Why does loading a circuit cause voltage drop in DVOM?

Question

So this question relates to cars but applies to all electronic circuits really.

I was watching a car diagnostic video on youtube and in the video he demonstrates the difference between doing voltage reading while the circuit has a load vs no load. https://www.youtube.com/watch?v=0UO-e7bLsY4 (skip to 6:37)

Why does the multi meter show a steady 13V when connected to the circuit without an electric load, but as soon as there's a load the voltage drops to zero immediately, that tells me that there's some sort of restriction in the circuit like for example corrosion. Shouldn't the multi meter show zero voltage regardless of whether there is a load connected or not?

Maybe I'm failing to understand how multi meter's actually measure voltage?

Answer 1

The voltage drop across a resistance depends on the current through that resistance, according to Ohm's Law.

A digital voltmeter or multimeter has a very high resistance - often 10 megohms - so will draw an extremely low current, resulting in an extremely low voltage drop across the resistive connection.

An incandescent bulb or electric motor will draw a much higher current than the DVM, so will produce a much larger voltage drop across the resistive connection.