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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&t=397s (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? Shouldn't the multi meter show zero voltage regardless of whether there is a load connected or not since obviously there's corrosion in the wire.

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

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  • Voltage was measured on a faulty connection (hidden) caused by intermittent electrical contacts. Video revealed the fuse bank was loosely connected to the terminal, the intermittent connection allowed voltage to show as long as a load, the test light didn't connect. As soon as the test light probed the left terminal, voltage dropped to zero, indicating a fault. The fault was a loosely connected terminal supplying power to the fuse bank. Voltage measurements are only part of electrical troubleshooting. Mechanical (physical) connections plays a major role in all electrical connections.
    – F Dryer
    Sep 17, 2023 at 18:19

2 Answers 2

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Ohm's Law which says: V = IR

As you can see from the relation, the voltage loss across a conductor (V) is equal to the Amperage (I) times the Resistance (R).

Since there are no 0 resistance conductors (aside from superconductors which are a special case) there is always going to be voltage drop across a conductor when current is flowing.

So when you measure the voltage with no current, i.e. when it's an open circuit, you get 0V drop. When current flows you always get some voltage loss. This is what you're seeing with your multimeter.

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Imagine you have a water faucet/tap with a crushed copper pipe supplying it. The crushed area is letting a very small amount of water passed it (equivalent to a bad connection in electricity). If you were to check the water pressure (equivalent to voltage in electricity) after the crushed section of pipe with the faucet/tap closed, you would find normal water pressure, because no demand for water is being made and some water is able to get passed the crushed area. If you were now to open the tap very slightly (drawing a load) you may get drops of water coming out, but the water pressure would drop to zero, because of the restriction caused by the crush.

The high resistance in the wire only starts to affect the voltage when a current is drawn. The meter is drawing a tiny amount of current, so its affect on the voltage is minimal. The test lamp draws more current, so it affects the voltage a lot more.

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