TheSamba.com :: View topic - How Do I Use a Multimeter to See if a Wire Has Power?

Wed Jun 23, 2021 4:59 pm

I am installing a stereo and see that the previous owner has tapped into a wire but I’m not sure if it has power. How do I use my multimeter to test if that wire has power? I have looked on YouTube and the Internet and there are various descriptions of how to use a multimeter for different purposes but nothing that specific. It would be great if you can tell me exactly which setting to put the multimeter on. I am guessing the red probe touches to the wire and the black one to ground-correct?

Thanks!

Wed Jun 23, 2021 5:29 pm

Test light may work better for that ,but yes black is ground set it on volts and probe the wire with the red one ,depending on setting it will show if you have voltage ,test it on fuses first to see if its working sometimes you dont have a good ground .

Also check it with key off ,and again with key in the on position. Hope that helps

Wed Jun 23, 2021 5:32 pm

It might help to see a pic of your multimeter but typically you put it on "DCV" power.
It might say DCV or V

Example (not my meter) of it set to read with a range up to 20 volts:

Quote: I am guessing the red probe touches to the wire and the black one to ground-correct?
Correct

Wed Jun 23, 2021 6:25 pm

Some good how to articles for using a multimeter:
https://www.hagerty.com/media/maintenance-and-tech/multimeter-part-2/
https://www.hagerty.com/media/maintenance-and-tech/measuring-resistance/
https://www.hagerty.com/media/maintenance-and-tech/measuring-current/
https://www.hagerty.com/media/maintenance-and-tech/measuring-voltage-drop/

And one to help understand typical autmotive circuit failures:
https://www.hagerty.com/media/maintenance-and-tech/automotive-circuit-failures/

Wed Jun 23, 2021 6:46 pm

Thanks, all! Will give it a try tomorrow.

Wed Jun 23, 2021 7:26 pm

68IHscout wrote: Test light may work better for that ,but yes black is ground set it on volts and probe the wire with the red one ,depending on setting it will show if you have voltage ,test it on fuses first to see if its working sometimes you dont have a good ground .

Also check it with key off ,and again with key in the on position. Hope that helps

I have one of these. Red to the wire I’m testing and black to ground?

Thu Jun 24, 2021 3:30 am

That may not work on low voltage, may be for home 120/240V
Test light will have a 12 V bulb in a handle a black wire with clip and usually a pointer, like a screwdriver with wire attached.

Thu Jun 24, 2021 5:48 am

that tester you showed a pic of looks like something you would use to test house electrical plugs, so I don't think it would glow enough with only 12 volts DC running though it. I would go out and buy and decent multi meter they only cost about $25 and you would use it for a lot of things on the car and the house.

Thu Jun 24, 2021 6:32 am

Tested the red power wire with multimeter and it shows around 12 V. Tested the ground wire by touching the black probe to the ground wire and the red probe to the battery positive terminal and that showed around 12 V also. Understanding that the yellow wire is for maintaining memory in the stereo, where can I temporarily connect the yellow wire just to make sure the unit is getting power?

Thanks

Thu Jun 24, 2021 6:40 am

Find a terminal on the trunk side of the fuse panel that has B+ with the key off and removed.

Thu Jun 24, 2021 6:48 am

Sorry, not following. How do I access that side of fuse panel? Mine is low just in front of shifter. I connected stereo to the wire that has power and ground to the ground wire - both of which showed 12V. No power to stereo. Does the yellow wire have to be connected to power for the unit to work?

Just attached the yellow wire to the bottom of fuse number 11 and still no power, even with key on. Stereo mini fuse on back of unit is also good.

Thanks

Thu Jun 24, 2021 6:55 am

Pekoe2053 wrote: 68IHscout wrote: Test light may work better for that ,but yes black is ground set it on volts and probe the wire with the red one ,depending on setting it will show if you have voltage ,test it on fuses first to see if its working sometimes you dont have a good ground .

Also check it with key off ,and again with key in the on position. Hope that helps

I have one of these. Red to the wire I’m testing and black to ground?

never used one of those ,i would imagine if it reads the fuses (lights up) it would ok .. did you get any instructions with that ?

