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  12 Volt Wiring
  There is no reason that any attentive, diligent person cannot resolve any problem arising with trailer wiring, which is very simple. Many of the techniques presented here apply equally to cars, but those are far more complex, there is more opportunity to screw up. First, a few Cautionary Statements. 

Caution - It is quite possible to ruin part(s) of the tow vehicle's electrical system by carelessly grounding something that should not be, or without correctly evaluating what is appropriate for every component affected by the project, or by mistakenly connecting the wrong wires. It is not acceptable to fool with these things without understanding and paying very careful attention to each detail. I don't allow "professionals" to touch my wiring - ever. Even those who know what good is are under pressure to complete the project as fast as is humanly possible, and that is not consistant with doing it well. Careful thought and careful detail work get it done right.

Some newer domestic models and imports may require a special interface that prevents damage to the onboard computer that monitors the vehicle's light system for faults. If wiring a vehicle that has such a system, special protocols must be followed, and I have no specific knowledge of those.

What is presented here must not be considered a complete explanation of anything - just some methods that may be useful. The sites linked on the "Trailer Links" page might fill in some of the gaps. Proceed with due caution.

  The basic layout - This schematic is reasonably "generic", and it helps to see things in these terms, rather than as "a bunch of wires" that go "somewhere". It assumes that the lights can all find a good ground in the sheet metal that they attach to. Also, it does not take into account the marker lights at the top corners/edges, or the three required by law in the middle rear on vehicles wider than - I forget - maybe 78 inches. Those are all pretty much self evident, they tie in to the tail/marker light circuit wherever convenient.

I strongly prefer a fairly heavy gauge of wire - 12, or even 10 gauge - for the main feeds in the tail/marker circuit because it pulls quite a bit of power, especially on larger trailers. Naturally, where it branches off to an individual light there is little need for that.


Generic diagram of trailer wiring
  The brakes must be wired with a heavy gauge, I would not consider less than a 10 gauge, all the way to the unit in the tow vehicle. They should have a separate ground wire, although it seems perfectly effective to allow the other functions to ground back to the tow vehicle through the hitch. (I have seen rubber receiver inserts for sale which might queer that deal with that kind of hitch.) 

In the case of a tow vehicle/trailer combo that both use dual filament bulbs for tail/brake/and turn signals, (usual) then the turn signal circuits also activate the brake lights - same wires, same filament.
Caution - In the case of a tow vehicle that uses separate bulbs for brake and turn signals there are issues outside of my experience, and I suggest that you take no action touching on those without understanding the matter completely.

  Troubleshooting - A decent meter is almost a must. It needs not cost much, Harbor Freight has some for cheap, although I am quite fond of this one, a "midrange" Sperry. The main thing is that it have at least high and low range readings for DC volts, and high and low ranges for testing resistance (Ohms).

  Commonly the problem is obvious, a burned out bulb, corrosion, etcetera. If not, testing is just a question of logically zeroing in on the problem, section by section. Burned fuses are in a grey zone. If it happens once it may not mean anything. If it happens instantly there is a serious problem. If only occasionally, it is likely that there is just a little too much load on that circuit, and it could be a candidate for a relay, which are discussed farther down this page. Lights that sometimes work and sometimes don't, or that are dim or flicker certainly have a poor connection. Sometimes it is on the feed side, usually corrosion in the socket, but that sort of symptom normally indicates a problem with the ground.

When checking for the presence of power in a circuit, or for bad insulation, (resistance - ohms) the meter must be clipped to a good (reasonably clean and well connected) metal ground or the reading will not be reliable - if it will read at all. The quality of that connection can be tested by setting the meter to ohms and touching some other bare metal part with the probe. A good ground will read zero, no connection reads "1", denoting infinity.

There is considerable overlap between car and trailer electrically, but for now I'll assume that there is no electrical problem with the tow vehicle itself. Naturally if it is only one or two running lights or one or two brakes not working, the trailer itself is implicated. On the other hand, if there are no running lights at all, or perhaps a turn signal doesn't work but the bulb and it's socket appear good, then start at the plug-in on the tow vehicle. A suspect area is where the trailer connector is tied into the vehicle's wiring - this is often done badly.

