Fixing the goofy gas gauge

I have owned my 76 308 GTB for 30 years and suffered in silence as the gas gauge presented me with a variety of readings when the tank was less than half full. But my patience came to an end and I decided to fix it. I have manged to do so without molesting the original wiring harness or its attendant connectors. The car can be returned to stock within an hour with no traces of this sacrilege. If there is interest, I will post the details. The work can be duplicated by anyone comfortable with using a soldering iron.

 

I'm willing to share everything, just wanted to know if anyone else cared. My car is a 76 GTB, but I think this will pertain to other years and types.

I had two problems: 1. a flicking of the needle when the turn signals were activated and 2. significant needle movement when cornering or stopping.

These require two different solutions. I could not just come up with one circuit.

The first is solved using a buck-boost converter.

DC DC Boost Buck Adjustable Step Up Down Converter XL6009 Module Solar Voltage | eBay

I usually avoid Chinese products, but these are made very well (quite impressive actually) and they only cost $1.80.

The second requires constructing a custom circuit and will take about an afternoon of labor.

The circuits are installed in line with the existing wiring (similar to using an extension cord between appliances). Not altering the original wiring is important to me.

There will be a module that shows, as can be seen in the picture. It is located in the vicinity of the gas tank.

I will break the actual details into two sections to follow. This is meant to be a primer of how to do it (a nuts and bolts approach). If people wish to know why things were done in a particular way, please ask.
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The buck boost converter must be modified to be useful. It will not operate properly if the desired output voltage is 13.7 volts and the input voltage is 13.7 volts. So the input voltage must be lowered so the device has "something to work with". I reduced the voltage by putting 6 diodes in series before the input to the converter. The forward drop voltage of silicon diodes is 0.7 volts; therefore the input voltage is reduced by about 4 volts. This is sufficient for proper boost up to 13.7 volts needed for the gauge. The diodes I used were 1N4006 - I had them here in my parts bins and they are an excellent diode. I mounted them on component carriers (I didn't have 6 position carrier so I used a 4 and 2). I used plastic foam to hold the carriers in place and to keep the pins from chafing other wires when mounted.

Attached are photos that show the converter and size and the converter with diodes at the front.

If one looks at the online picture of the converter there is a blue device with a screw head. This is the pot used to adjust the output voltage. I don't like potentiometers in circuits because people always "fiddle" with them and can't get back. This is dangerous in this instance because over voltage can kill the gauge. I removed the pot and replaced it with discrete resistors. On the picture at between 11 and 11 1/4 inches there is a rectangular block (outlined in white) where the pot resides. When the pot is removed there will be 3 holes which I will number 1,2,3 from left to right. Install a 3.9K ohm resistor between holes 1 and 2; and a 27K ohm resistor between holes 1 and 3. This will give 13.7 volts out.

Now all that is needed are the wiring connections for the gauge. There will be one wire with a male connector for the 12v (green in my picture), there will be one wire with a piggyback connector for the ground (black in the picture), and one wire with a female connector to the gauge (also green in my picture). The unit is installed between the 12 volts that drive the gauge and the gauge itself.

The final picture shows the unit mounted behind the dash. I affixed it with tie wraps to keep it in place.

Hope this is clear for phase 1.
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This requires building a custom circuit that is installed between the wiring harness to the tank and the fuel level sender.

The schematic of the circuit to be built is attached. All components are mounted on carriers so they can be easily removed
and/or altered. The circuit board is a perforated prototype board. An example would be a Vector Board type 8029. You can see them here.

8029 Vector Electronics | Prototyping Products | DigiKey

The wiring is point-to-point. I would suggest Kynar wire 30 Gauge, it can carry 0.8 amps; the maximum that the circuit uses is 0.5 amps.

The circuit board is mounted inside a zinc Bud enclosure P/N CU-123. It is mounted on little standoff feet. Four are required; I suggest
RAF 2103-440-AL-7. The 2103 type is 7/16 inch high and you want no more than that. You can see them here.

2103-440-AL-7: Standoffs Hex Female to Female 1/4 Inch 4-40 7-16 Inch Aluminum Iridite: RAF Electronic Hardware

The attached photos show a plan view of the board with dimensions and an elevated view showing the standoffs. There is also a plan view of the board inside its little box. The final photo shows the interconnect wires. The black wire has a piggyback connector that takes ground from the existing wiring harness and provides ground for the level sensing mechanism in the tank. The yellow and green wires have male connectors that plug into the female connectors of the existing wiring harness. The white wire has a female connector that goes to the level sensing mechanism in the tank. The red wire attaches to another red wire that picks up 12 volts at the fuel pump.

A few words about component choices. On the schematic U1 and U2 are contained in a single integrated circuit, an LM358. These are single sided op amps that I have used before in the engine compartment. They have been in there for 25 years with no failures
The large yellow blobs are 25 microfarad capacitors. I used Vishay Sprague type 199D256X0025D1 (the rest of the letters don’t matter). These will just fit the space in the box.
The 66 Ohm resistor is formed from two 33 Ohm ½ watt units. 33 Ohm is a common value 66 Ohm is not.
The 0.22 microfarad capacitor is ceramic and must be placed close to the 7808 integrated circuit (this is manufacturer’s recommendation for stability). The capacitor I used was the smallest ceramic part I could find.
The transistor choices are critical. The PNP ZTX551 was chosen because it has the lowest base-emitter saturation voltage (VBE sat) I could find. This is necessary to be able to drive the needle all the way to the full indication. The NPN NTE48 is a small package Darlington pair that has the required gain to drive the low fuel light bulb. The point of the previous discussion is to emphasize that just “garden variety” transistors will not work. Ask me how I know

Hope this is sufficient for those willing to build. If there are questions feel free to ask.
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