Thu Jun 24, 2021 6:57 am

There will usually be three wires necessary for the radio. One is a positive wire that gets power all the time. One is a switched positive wire that gets power only with the key in the "Run" position. The third one goes to ground, as in any grounded metal part of the car, but usually search for an existing ground terminal.

The terminals on the fuse panel I'm referencing are on the exact opposite side of the fuses. So, wherever your fuse panel is under the dashboard, look at the exact opposite side of it (in the trunk) and you will see the wires for the rest of the car going to the terminals on the other side of the fuse panel. Some of those terminals are powered from the battery all the time, some of them are powered with the key in the "Run" position.

Thu Jun 24, 2021 7:02 am

68IHscout wrote: Pekoe2053 wrote: 68IHscout wrote: Test light may work better for that ,but yes black is ground set it on volts and probe the wire with the red one ,depending on setting it will show if you have voltage ,test it on fuses first to see if its working sometimes you dont have a good ground .

Also check it with key off ,and again with key in the on position. Hope that helps

I have one of these. Red to the wire I’m testing and black to ground?

never used one of those ,i would imagine if it reads the fuses (lights up) it would ok .. did you get any instructions with that ?

Those are generally only for home receptacle testing. It is designed for 120-240 volt AC use. The leads are not long enough to be very useful for automotive purposes, and might not light up with 12 volts DC.

If you want a test light, get something like this:

https://www.amazon.com/JASTIND-Automotive-Extended...5588546775

Thu Jun 24, 2021 7:14 am

Connected up my old stereo to the red power wire and the black ground wire, both of which shows 12 V on the meter and it does not work either! It is so old it doesn’t have a yellow wire.

Thu Jun 24, 2021 7:45 am

If you have 12V at both wires, something is wrong. One of two likely scenarios.

1) The "Ground" wire is not actually a ground, it is another 12V source.
2) The Ground wire is not actually grounded at the other end.

Disconnect the wires from the stereo and check the wires for voltage again. If the positive wire has voltage (it should) and the ground wire has no voltage (it shouldn't), then the next step is to switch the meter to read resistance (Ohms, Ω) and check the ground wire for continuity to ground. Put one meter lead into the ground terminal at the connector, and the other meter lead to a good ground. There should be almost zero resistance, like less than 0.5 Ω. If it is substantially higher than that, or the meter read "OL" (Out of Limit), then the ground wire is broken somewhere.

Thu Jun 24, 2021 8:28 am

Paul Windisch wrote: If you have 12V at both wires, something is wrong. One of two likely scenarios.

1) The "Ground" wire is not actually a ground, it is another 12V source.
2) The Ground wire is not actually grounded at the other end.

Disconnect the wires from the stereo and check the wires for voltage again. If the positive wire has voltage (it should) and the ground wire has no voltage (it shouldn't), then the next step is to switch the meter to read resistance (Ohms, Ω) and check the ground wire for continuity to ground. Put one meter lead into the ground terminal at the connector, and the other meter lead to a good ground. There should be almost zero resistance, like less than 0.5 Ω. If it is substantially higher than that, or the meter read "OL" (Out of Limit), then the ground wire is broken somewhere.

First, thanks very much for your guidance! So when someone says that if you touch the red probe to the positive battery terminal and the black probe to the ground wire, it should read - that is not correct?

Thu Jun 24, 2021 8:34 am

Pekoe2053 wrote:

First, thanks very much for your guidance! So when someone says that if you touch the red probe to the positive battery terminal and the black probe to the ground wire, it should read - that is not correct?

It is correct. One lead on the positive post, and one lead on the negative post, and you should have whatever battery voltage is (usually 12.6 volts on a healthy battery).

I was assuming you were leaving the negative lead on ground and testing each wire at the radio individually, at which point you should NOT have 12 volts on the ground wire. However, if you had one lead attached to the positive wire and the other attached to the ground wire, and it showed 12 volts, then it sounds like the circuit is fine and there is something wrong with the radio.

Thu Jun 24, 2021 8:45 am

Loose ground-glad it was a simple fix. Thanks all!