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87 carburated motor in 88 efi harness truck won’t charge is onltvissue alt is good starts runs drives pullnpositive cable dies at first it would charge pull cable off put back on like polarizing any input any one pleasev
Disconnecting the battery of a running automobile is a good recipe for blowing the diodes out of the alternator. If I misunderstand what you are saying my apologies. If you are not getting 13.5 volts at the battery with the engine running your alternator is probably failed and will need repair or replacement.
