Let's talk a little bit about trailer wiring.

   Those of us who own and operate horse trailers know that the electrical system is probably the most trouble prone part of the trailer. It should be assumed that your lights and brakes are not working unless you verify that they actually do work.
   Why is that. The answer is primarily that trailers spend a great deal of time just sitting around waiting for the next time you and I decide to take our horses somewhere. While the trailer is sitting out in the elements the electrical connector (or plug) tends to get corroded. The pins in the plug get a coating of corrosion on them that prevents a good electrical connection from being obtained when you plug it in. The socket (female receptacle) on the tow vehicle that your trailer's electrical connector plugs into also falls victim to the same corrosive effects. Both the plug and the socket should be cleaned whenever you hook up your trailer and the lights and brakes should be tested before you head down the road. Always keep a piece of sandpaper handy for this purpose and clean the tow vehicle's socket with the vehicle turned off. After cleaning, plug in your trailer, start your tow vehicle and then check that all your trailer lights are working. If all is well, drive ahead slowly and apply the trailer brakes to make sure that they are working properly.
    (A light coating of dielectric grease on your connectors and in your light sockets may help in reducing corrosion.)
   Trailers tend to have rather cheaply constructed light fixtures. If a particular light is not working, try opening the fixture (usually by removing the lens with a screwdriver) and then removing the light bulb. Check for a burned out light bulb or corrosion or broken wires in the fixture. Repair or replace as necessary.
   Another problem that seems to pop up fairly regularly with trailer wiring stems from the wiring techniques employed by many, if not most, trailer builders. Whenever a wire (or wires) needs to pass through a piece of steel or aluminum, a hole is drilled. Drilling a hole in metal results in a sharp edge being created around the hole that should be reamed or filed smooth in order to prevent the plastic insulation on the wire from being cut. This is often not done and the wire (or wires) inserted in the hole is often damaged by the sharp edges resulting in a grounded wire. If you ground a wire that is "hot", then you have created a "short circuit" which will immediately blow a fuse in your tow vehicle. Depending on how your tow vehicle is wired you may find that you have no brake lights or turn signals or running lights on either your tow vehicle or your trailer.
   Not to worry though as all can be made well with a trip to your friendly automobile or trailer dealer and they can solve your problem for about $85.00 an hour.
   If however, you have the time, you may want to consider trying to repair the problem yourself.
   So what is the best way to go about it?

   First, read this caution and disclaimer.

    Caution, if your vehicle has a diagnostic indicator that tells you when a lamp is burned out or not functioning, you may need special wiring adapters and testers. Not using these adapters may cause damage to the vehicle wiring or computer.

   And:

   This article is provided for reference and informational purposes only.
The author and/or www.Kateshorses.com assume no liability nor responsibility for any damage or injury which may occur while attempting any of these procedures and offer no guarantee that any of the information contained herein is accurate. If you are unsure of your ability to complete a task, please retain the service of a qualified and/or licensed technician.

   It is always possible to stumble onto the problem by just sort of poking around in your trailer, however a step by step approach may be the way to go.
You will need a vom  to start the job. A vom is a volt ohm meter, and is available at any radio shack or similar store. A cheap one will cost less than $10.00. There are several scales (you want the 0-50 volt dc scale or something similar for checking voltage and the x1 (pronounced times one) scale for checking continuity) to choose from. By connecting the wire leads to the meter and using the probes to test the wiring you can make all the required tests. By connecting the black probe to ground and then touching the red probe to your wiring, you can observe the deflection of the meter's indicating needle to tell if you have voltage. By touching the 2 probes together and adjusting the needle to 0 with the zero adjusting knob and then touching either probe to ground and the other probe to one of the trailer wires you can test for continuity.
    Let's assume that your left turn signal is not working on your trailer. Take the light bulb out of the socket and see if it is burned out. If it appears to be ok you can test it with your vom as described below. If the bulb is ok you can test the fixture with your vom.  Make sure that the key is turned on in your tow vehicle and that the left turn signal is flashing. Then take your vom and switch to the 0-50 dc volts scale or similar. Connect the black probe to a metallic part of your trailer (chassis ground)  and touch the red probe to one of the terminals that normally touch the bottom of the light bulb. There may be one or two connection points and usually with turn signals the light bulbs are dual filament bulbs such as the 1034 (pictured) or the bulb may be a single filament light bulb such as the 1156 (also pictured).

    You should find 12 volts pulsing on one of the connection points. If you do then check the light bulb with your vom by switching to the x1ohms scale and while touching the 2 probes together adjust the needle to zero. Then touch one of the probes to the brass case on the bulb and the other probe to each of the solder connection points on the bottom of the bulb. In both cases you should read between 1/2 to 4 ohms. If you do then the bulb is good. Clean the bulb and clean the socket and reinsert the bulb and the bulb should light. If you have a ground and voltage the bulb must light if it has good connections. If you do not find pulsating 12 volts at the socket then it is time to dig deeper yet.

