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Accelo · 08-22-2022, 01:42 AM

What truck are you working on? The fuses for the shunt (amp gauge) is not fused in the fuse box but in the harness on 67-72 trucks.

As electric current flows through wires their is some resistance to the flow.
So if you measure voltage on a long piece of wire the voltage goes down as you move further down the wire. A shunt runs most the current along a wire and measures the difference in voltage along two points on the wire. Since the length of wire used is short the voltage drop isn't much. So the gauge in the dash is marked (Amps) is in reality measuring mil-volts of voltage drop. the gauge is really a sensitive voltage gauge.
This allows the manufacture to run a smaller wire diameter into the cab that would otherwise be possible. The short fall is any additional resistance, anywhere in the circuit, will throw the mill-amp gauge off, or typically it will not work at all.

The additional circuit resistance is responsible for 90% of the gauges that are not functioning.

The issue with small changes in resistance changing the gauge function is why "GM" went to a voltage gauge.

08-22-2022, 01:42 AM	#2
Accelo Registered User Join Date: Mar 2015 Location: washington Posts: 2,180	Re: Someone explain a shunt please What truck are you working on? The fuses for the shunt (amp gauge) is not fused in the fuse box but in the harness on 67-72 trucks. As electric current flows through wires their is some resistance to the flow. So if you measure voltage on a long piece of wire the voltage goes down as you move further down the wire. A shunt runs most the current along a wire and measures the difference in voltage along two points on the wire. Since the length of wire used is short the voltage drop isn't much. So the gauge in the dash is marked (Amps) is in reality measuring mil-volts of voltage drop. the gauge is really a sensitive voltage gauge. This allows the manufacture to run a smaller wire diameter into the cab that would otherwise be possible. The short fall is any additional resistance, anywhere in the circuit, will throw the mill-amp gauge off, or typically it will not work at all. The additional circuit resistance is responsible for 90% of the gauges that are not functioning. The issue with small changes in resistance changing the gauge function is why "GM" went to a voltage gauge. Attached Images Last edited by Accelo; 08-22-2022 at 01:05 PM.