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Old 11-21-2012, 08:17 PM   #1
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Basic Wiring Question

My son who knows electrical but never worked on trailer wiring needs to know what the wires are. He tried to remount the interior front lights for me today and something is not right, they don't work. Red & yellow wires going to the interior lights - which is 12 volt? And if someone could kindly just post a few basic "trailer wiring 101" for us it would be helpful. All he needs to know is what are the wires by color. I got lost in the black hole of this section of the forum trying to find an answer before I started a new thread. Thank you!

It's for a 1970 Streamline Duchess.
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Old 11-21-2012, 10:27 PM   #2
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First I'd look at the 12 volt fuse panel. That should show you which wires are "hot" coming off of the battery and how many circuits there are.

Second, I've found one of those test light things (looks like a screwdriver with a light in the handle and a wire with an alligator clip coming out of the handle) very useful to see if a wire is "hot". They cost about $5.
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Old 11-21-2012, 11:44 PM   #3
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it's probably going to be lots of trial and error.

There are lots of us here that can give you that kind of help on an Airstream TT, but you will need to find someone that has detailed knowledge of your Duchess. I don't even know if they put out a Service Manual, but if they did, that would certainly have much information invaluable to doing wiring work.

Other than that, basic detective work will be needed. For example, if you suspect that a certain light fixtures has a certain wire coding, take the cover off, and if you have to, remove the fixture from its mount to see what wire was used.

Keep lots of detailed notes as you go around the trailer doing this type of thing. Then like DanielB said, go to the fuse panel asnd see where that wire connects into the wiring.

Sure wish I cold tell you more than that...
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Old 11-22-2012, 09:25 AM   #4
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I have no knowledge of your brand of trailer, but here's some basic electrical guesswork--very basic, so I hope I don't offend your son who knows electrical.

Get an inexpensive volt/ohm meter.

With the meter set to read volts, connect the leads between each of the wires and a metal part of the trailer. One wire should read 12volts, and the other should read nothing (that's the ground wire).

If this turns out to be the case, set the meter to read ohms and measure between the ground wire (stay away from the hot wire or you'll blow the meter) and the metal of the trailer. It should read very near zero ohms. If you find resistance (ohms) in the ground wire, that's your problem. Reattach the ground to the metal in the trailer (maybe back near the battery) after cleaning away any corrosion.

If you don't find 12v on either wire, check for blown fuses.

Another possibility is that both wires are hot, and the fixture is designed to ground by being in contact with the metal frame of the trailer. There may also be a short wire that runs between the light fixture and the metal of the trailer.

Most vehicles are wired so that the metal of the vehicle can be used as a return path for the current. Almost all ground the negative side of a DC system to the frame and use wires for the positive (hot) side.

One more thought is to use the ohm meter to check resistance in the light fixture itself. If it's anywhere as old as the ones in our Airstream, just fooling with it may have opened up something.

I'll second Aage's advice to keep notes, and then keep the notes for as long as you own the trailer.

Good luck with this, and have a great Thanksgiving.
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Old 11-22-2012, 09:36 AM   #5
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Thanks everyone! This is defiantly a helpful start. I'll be back if we have more questions after we try these suggestions. Happy Thanksgiving!
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Old 11-22-2012, 10:47 AM   #6
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The general rule of thumb on 12 volt systems is the wire conducts the Positive side and the trailer body conducts the Ground side.

Now that said if you have 2 wire going to a fixture that fixture could have a High and Low setting or the second wire could be the ground.

First thing to do is remove the fixture and with a voltmeter test each wire to the trailer body for 12 volts while the switch is on, each position if a 2 position switch. If you have voltage on one wire and not the other, having switched the switch to each position if so configured, the wire without voltage is the ground. If you have voltage on each wire as the 2 position switch is moved from one position to the other the ground is through the trailer body. If thing check good at this point the problem is in the fixture.

With 12 volts lighting there is NO Polarity as current will flow in either direction.

If you do not have voltage on either wire the problem could be a broken wire, at the fuse panel, or a dead battery. Start at the battery and test for voltage. If good move to the fuses panel and check Both sides of the fuses for voltage. If there is no voltage on one side of one of the fuses that is the bad one. If all checks out the go to the switch and check for voltage on both sides of the switch while it is turned on. If that is good go to the fixture and check for voltage on the wires while the switch is in the on position or positions.

The point that you lose voltage indicates the problem is between that point and the last point you had voltage. Switches can go bad as can wires brake, fuses blow, batteries go dead thus the need to start at the source and check from there.
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Old 11-22-2012, 11:39 AM   #7
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Quote:
Originally Posted by HowieE View Post
The general rule of thumb on 12 volt systems is the wire conducts the Positive side and the trailer body conducts the Ground side.
True

Quote:
Originally Posted by HowieE View Post
With 12 volts lighting there is NO Polarity as current will flow in either direction.
Not necessarily true. Current will flow in either direction, but you still want to hook things up correctly. LED lights are polarity sensitive and will not work hooked up backwards (incandescents will, that's probably what HowieE meant). OK, a qualifier on that, I have some LED bulbs designed to go into sockets that hold incandescents that will work backwards, something about how those are made. The LEDs that only have leads are polarity sensitive. The strip and roll lights have their polarity clearly marked. The "D" in LED stands for diode, and diodes by their nature are one-way devices.

Also, you really don't want to hookup electronic gear backwards. Fuses will usually save the gear, though. And HowieE did say that about lighting specifically.
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Old 11-22-2012, 12:00 PM   #8
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Problem solved Yipee! It was 2 blown fuses. They blew when I was "welding" in the bathroom around the 12 volt receptacle. My son said no more welding in the bathroom! LOL Next time I'll turn off the power when trying to disconnect the 12 volt to paint around it.

Something new we learned... 110 lights work on a 12 volt system. My son thought the fixtures and bulbs needed to be 12 volts to work.

OK well I hope someone else finds this helpful in some small way. Thanks again for all your input!
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Old 11-22-2012, 12:09 PM   #9
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Glad you solved it while I was writing. I don't see how it does any good for a 110 v. AC bulb to be used in a 12 v. DC system; they are made differently.

My original post for what it's worth: Auto and trailer wiring does not necessarily have the same color codes 120 v. wiring has. This leads to lots of confusion for people who are only used to residential or commercial wiring.

120 v. wiring uses black and red for hot, white for common and green or bare for ground. Common is often called ground also, but is not quite the same. In auto and RV's, black and red tend to be hot and white tends to be ground, but many things use any color short wire to the body for ground. There may be a wire going back to the distribution panel that serves as common or ground, or not. You'll also find wires that have a solid color with a different color stripe that may be hard to see. 12 v. wiring can drive people looney.

Either the red or the yellow could be hot. Use a voltmeter set for 12 v. DC and test each one using the body for a ground. Each wire may be for different fixtures, or one may be a ground. If there is no voltage in the wires, there is a problem elsewhere—broken wire, bad connection, fuse blown, bad switch, bad converter, bad battery, bad something. Or the light fixtures are bad or their bulbs are bad. The key to this is to check it with a voltmeter or test light as described in post #2.

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