  Set the meter to the high range DC volts and activate the functions one by one. (It will be helpful to take notes - a map of which contact does what.) Don't expect every contact to do something necessarily. One will normally be a ground, and another unused. (The multiple bulb issue mentioned above would be a likely exception.) A helper is convenient, but not essential. Any number of primitive expedients can be devised to hold the brake pedal down, or to activate the trailer brake control. Plug-in socket on tow vehicle

  It helps a lot to find a way to prop the cover open while getting readings - the tiny vice grip works nicely in this case. 

  Following the no running lights scenario, in all likelihood most/all of those lights on the tow vehicle will also be out if a burned fuse is involved. If yes, the choices are to replace it and see what happens down the road, a little questionable, or better, to figure out why this happened. Conceivably just a bad quality fuse, probably something grounding in the trailer, possibly an overload (too many lights for that circuit). 

If there was no problem with the fuse, no problem with the lights on the vehicle, and no indication of that function being present at the plug-in above, it's time to get dirty tracing the wires from the socket to where they are joined into the vehicle's wiring. As mentioned above, this is a good bet. 

Sometimes these connections are made using a "power tap", which is almost as bad as quick and dirty gets. The problems with them are explained in the "splices" section below.

If those splices have been done well, and really do look good, don't rule this out entirely, but shift suspicion to either the connection inside the socket, (wires can pull loose) or to wires damaged badly enough that this should be visible upon inspection.
The wires are clamped in their sockets by screws. The insulation on these should be stripped only far enough that when inserted fully the bare wire does not quite emerge, or is just visible. "Tinning" the ends with solder could help a little, but is not essential. Just be sure that there are no loose strands, period. Interior componant of plug-in socket

If there is a fuse that burns regularly (or instantly) set the meter to ohms, high range.

[Using the low range could be misleading, because there is sometimes a bit of "leakage" in cars - a sensitive meter will detect some slight connection between many circuits that, in principle, should be completely separate. To digress a little more, a sensitive meter can take a reading from people, when on the lowest range DC volts, and by concentrating "just right", it is possible to increase the reading. Place both contacts on either side of your head, and experiment a little. I find that I cannot make it go down, but only up, unless I ground myself, touching the faucet of a kitchen sink will accomplish that.]

Anyway, to check whether that fuse burned because the wire(s) coming from it have grounded somewhere in the vehicle, turn all functions off, and probe each contact as in the photo above. In theory, only one should be a ground. If more than one is, don't jump to conclusions, just set about understanding why.

Another thing to check at this point is whether the problem arises of some of these circuits becoming "cross-connected". With everything still turned off, and the meter still set to ohms, connect the meter to one pin in the plug, and probe all the others. Then connect to the next pin, and probe the others, and repeat this until all have been checked this way.

  For that purpose, an item like the one on the right is very helpful, also for testing continuity within an individual wire, and for identifying a circuit from the opposite end. I forget where I found a container of these clips, maybe at Home Depot, auto parts houses would also be a likely source. Insert a piece of wire into the end, crimp it in, and tape it.

The nose of this clip being narrower, it can attach in places the meter's clip cannot, simply
connect the meter to the bare end of the wire. Just take care if connecting this to anything "hot" that it cannot even possibly contact a ground - this would probably only burn a fuse, but don't count on it. Not all systems are protected as well as they should be.

  Assuming there is no indication of inappropriate grounding, or cross-connections, move to the trailer. It is usually possible to see which side of the light fixture is "hot" and which is the ground. Ordinarily it is possible to test for continuity between the hot side and a sheet metal or structural ground. But - sometimes, as in the case of the running lights, shown in the schematic above, these wires are all tied together. Further, all the other bulbs on this circuit provide a path to ground through their filaments. Identifying the problem here may not be easy. Tedious persistence pays off, isolate suspect portions, and test them one by one.

  If some lights work perfectly and only one or some don't, naturally, start at the light itself. On the whole, I have had more trouble with bad grounds/corrosion than with bad bulbs or bad wires.

  Matching new trailer vehicle combo - or - where does this wire come from?
Find a wire long enough to easily reach from the trailer's plug to the rear lights and strip an inch of insulation from each end. Fold a bare end back and insert it in one of the holes in the plug. Remove the tail/turn signal bulbs, set the meter to ohms, high range, and connect it to the other end of the long wire. Probe the contacts in the the the bulb sockets. There is a better than average chance that one of them will complete a circuit without having to move the wire to another hole in the plug. If they all complete a circuit there is really a problem, so check them all just in case.
  Make a sketch of the plug, and note which contact in it corresponds to which contact at the rear. The tail light's contact can be differentiated from the turn signals at the plug because the sockets at the rear both have a tail light contact. In other words, one contact in the plug will make a complete circuit to both light sockets, whereas the signals will only complete a circuit once in each socket.

