Multi-tester Use

[blazeorange]

How do I use a multi-tester to determine which is the negative and positive side on 12 Volt products (1-Hot, 1-Ground) when I don't have the red and black colors available to know by sight?

I have a nice multi-tester but haven't learned how to use it very well and was wondering how to determine the polarity of wires before I hook them up in order to not hook them up backwards.

Thanks.

Blaze.

[pap4]

1 of the posts coming up from the battery is larger than the other that is the positive or hot one

[perchjerker]

If I understand your question correctly, you could use the ohm meter function and connect it between your fuse box and one of the wires going to the battery. Since the fuses are normally on the positive side, a zero (or close) reading here would indicate that wire is the one that goes to the positive battery terminal.

We really need more details in order to help you out properly.

[Trailerguy]

When you say multi-tester I assume you are talking about a volt/ohm meter? You would plug the probes into the meter, black -, red +. Then turn the dail to DC voltage, which ever setting is slightly more than 12, mine has a 0-25 scale. Test it directly on the 12V battery, you'll see the needle move to about 13.8 volts on a good battery. Then if you want to check two wires, say they are going to your depth finder, disconect one wire, put the probes across both wires, if the needle goes up you have correct polarity, if it goes down you are reversed. So switch your probes around. Now that you know you have found a complete circut, you can leave the ground - probe on that wire and use the other probe to find the hot + wire in other circuts, say your radio for instance.

[blazeorange]

That answers my question if the equipment is hooked up already but what if I have a bilge pump (or other item) that I want to install and the wires were painted over and then later extended/chopped, etc. Now I don't know what goes where and I don't really want to tear the pump apart to see what was what. Is there a way to check as such without the bilge pump in the boat yet? The proper way would be to tear it apart and run new wires but for future reference I'm curious if this can be determined?

Thanks for your posts. They indirectly answered some other questions I had.

BLAZE.

[Silentsixty]

Blaze,

With bilge pumps or livewell pumps, toss them in a bucket of water hooked up both ways. Whichever way pumps the most water is right. It will be obvious. All the electronics I've fooled with had polarity protection & just won't work if hooked up wrong + they are marked. I'd ask on here about a specific model of something if lacking instructions. Don't know about lights.

I was going back to check, because I suddenly realized what you were asking when I saw your most recent post. Leaving the rest in case it has any value.

Set on 25 or 50 volt DC. Touch one wire with red test probe, other with black. If you read + 12-13 volts red probe is connected to positive. If neg number, red is on neg.

No needle response on a analog tester means no juice.

A negative reading will peg the needle on my cheap analog multi-tester and pegging needles in general is bad. A better way for analogs would be to run a neg lead from the same DC power source as your testing. Touch the known grd to the neg probe and touch the pos probe on one of the two wires to be tested. +12-13 volts = pos. Repeat with other wire. No reading on both suggests a broken connection on the +.

Its my impression that digital testers are ok with negative numbers. Trailerguys post seems to indicate this.

Extra tip - locate fuse close to power source whenever possible.

Hope this helps,
SS

[boat nut]

In the case of a bilge pump installation, bare the wires you just removed from the pump. Connect the multimeter across the two wires, turn the bilge pump switch on. In the case of a digital meter, a positive reading means the positive lead is on the positive voltage source, a negative reading means, of course, the opposite. In the case of an analog meter (if you can still get one), as previous posters indicated, an upscale reading means proper polarity, while a pointer going downscale (bad for the meter) indicates reverse polarity.