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'70 & up "late" system I will guess. (2 field terminal alt/flat reg with triangle connector). (1) turn the key on & see if you have voltage at both alt field terminals and at the blue/green wire female terminals in the reg triangle connector. (2) replace the triangle connector back on the reg & start it to a fast idle & pull the green wire connector off of it's alt field terminal & with a jumper wire with alligator clips ground the male alt nub (not the female connector/wire you removed from it) for 5 seconds max & have a helper in the dr seat holler if the voltage jumps up & you can hear if the alt kicking in (if it is good) pulls the RPM down a hair (not much but you can feel/hear it). If so the alt is good. (3) if alt is good unbolt the reg & clean it's backside/firewall & reinstall to confirm continuity to ground (4) if the alt is bad pull/check the 2 brushes/brush holders & if they are OK then replace the alt (an 85 M body 78 amper is the best OE one). (5) if the alt is good & cleaning the reg ground does not fix it then replace the reg. You might recheck the voltages at the reg connector/alt field terminals (4 terminals) when running but if they are hot with eng off/key on they should still be hot with eng running. Holler back

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