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Given my mom's car, I measured a voltage in an electric connector equal to the voltage across the car's battery. This is when the multimeter is connected to the connector supply pins. If I take the negative terminal and put it on the car body the reading is now 0. Somebody told me on the phone why this is happening, I didn't understand very clearly so I came here to ask for help. Can someone please explain to me why is this happening? Why is the multimeter giving me that voltage measurement even though it doesn't truly exist?

Thank you in advance and sorry if the answer is obvious.

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  • \$\begingroup\$ Not an expert on car electronics, but doesn't the car have a seperated signal gnd and a chassis gnd? \$\endgroup\$ – Remco Vink Jun 7 '18 at 8:09
  • \$\begingroup\$ You need to show an image if exactly what you had connected where so we can follow what you did... otherwise we cannot help, What is the connector for? Or what does it supply? \$\endgroup\$ – Solar Mike Jun 7 '18 at 8:12
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    \$\begingroup\$ Paint is an insulator. Don't mess with your mom's car unless you know what you are doing. \$\endgroup\$ – Andy aka Jun 7 '18 at 8:28
  • \$\begingroup\$ @RemcoVink as far as I know indeed allmost all cars have GND "signal" wires but they are all "star" connected at the battery - (minus) pole. The car's chassis is grounded. \$\endgroup\$ – Bimpelrekkie Jun 7 '18 at 8:33
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If I take the negative terminal and put it on the car body the reading is now 0.

Then very likely your meter didn't make a proper contact to the chassis (there might have been paint/oil/grease/dirt preventing a proper connection).

or

There is something wrong with the battery - (minus) to car chassis connection.

Normally cars have the battery - (minus) connected directly to car chassis.

So measuring the voltage across the + and - battery terminals should give about 12 V.

Measuring between the + battery terminal and car chassis should also show about 12 V.

Car batteries can deliver extremely high currents which can evaporate (after melting and catching fire) even a modestly thick wire when connected across the battery terminals. So normally you should avoid messing with car batteries if you're unsure what you're doing.

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    \$\begingroup\$ Come on, a little spark, a little hydrogen gas explosion, some acid in the eyes, these are just rites of passage. \$\endgroup\$ – Misunderstood Jun 7 '18 at 10:22
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Possible readings.

Apart from the bad contact possibility offered by Bimpelrekkie, you have three ways of taking the measurement. One, Figure 1b, will give you a zero volt reading.

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