    The first step in troubleshooting your system is to familiarize yourself with it. To accomplish this you must identify the terminals in the female receptacle that is attached to your rear bumper or trailer hitch. The reason for testing is that no auto manufacturer has ever published a reliable wiring diagram and there is absolutely no industry standard as to which terminal does what. Ok, lets begin.
    Look at the female receptacle. It should look like one of the pictures below. Remember that the female receptacle is on the tow vehicle.
    It is also very common for manufacturers to use a 7 way plug and receptacle but to only use 6 wires, usually leaving the center terminal unutilized.

6 Way Systems

Round 1 1/4" diameter metal connector allows 1 or 2 additional wiring and lighting functions such as back up lights, auxiliary 12v power or electric brakes. Note: The black (12v) and blue (electric brakes) may need to be reversed to suit the trailer. Check with a test light or VOM.

7 Way Systems

Round 2" diameter connector allows additional pin for auxiliary 12v power or backup lights.

    Keep in mind that if your system is wired the same as any of these pictured connectors, it is purely coincidental.
    To begin with, get a piece of fine sandpaper and clean off the corrosion on the connectors. They should be bright and shiny.
    Next, using your vom, connect the black probe to a suitable ground. (a steel bumper or part of the frame after you have cleaned the connection point with sandpaper makes a suitable grounding point.) Get yourself a couple pieces of paper and a pencil and draw a picture of your receptacle as you are looking at it from the rear. With the tow vehicles key turned off and being careful not to touch more than 1 terminal at a time; check all the terminals inside the receptacle for voltage with the red probe and record your findings on the sheet of paper by labeling each terminal as either hot or not hot. Any terminal that is hot with the key off is being used for some purpose other than vehicle lighting. Perhaps it is being used for a switchable interior light or something like that.
    Now turn the tow vehicle key to the run position. Draw a new picture of your receptacle and again check all the terminals for voltage. Again record your results on your drawing. You should find that 2 additional terminals are now hot. Those are for your electric brakes and for your 12 volt dc which is usually used for interior lighting and as a battery (for the electric brake breakaway circuit) maintenance voltage.
    Ok, so far so good. Now turn on your left turn signal and again check for voltage. This time you will find, in addition to the voltage you have already found, a voltage on 1 terminal that is pulsing on and off. This is your left turn signal voltage. Label your drawing accordingly. Repeat the procedure with the right turn signal. Remember that the left and right turn signals are usually also the brake lights so it is not necessary to test the brake lights. It is necessary to repeat the test for the tail lights. Turn the turn signal off and turn on the tow vehicles parking lights. This will result in another terminal becoming hot and again label it accordingly on you paper. Now, with the help of another person, put the tow vehicle in reverse and check that the tow vehicle's backup lights light (it may be necessary to start the tow vehicle's engine) and then repeat the voltage test and see if any other terminals have become hot and label your paper accordingly.
    You should now have identified all the terminals inside the receptacle except for 1. That 1 is vehicle ground. Again, using your vom, turn to the resistance or ohms scale. Touch the probes together and adjust the zero scale, and then with the tow vehicles key turned off attach the black probe to vehicle ground and touch the red probe to the last unidentified terminal. The meter's needle should swing to zero indicating zero ohms (which means that the terminal in the receptacle that you are touching and chassis ground are connected together electrically). If this happens then you have identified the receptacles ground terminal.

    If you have a 6 way plug and receptacle you should have located 2 terminals that are hot with the key on. 1 terminal pulsing hot with the key on and the left turn signal turned on. 1 terminal pulsing hot with the key on and the right turn signal on. 1 terminal that is hot with the key on and the parking lights on. And finally, 1 terminal that is never hot but which has continuity to ground when tested with the ohms scale of your vom.
    If you have a 7 way plug you have probably found that you have an unused terminal that is never hot or grounded. It may however be hot when the key is off and used for interior lighting or some other auxiliary circuit or it may be used for backup lights.
    Remember that we have identified 2 wires as hot with the key turned on? We now want to determine which 1 of those 2 wires is used for the electric brake circuit. In order to accomplish this task we will have to verify that the wire is hot with the key on and then disconnect the electric brake controller from it's power source. If you are unsure of the connections at the electric brake controller, just take note of how the device is hooked up, and unhook it. The voltage at one of the connectors at the receptacle should no longer be hot indicating that the connector is for the electric brake circuit.
   Congratulations, you have identified the position and usage of all the connectors located in your female receptacle. This means that your tow vehicle seems to be working correctly and has no blown fuses or broken wires.
 
