Make sure you touch the leads of the meter together first, they should go to zero or nearly zero resistance. Test the test tool first.

I never did get a cord that worked for my block heater. I have a magnet mount heater but have never used it. 5w-40 oil and good glow plugs and battery and I've never had any trouble starting, even with temps in the teens and with a glow plug out.

Where are you located, OP? You may not need a block heater.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap