How to use a DMM

Ok, I finally got a DMM, but I'm a little unsure of how to correctly use it to test what I want.  I've used test lights in the past for basic things, and have also used DMMs for basic concepts.  But now I have a new one, and more complex (well, to me at least) concepts to check.

Here's the case in it's simplest form:  I need to use a car wire (let's say a GREEN one), but I have to check if it's (+) or (-) first.  I put the meter to DC, put the RED probe on the GREEN wire and the BLACK probe to chassis GROUND.  The meter reads 1V (+).  So, then I put the BLACK probe to the GREEN wire and the RED probe to ground and it reads 1V (-).

So my question is, how do you know when it's (+) or (-)?  Do you ALWAYS put red to the wire in question and black to ground...and if the wire is in fact (-), it will read with a (-) sign?  Or do you switch the probes when you're testing for different things?  Arg.

Any help would be great....THANKS!

Usually when you have a wire in question you will already know if it's a (-) or (+). You just want to make sure that it's what you think it is. If you think it's a (-) wire then you put the meter to DC, put the RED probe on the target wire and the BLACK probe to chassis GROUND. The DMM should read 0.000 +/- .1 Then you verify this wire by turning on the ignition and test it again. Or you can switch the RED lead to 12 volts + and use the BLACK lead to the target wire. The reading should be 11.5 - 12.5 volts if you are testing for a constant wire and 14.2 - 14.6 volts on a running ignition wire.

Here some good sites for how to use a DMM and also a AMM:

• https://www.doctronics.co.uk/meter.htm#digital
• https://www.doctronics.co.uk/meter.htm#analogue
• https://www.acmehowto.com/howto/homemaintenance/electrical/multitesteruse.htm
• https://mechatronics.mech.northwestern.edu/mechatronics/design_ref/tools/multimeter.html

Wow, thanks Jeff.  I understand the beginning part, but not the following:

The DMM should read 0.000 +/- .1 Then you verify this wire by turning on the ignition and test it again.  - Ok, so if it's negative it will show 0.000 +/- .1.  If I verify it with the ignition running, will it show the same reading?

Or you can switch the RED lead to 12 volts + and use the BLACK lead to the target wire. The reading should be 11.5 - 12.5 volts if you are testing for a constant wire and 14.2 - 14.6 volts on a running ignition wire.  Is that just for (+) wires?....and then for the negative I'll get 0.000 as in the first part?

Thanks for the sites.  (The first two were giving me errors....wasn't sure if it was me or the address).

If the wire is in actuality a ground wire it should read as close to zero as possible with the igniton off & on. Just because it's a (-) wire when the igniton is off doesn't mean that it willstay that way after the ignition is on... just double check this and you'll be safe.

The second part of the testing is so that you can confirm that the wire is infact a (-) by placing the RED lead to 12 + and the BLACK tot he target wire. If it registers 12+ then you know it's a ground, but if you turn the igniton over and the reading flucuates, then you know it's a power wire.

These are just two different ways of getting the same result from the target wire. You can use euther or both just to be sure of the polarity of the wire. Good luck !