Initial experimentation with external shunt ammeter

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Hey Del, I have a complete dash out of a '76 Dart, IP and dash harness down to the bulkhead connector. Would any of that help you?
 
A few posts requested more technical information on what this thread is all about. Here is my attempt at shedding some light. The concept stems from Ohm's Law where V=I/R; V=Volts, I=Current, and R=Resistance. Shunts are highly accurate small value resistors, often <1 Ohm in resistance. Using V=I/R, a 1 Ohm resistor conducting 1 amp of current will have a voltage drop of 1 volt. A 0.2 Ohm resistor will have a voltage drop of 0.200 volts (200 mV).

With that said, factory amp gauges that used the wiring harness as the resistance source are doomed to inevitable calibration drift, as connections corrode and the wire degrades. They should comfortably make it through the warranty period, though. Digi-Key has a supply of shunt resistors in the micro-ohm range (designated UOHM).

Displaying the amperage was done with an analog volt meter in the older Mopars. Instead of reading volts, it approximated amps. If the alternator is charging, the voltage is positive and swings the needle to the right. If engine is not running & headlights are on, the voltage is negative, pulling the needle to the left. A digital VOM could also be used to calculate amps across a shunt. I hope this helped some.

four_wire_shunt-featured.jpg
 
mpgmike said:
A few posts requested more technical information on what this thread is all about. Here is my attempt at shedding some light. The concept stems from Ohm's Law where V=I/R; V=Volts, I=Current, and R=Resistance. Shunts are highly accurate small value resistors, often <1 Ohm in resistance. Using V=I/R, a 1 Ohm resistor conducting 1 amp of current will have a voltage drop of 1 volt. A 0.2 Ohm resistor will have a voltage drop of 0.200 volts (200 mV).

With that said, factory amp gauges that used the wiring harness as the resistance source are doomed to inevitable calibration drift, as connections corrode and the wire degrades. They should comfortably make it through the warranty period, though. Digi-Key has a supply of shunt resistors in the micro-ohm range (designated UOHM).

Displaying the amperage was done with an analog volt meter in the older Mopars. Instead of reading volts, it approximated amps. If the alternator is charging, the voltage is positive and swings the needle to the right. If engine is not running & headlights are on, the voltage is negative, pulling the needle to the left. A digital VOM could also be used to calculate amps across a shunt. I hope this helped some.

View attachment 1715798527
Click to expand...

This is a great explanation, however I think the shunt resistor values should probably be in the milliohms, depending on what you want to measure. For example, 10 amps across a 10 milliohm shunt is only 0.1. Volts. Unless you have a good meter really close to the shunt, it may be hard to measure really low voltages.
 
mpgmike said:
A few posts requested more technical information on what this thread is all about. Here is my attempt at shedding some light. The concept stems from Ohm's Law where V=I/R; V=Volts, I=Current, and R=Resistance. Shunts are highly accurate small value resistors, often <1 Ohm in resistance. Using V=I/R, a 1 Ohm resistor conducting 1 amp of current will have a voltage drop of 1 volt. A 0.2 Ohm resistor will have a voltage drop of 0.200 volts (200 mV).

With that said, factory amp gauges that used the wiring harness as the resistance source are doomed to inevitable calibration drift, as connections corrode and the wire degrades. They should comfortably make it through the warranty period, though. Digi-Key has a supply of shunt resistors in the micro-ohm range (designated UOHM).

Displaying the amperage was done with an analog volt meter in the older Mopars. Instead of reading volts, it approximated amps. If the alternator is charging, the voltage is positive and swings the needle to the right. If engine is not running & headlights are on, the voltage is negative, pulling the needle to the left. A digital VOM could also be used to calculate amps across a shunt. I hope this helped some.

View attachment 1715798527
Click to expand...

You are right in theory but calibration drift may not be that big an issue. The main thing we are looking for is balance of zero at center, and a system that is sensitive enough that we can actually see that. ----Which is my complaint about many Fords and even others---that the entire system was so "numb" that it took a 30A load to see any change LOL

Actually I've thought of using a descrete external shunt

I was really hoping I'd get more chance to play with this but combo of heat, fire smoke, and continuing rib problems--plus I just don't have the "soup" I used to...................AND to finish it off,.............an extended BS with trying to get the glass company to deal with the glass leaks in the Scamp. That used up 3 weeks or more right there
 
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