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amp gauge and shunt

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John Justice
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amp gauge and shunt

Post by John Justice » Tue Feb 07, 2017 9:18 am

Is the amp gauge used with the large shunt so common on 1960's Chris Crafts different than a garden variety amp gauge? Have an amp/oil pressure combo gauge that doesn't seem to work. VOM shows 14 volts when running so I think the alternator is working, but the amp gauge shows nothing.

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mbigpops
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Re: amp gauge and shunt

Post by mbigpops » Tue Feb 07, 2017 10:24 am

What engine are you using in your application ?

I know on my KL the generator has cutout relay that only kicks in about a certain RPM so I could be idling and measure 14V at the generator output and see nothing on the ammeter as the relay would still be open.

Need more info on your setup.

Mark
1953 CC Rocket Runabout "Rocket Man"

jim g
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Re: amp gauge and shunt

Post by jim g » Tue Feb 07, 2017 7:47 pm

John Justice wrote:Is the amp gauge used with the large shunt so common on 1960's Chris Crafts different than a garden variety amp gauge?

Have an amp/oil pressure combo gauge that doesn't seem to work. VOM shows 14 volts when running so I think the alternator is working, but the amp gauge shows nothing.

Yes. The gauge is different from a common amp gauge. If the gauge is used without the shunt. The gauge will get burnt up inside.

I would recommend calling Mark at Clawson Classic Instruments. He can explain how it works. Also if just your amp gauge is bad. He can repair it without doing a restoration to it.

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Bilge Rat
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Re: amp gauge and shunt

Post by Bilge Rat » Wed Feb 08, 2017 7:29 am

A gauge shunt is a calibrated resistor that accurate reduces the current through itself and those reduced current points are tapped.The gauge connects to theses taps and sees a fractional ratio of the actual current (actually voltage drop). The gauge is calibrated to read the reduced current and display the actual larger current on its scale. High current from a 40 amp alternator passing through the fine gauge magnet wires in the gauge could and will cause gauge heating problems and the potential for an un-fused battery voltage to short out to the gauge housing. That would create a spectacular condition you want to avoid. Also allows for a smaller sized conductor to the gauge rather than what would probably require at least a 8 or 6 gauge cable to handle the alternator's high output.

The shunt/gauge combination is a matched set to allow the gauge to read accurately. It is a much safer way to measure high current than a direct reading gauge.
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

jim g
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Re: amp gauge and shunt

Post by jim g » Wed Feb 08, 2017 8:08 am

Bilge Rat wrote:A gauge shunt is a calibrated resistor that accurate reduces the current through itself and those reduced current points are tapped.The gauge connects to theses taps and sees a fractional ratio of the actual current (actually voltage drop). The gauge is calibrated to read the reduced current and display the actual larger current on its scale. High current from a 40 amp alternator passing through the fine gauge magnet wires in the gauge could and will cause gauge heating problems and the potential for an un-fused battery voltage to short out to the gauge housing. That would create a spectacular condition you want to avoid. Also allows for a smaller sized conductor to the gauge rather than what would probably require at least a 8 or 6 gauge cable to handle the alternator's high output.

The shunt/gauge combination is a matched set to allow the gauge to read accurately. It is a much safer way to measure high current than a direct reading gauge.
Thats pretty much it. Also it requires a specific gauge wire and a specific length. Which is why you will find the excess length bundled up under the dash.

John Justice
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Re: amp gauge and shunt

Post by John Justice » Wed Feb 08, 2017 7:51 pm

Great information. Thank you. I suspect the gauge was restored using a standard amp gauge mechanism.

sproat3
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Re: amp gauge and shunt

Post by sproat3 » Wed Feb 08, 2017 11:43 pm

John-
Just because the VOM is showing 14 volts with no deflection on the gauge does not mean the amp gauge is not working. The gauge deflects to show the magnitude of current either charging the battery or being drawn from the battery. If the battery is fully charged, the gauge will not deflect when the generator is above the cutout voltage. It should show some deflection after a battery draw down from cranking the starter. That said, sending it out for a check and calibration before we get to summer is probably a good idea.
Ward
1960 Chris Craft Capri (Tahoe Heidi)

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