Resistance to ground increase when key is on

Mattax said

Correct. The problem with a votlage drop on the sending unit is the current is very low.
Voltage drop only occurs when and where current is flowing through resistance.

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I agree but if there was a main ground connection issue, he may see more than 0.2 volts at the sender. Not the senders fault, just a grounding issue, and the voltage will be from everything using that ground, not just the sending unit circuit. Less than 0.2 puts his mind at ease that no problem exists.