Major Electrical Issues and Malfunctions

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I would first make sure the polarity is good on everything. I would then take the old multimeter and check each step of the way starting from the battery. Once you find something that looks wrong, address that problem.

Again, I cannot stress the concept of logic. On my team, sometimes we just map out all the problems and alter them all at once. This doesn't usually solve the problem and also fails to help you learn anything or get closer to it. Change one variable at once!