Intermintent Volt Gauge

[GreenRiverCummins]

So on a 150 mile round trip with my family at about 40 mile's in my Check Gage light came on and i seen the Volt Gauge go from 14 to 0. We were unable to turn around due to Family health reason's but we did pull over and check the Belt, pop the belt loose, and checked the alt bearing. Checked the terminals all good so we we had to drive the remainer of the trip with no trouble batterie's stayed fully charged/Sparked when replaceing the terminal's. So today i removed and recleaned the terminal's sanded all the grd's i could reach, replaced all the fitting's on the wire end's, removed the alt had it tested at 2 store's it checked out good had the batteries tested as well all checked out checked for exposed wire's, bad fuses, and so on then added a second volt gauge it read's 14 while the fac read's 10-11 So i'm lost is it the PCM or the Gauge cluster going bad will it cause an issue to ignore the fauilty gauge cause i'm gonna leave the new gauge in place as a safety can never have to many gauge's

Thanks in advance for any help or info

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Update took the alt back off and took it to a friend who is the best alt/starter man I've ever seen looked it over tore all the way down and found piece that was wore in half and in 5 mins and 30 dollars later he had her Running awesome on the truck took it for a 50 mile round trip to town and no worries she's holding 15 volt on the fac gauge and 14 1/2 on the aftermarket gauge. I asked him why it passed in the test at 2 store's he informed me that those type machines can't test the equipment a fraction of what is required to diagnose anything other then a a loose wire. I'm inclined to believe him seeing how he's been doing this for almost 45 years now.