Alternator upgrade question

[laws69cars]

I have a question guys I've read a few alternator upgrade threads but I'm stumped with my switch? I have my original Delco 120amp alternator that charges great when cool between 14.0 and 13.5 but once it warms up the volts on the scanmaster start to go up and down between 11.0 to 13.0. I purchased a new LT1 140amp and it bolts right up but from the minute I start it it's only giving 11.0 to 11.5 volts on scanmaster and voltmeter? Any ideas what I'm doing wrong or is there wiring i need to upgrade?

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[bpman1234]

check all your grounds make sure clean and tight take them off and and clean all the paint use sand paper and make sure there are no frail wires. fender fire wall and the back of the block .