The ground contact in the plug is easily identified by probing both bulb sockets themselves, not the contacts within them. (The lights ground through the sockets.) Like the tail lights, both sockets will complete a circuit through the long wire to one contact in the plug. (Of course, the whole problem may be that some wires are grounded which should not be, or which are contacting one another and should not be, so checking all the contacts is good policy. Making notes about the relationships discovered this way helps to clarify exactly what is going on.) 

A helper will save many trips to relocate the wire end in the plug, but unless this person is experienced, or a quick study who gets it right, this may only create confusion. In some instances the clip shown above right may be useful, twisted together with the long wire. 

  Wiring: Splices -  The insulation must be stripped with some care at splice points to avoid nicking the individual strands with the tool - this not only weakens the wire but increases electrical resistance. Probably not very significant, but why do it carelessly when it is easy enough to do it right?

  The wires are laid across one another to form an "X", then the end of each is twisted around the other, in opposite directions. Each turn should be tight, smooth, and regularly spaced. There must not be any loose ends, and the ends must not be "bristly". Particularly with larger gauge wires, this will require smoothing the ends down with "alligator nose" pliers. Remember how some years back fighter jets were crashing because of chafe in the harness bundles?

  This is an acceptable join - not perfect. It would require considerable strength to pull it apart, even before taping, and the ends will not chafe if taped well.

  The "Y" configuration is needed fairly often. Using the insulation tool above, make two cuts an inch or so apart, (longer on large size wires) and carefully slice the insulation between lengthwise. Strip a long end off the joining wire, wrap and smooth it tightly with the pliers.

  Connectors - I have a very low opinion of these. There is the chance of the wire pulling out, corrosion is always a problem, and even taped they do not merge smoothly in a harness.

  There is no getting around the need for this sort now and then. Crimping the wire securely in place is the trick with these. Care is needed.

  I prefer this design if there is any tendency whatsoever for the wire to pull on the connector. With these, it is possible to push some extra length of bare wire through the center and wrap it tightly around the neck before crimping. No loose ends, and then tape it to be certain it stays that way.

  The "power tap" is placed over an existing wire, and a new one is also inserted. It has a little guillotine that slices through the insulation (and wire strands) on both of the wires when it is closed, achieving a connection. Of sorts. This does nothing to shield the wires from air and moisture, and so the connection is prone to corrosion. 

  Further, here are the autopsy results, showing enlargement of some individual strands of wire carefully removed from the power tap above. These are only some of the damaged strands, and are not only high risk candidates for breakage, but electrical resistance is increased even before they break. This will cause heating in that area. (The colors in this image are inaccurate - photographing things this tiny seems to present special challenges.)

  Taping - Before working with the wire at all the hands must be washed with great care. Even the oils that make finger prints will prevent the tape from adhering and sealing as well as it might. Especially when handling the tape itself this will make a dramatic difference. (Never touch the sticky side.) Taping is usually seen only as a method of preventing the electricity from escaping, but corrosion is always an issue, and can be completely prevented by the correct taping method. 

Borrowing from deep well submersible installation/maintainance, the most reliable way to seal the splices underwater is with a very careful job of taping, using good quality tape. (There are products on the market designed for greater speed and ease of installation, but these sometimes leak.) In the case of the car or trailer there is no need to use quite as many layers of tape, but the same rules apply. Tight, smooth, regular wrapping, with an overlap of almost half the width of the tape. Generally - different situations call for different strategies, but always enough layers of tape to ensure that the wire will never chafe through. Of equal importance is the quality of tape used. 
The 33M is not cheap, and is not necessary in many instances. BUT - when it comes to sealing the splices, wrapping smoothly and tightly around irregular surfaces, it is the number one product, period.

On the other hand, some parts of any large project will eat up a lot of tape, and Harbor Freight has some that is satisfactory for general purpose at fifty cents per roll when on sale.

  This is one of the places where the toughness and elasticity of the "super 33" is invaluable. Wrap around tightly enough to stretch the tape into position to come back around without wrinkling or folding.

  Then under and along the wire, back up it past the juncture, back down and around and up the other branch, and back down, etcetera.

  Eventually it is like this. I would not advise it, but feel confident that this could be immersed in water for quite some while without loss of function.

