I'm working on trying to figure out how much volts or amps my cb and lights pull from my battey and if they pull when the truck is off but I dont know how to test. I have a multimeter but really dont know how to use it. My question is how do you measure how many volts or amps a circuit is pulling when its on and when its off?

You can use an Amp meter. They typically have a loop that you open and insert ‘one’ side of the circuit. Most multi-meters do not have this feature but you can find them if you look. Hopefully, someone can loan you one.

Instructions:
Find the 2 wires that go into the back of the CB or whatever. Remember this is DC current so chose that setting on the meter. Place the + wire into the loop and push the PTT button on the mike and read the meter. The amps that it will draw will not change with the truck running or not but test it with it running so it can draw what it needs. If the battery is low it will pull less amps only because there are less available. A CB, if not altered, would be 2-3 amps I think.

For the lights just copy the above, hope this helps.

Roger.

you can figure amps if you know the watts rating of the piece

watts=amps x volts

Most multimeters top out at 10A max in a series connection. Divide the Watts rating of the equipment by 12V to see if you've still within the safe range of the meter.

A clamp-on ammeter have to be marked specifically to be able read DC amps. Most are AC only.

Volt meters are hooked in parallel with the load and amp meters are hooked in series. parallel means the red lead goes to the + and the black lead to the -. Series: the + lead is disconnected from the sorce and the red lead of the meter is hooked to source and the black lead is hooked to the wire you disconnected. Make sure you have your multimeter on the highest amp scale to start with. As said before user the formula POWER divided by VOLTS = CURRENT. dO THIS TO ENSURE YOUR METER WILL HANDEL THE AMPS YOU ARE TRYING TO READ.