    If you are unable to find a certain function (for instance you can not find a connector that gets hot when you turn on the tow vehicles lights) then you will have to correct that condition at the tow vehicle before you can proceed to the trailer. Hopefully the problem is only a blown fuse or a broken wire because solving problems more serious than these will probably be beyond the abilities of most novice electrical troubleshooters.
    Get out your owners manual and locate the location of the fuse panel on the tow vehicle. It is usually located under the dash or under the hood and sometimes there is a fuse panel in both locations. The owners manual will locate the location of the fuse that supplies power to the connector that is not working. Remove that fuse and check it for continuity. You can do this by switching your vom to the x1ohms scale and after zero adjusting your meter touch one of the probes to one of the pins of the fuse and the other probe to the other one. The needle should swing to zero indicating electrical conductivity and hence, a good fuse. If the needle does not deflect, then the fuse is blown and needs to be replaced. Once the circuits have been repaired and all is well at the trailer receptacle you will then move on to the trailer wiring. If you did discover a blown fuse, the question is then raised; why did the fuse blow. If you have replaced a blown fuse and the fuse blows again that indicates a tow vehicle wiring problem, but if the replacement fuse does not blow again, with the trailer unplugged, then more than likely the trailer has a problem that causes the fuse to blow when you plug the trailer in and turn on the device that the offending fuse powers.

   OK let's review.
You should have located these functions:  
1. A wire that is hot with the key on.
2. A wire that is hot with the key on and the electric brake controller hooked up.
3. A wire that is hot (and flashing) with the key on and the left turn signal turned on.
4. A wire that is hot (and flashing) with the key on and the right turn signal turned on.
5. A wire that is hot with the key on and the parking lights turned on.
6. A wire that is never hot but has continuity to chassis (or frame) ground. (continuity means that the wire is electrically connected)

Optionally you may have found:
1. A wire that is hot all the time. (may be used for trailer internal lighting)
2. A wire that is hot with the key on and the tow vehicle shifted into reverse.

   The great majority of all horse trailers produced in the last 20 years or so will only use the first group of circuits (1-6) and those are all that you need to verify.

The trailer
    You have verified that the tow vehicle's trailer receptacle (female plug) is working correctly. All the necessary voltages are present when and where they are supposed to be.
    Let's make another picture on another piece of paper. We will illustrate the male trailer plug and define the connections and the wire colors attached to each connection. Each wire has a position on the plug corresponding to a position on the tow vehicles female receptacle. Begin by orientating the plug with the tow vehicle's receptacle and observing how it would plug in and which connectors connect to which connectors. Now that we know how the plug is supposed to plug in, remove the cover from the plug by removing or loosening the screw or screws and sliding the cover back over the cable. This will reveal the wires and the way that they are connected to the pin terminals in the male plug. Make sure that there are no severely corroded or broken wires. Make a picture of your trailer plug as viewed from the rear of the plug when it is plugged into (this is the least confusing way) the female receptacle. It should be apparent that each wire corresponds to a tow vehicle wire. Illustrate each trailer wire on your paper and define each wires position, color and function.
    It is now time to begin the actual troubleshooting. Up to this point you have been defining and familiarizing yourself with your particular trailer wiring. To begin troubleshooting you need to ask yourself what are the symptoms of your problem. You obviously have a problem with your trailer lights or you wouldn't have started this process to begin with.
    Let's say that everything works except the tail lights. Since the other lights are working, you know that the ground wire is ok. If you had no ground then none of your lights would work. Plug in your trailer plug with the cover still removed. Locate the wire that you previously determined to power the tail lights. Locate the wire that you previously determined is the ground. Switch your vom to the 0-50 volts dc scale and touch the black probe to the ground wire and the red probe to the tail lights wire. Do you read 12 volts?
    If no, then you probably blew the fuse again when you plugged in your trailer. Unplug your trailer and check the fuse again. If it is again blown then you must trace the tail light wire through your trailer to find out where it is cut and touching chassis ground resulting in the short circuit which is causing the fuses to blow. You will probably find that the insulation on the wire has been compromised, probably in a place where the wires pass through a structural part of the trailer. Find the bare wire and reinsulate it with some electrical tape and file off the sharp edge that cut the insulation in the first place. Replace the fuse and try again. Your problem should be solved and if not just repeat the procedure until you have solved the problem.
    If yes, then the fuse and the tow vehicle are still ok and you will have to trace the wire through your trailer until you find the broken wire or loose connection that is preventing the 12 volts from getting to the tail light fixtures. Repair the problem when you find it and retest your lights.
    See, it is not that hard after all. Like most jobs that appear very complicated, once you begin systematically and patiently familiarizing yourself with the system, it is really not all that complicated at all.

    Here are some sites offering wiring diagrams of interest:    offroaders.com/tech/trailer_wiring-diagram.htm
                                                                                           accessconnect.com/trailer_wiring_diagram.htm  
                                                                                           horsetrailerworld.com/home/wiring

    Below are some interesting charts and diagrams which may or may not help you in your task.
Note:  You normally will not see 4 or 5 connector plugs associated with a horse trailer as they have not enough pins to accommodate the needed accessories (interior lights or electric brakes for example).

sources: accessconnect.com/trailer_wiring_diagram
              horsetrailerworld.com/home/wiring

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