  Making new harness - In two cases I found that insulation had gone bad in a very obscure way. There was power escaping from some wires, and finding a way into others. Disconnecting them at all points did not stop this. If power was sent into one, others would read voltage. In both cases they were more than twenty years old, but there was no visible deterioration of the insulation. The only solution was to replace those portions of harness. The sort of "harness" usually offered at auto parts houses will not do for a "real" trailer, and the better cables from trailer supply houses is all right. The project outlined below involves a little more effort, but there are the advantages of heavier wire gauges, otherwise unavailable, and conduit to protect the wires from damage. 

  The spools unwind from a rod as with conventional (120-240v.) wiring jobs. Taking care to keep the wires in a straight line, and straight relative to one another, tape them together at intervals of about one foot.

  A good grade of automotive heater hose makes an excellent protective conduit. Extra effort in keeping the bundle straight (it will try to curve) is rewarded when stuffing it into the hose.

  The hose will also try to curve while inserting wires, and putting something heavy on an end allows it to be stretched straight. Even so it will prove quite difficult to push a bundle of wires through much more than a five foot section at a time

  After the wires are in these sections are taped securely together to prevent any possibility of them pulling apart, or of moisture entering.

  The finishing touch is sealing the ends. Pulling the hose back, and using the cheap tape, wrap tape around the bundle to form a plug with a little silicon gasket maker in the middle..

Then, still using the cheap tape, apply an outer covering to join it to the hose.

Relays: New loads on the tow vehicle's electrical system (which it is not designed for) are created when the numerous electrical functions of a trailer are added on. The trailer's marker and tail lights would normally be on one circuit, and the power that feeds them comes through the headlight switch, in addition to the tail and marker lights of the tow vehicle. Even with a two horse this will approximately double the amount of power drawn through that part of the headlight switch, and larger trailers pull significantly more, having more lights. This is one place where a relay seems called for. That is essentially just a switch that closes when power is sent to the correct contact. Caution - Installing a relay is something that must be done right, otherwise there is some risk of causing serious problems.

   The indicated contacts (instructions come with it) are connected to a switched circuit, a ground, and also to separate source of power. When the switched circuit is activated, the relay draws only a very small amount of power from that source to close it's switch, which sends power from the separate source to the fourth contact. This way the power sent to the trailer's lights is taken from the separate source, bypassing the headlight switch entirely, and eliminating any risk of overloading it.

  Protect that with a suitable fuse on the "feed" end of the wire from that separate source. There is risk of fire or other damage without that. Fuse holders are available for both the glass tube and spade style fuses at any auto parts house. The wrong choice in fuses might have serious consequences... Be sure of making the right ones or leave it alone.
Fuses must have enough amperage capacity to provide plenty of power to do the job - BUT - little enough capacity that the fuse will burn, (this occurs internally only) opening the circuit before anything else starts to burn externally. For all the billions of our tax dollars spent to assure the flame retardant properties of vehicle interiors, etcetera, a wire that starts to burn for lack of the right fuse can take the whole car with it in short order. So choose wisely, young Skywalker.

In General - There are many types of fasteners that will improve the look and ruggedness of the system. It might be wise to put a slight "S" bend or a little extra length between hard fasteners such as these, to allow for vehicle flexing, although things must not be allowed to move into positions where they could get pinched.
The nylon zip ties are available in a broad variety of sizes, and can be very helpful. The wires need support, including where they approach lights, and so forth, so they do not pull on the connection inside the light socket.

Single wires, for example, those that run up to the top marker lights, can slide into quarter-inch vinyl drip tubing, which will protect them from damage. Push it through sections of maybe eighteen to twenty-four inches at a time, and tape them together. 

Use a rubber bushing any time a wire runs through sheet metal. These can be found in almost any auto parts house. Take care that the lips of the bushing are seated fully out on the sheet metal around the hole on both sides. Sometimes they don't go all the way through, and may work loose in time.
Dielectric tune up grease can be put very sparingly into light sockets to prevent corrosion.

I am not recommending that anyone else should, am just mentioning that I often use multi-strand THHN, rather than "conventional" automotive wire, for better or worse. This is gasoline and oil resistant, and so it is suitable from that point of view - BUT - the strands of wire are not as flexible, which might be a poor choice in some cases, such as sudden bends or too much vibration, and it should not be subjected to much heat. Also, it is harder to make a really smooth splice with it, as discussed above, and its abrasion resistance may be less. We makes our choices and takes our chances.

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