Help! Electrical issus

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I installed a gen3 3sgte two years ago from a gene 3sgte. Everything ran fine before and after the swap. After 2 years of driving with the gen3, the electronics started giving out. After initially installing the motor, the gauge on the dash read a low voltage of the batter but the battery still read 10-12 volts while the car was running using a voltage meter. One day I