How to test a wires polarity using a multimeter?

[Brett's_daddy]

I have a miscellaneous red wire that I want to find the polarity of using my multimeter but am not quite sure how. I know most red wires are positive but I do have a red wire that is actually negative so I want to be certain before connecting this other miscellaneous red cable. The multimeter I’m using is in the below picture. Any help would be appreciated!!!

[Brett's_daddy]

Mods please move this post to the Bass Boats forum!

[padlin]

You could ohm it out to see if it’s connected to the + or -, if that’s your concern. Put the meter on ohms, one lead on one of the battery posts, the other to the red wire, 0 ohms means that’s where it goes. That’s as easy any any.

[Brett's_daddy]

47 minutes ago, padlin said:

one lead on one of the battery posts

So which lead would you connect the probe to that isn’t connected to the wire you’re testing?

[Scott F]

Set the meter for DC voltage. Attach one lead to your battery negative. Attach the other lead to the red wire. If you read 0 volts, the red wire is either negative, or not connected to a power source. If 0 volts, Take a resistance reading between the battery negative and the red wire. O ohms means for sure it’s a negative lead. If you read battery voltage, then it’s a positive wire. 

[padlin]

7 hours ago, Brett's_daddy said:

So which lead would you connect the probe to that isn’t connected to the wire you’re testing?

I’m a little lost with your question, a “lead” and a “probe” are the same thing. One lead to the wire in question, the other to either battery post. If it’s too far away you can use a long jumper of some sort or find a known + or - point within reach. 
 

if Scott’s write up is more clear, use it instead.

 

[FrnkNsteen]

9 hours ago, Scott F said:

Set the meter for DC voltage. Attach one lead to your battery negative. Attach the other lead to the red wire. If you read 0 volts, the red wire is either negative, or not connected to a power source. If 0 volts, Take a resistance reading between the battery negative and the red wire. O ohms means for sure it’s a negative lead. If you read battery voltage, then it’s a positive wire. 

^^Agreed^^

 

A voltmeter measures voltage differential, meaning it measures a voltage drop across test points. 

 

If you put meter on DC Volts then put the black lead/wire on the battery negative and the red lead/wire on the positive, you will read the voltage difference between the two at give or take 12 volts depending on battery charge.

 

So test as Scott suggested and set meter to DC Volts, put the black lead on negative post and the red lead to your wire. If it is hot, you will get some amount of reading around 11 - 13 volts. If it is a ground, you shouldn't get a reading as there won't be any voltage difference.

 

DC (Direct Current) volts need red lead to power and black to ground. AC (Alternating Current) volts can be either way.

[FrnkNsteen]

Just thought of something,... 

 

The other possibility is it could be a hot lead that is tied to a switch, so if there is no reading, it could still be a hot wire that is just switched off.

 

Best way to confirm it is not a ground is to use the continuity alarm or ohm meter to check continuity to ground. If the meter reads OL (overload) it is not connected to ground. (I would have to wonder why you have red ground wires though! Not a good practice!)

 

But make sure it is not a powered wire first or testing ohms could damage your meter.

[Brett's_daddy]

5 hours ago, padlin said:

I’m a little lost with your question, a “lead” and a “probe” are the same thing. One lead to the wire in question, the other to either battery post. If it’s too far away you can use a long jumper of some sort or find a known + or - point within reach. 
 

if Scott’s write up is more clear, use it instead.

 

I meant which post would you connect the probe or clip to? I’m assuming the negative battery post? Also would you use the negative battery post as a connection point for testing both polarities?

[Alex from GA]

Yes on both questions.  The black lead to the battery or another closer negative source.