tranny does not shift at night with the lights on on 1993 Lexus LS400

why my transmission does not shift at night with the lights on?

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2 answers
check the wiring harness in your trunk hinge. open it up and check for bare/crossed wires. this is really common and
causes voltage "feedback" when the lights are on.
It sounds like perhaps a ground is bad but all electrical connections would have to be looked at to see if when the lights or other electrical loads are on is there a voltage drop due to bad connections or grounds.