The factory gauge sends a positive electrical charge down the single wire to the sender, which is grounded to the motor through the passage it is threaded into. The sender acts as a resistor, giving different readings based on how hot it is. Since your voltage gauge shows a drop when your water temp sender is connected, the problem could be rather significant electrically, especially when you factor in the dead battery. Rather than spending time tracing the wires and pinning it down, you could remedy the situation and greatly improve reading accuracy by axeing the factory setup and installing a nice 2 5/8 autometer electrical gauge.
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