more involved, but will save that for later.

The installation will not be difficult, the sender wire in the OEM circuit will be removed from gauge terminal and placed on measurement board terminal. The gauge drive from board will be connected to the gauge terminal where the OEM sender wire once connected.

The other feature is a solid-state regulator replacing the primitive and now expensive OEM IVR. The same regulator will also supply the micro controller. Many have already done some sort iof IVR conversion, and know about that.

My car does not have gauge problems at this time, so I need to find a fuel gauge and sender for development. I am doing this to help others. I plan to build and test a prototype, but I do not plan to make and sell units. I will work with others and perhaps receive a small royalty for each unit to recoup my development costs. I am thinking RedFish or others may have interest?

Attached is a first draft of the circuit. There are ways to shrink and cost reduce, but the suggested circuit will be easy and robust for development work.