# 7 prong trailer plug questions



## mmcmdl (Apr 14, 2022)

I have to replace the 7 prong plug on my larger trailer , half of it is worn away from road rash on a New York trip . Tractor Supply sells these plugs . My question , how to find out which wires go where as I only have half a plug and the plug is sealed ?


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## DAT510 (Apr 14, 2022)

Maybe this will help.






						7 Way Diagram - AJ's Truck & Trailer Center
					

We're happy to help guide our customers to the right trailer or snow plow for them. And we offer so much more than that! Stop into our Harrisburg, PA dealership today or call to learn more! (877) 855-5149



					www.ajtnt.com


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## ErichKeane (Apr 14, 2022)

Check out DAT510's diagram there, that is a good one!  At one point I found a much more complicated diagram and just figured it out with a multimeter.  It would have been 95% easier with someone to sit in the truck to turn the blinkers on though.

My truck's harness didn't have colored wires when i did mine, which made it particularly fun.


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## Cadillac (Apr 14, 2022)

Easiest way would be a meter test each wire. Don’t want to have to do twice.


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## Cadillac (Apr 14, 2022)

Usually it will tell you the wire breakdown on the plug cover on truck side. At least mine does


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## mmcmdl (Apr 14, 2022)

OK , going to head up to Tractor Supply and try to do this . Got to get this trailer road ready once again . Firewood for the campers may take off quickly !


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## RJSakowski (Apr 14, 2022)

My OEM MOPAR  plug is wired the same as Cadillac's.


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## Cadillac (Apr 14, 2022)

The plug wiring is gonna be universal for any seven wire plug. Not manufacturer specific.


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## mmcmdl (Apr 14, 2022)

Cadillac said:


> The plug wiring is gonna be universal for any seven wire plug. Not manufacturer specific.


I got the plug and will try by the diagram . Great news is , I saved $450 dollars !   I bought another pair of wire strippers and saved 15 hours of searching for my other 20 pairs . Life's good !


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## mmcmdl (Apr 14, 2022)

Got everything ready and had a terrible thunderstorm . Figured out there is only 5040 different combinations with wiring the plug . How bad could it be ?   Usually I have a 50/50 shot at wiring and see magic smoke appear before my eyes .


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## rabler (Apr 14, 2022)

mmcmdl said:


> Figured out there is only 5040 different combinations with wiring the plug . How bad could it be


What I do is try to confirm the   - (ground) pins on both ends.  As long as that is hooked up correctly pretty much anything else may cause odd functioning but won't damage anything.  (Famous last words)

 If you have a voltmeter, to confirm the ground pin, with the truck off: first confirm that there is no voltage between the pin in question and the truck frame (find something unpainted, try a few spots).  Then check to see that you have zero ohms (or at least something less than say 1/2 ohm) between that pin and the frame.  That verifies the ground pin on the truck.  Repeat for the trailer.


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## Dan Krager (Apr 15, 2022)

Oh my!  DON'T LET THE SMOKE OUT!  All things "electrical" run on smoke and if you let the smoke out, they just don't work. It's really hard to put the smoke back in. One usually has to start over with new stuff where the smoke is already sealed in.

In my experience, there are two "standards" for wiring a 7 way plug.  One is a "common universal" and the other is for RV.  RV trailers usually have a battery on board that needs charging, so the power to it is constant. It turns out that it's a matter of switching two wires.  Which two escapes my memory.  I have bad memory....

DanK


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## vocatexas (Apr 15, 2022)

In my experience, if you trace the plug wiring up into the rear frame, there is a 'T' plug-in there that the trailer light wiring plugs into, or a connector there to allow this. This is done because some pick-ups get their beds removed and replaced with custom flatbeds or boxes. It un-plugs the wiring for the pick-up tail lights. Even my old '76 F-250 is wired this way. If you need to replace the wiring you can go to NAPA and buy the plug harness and just plug it in. No stripping, no crimping.


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## Jake M (Apr 16, 2022)

mmcmdl said:


> I have to replace the 7 prong plug on my larger trailer , half of it is worn away from road rash on a New York trip . Tractor Supply sells these plugs . My question , how to find out which wires go where as I only have half a plug and the plug is sealed ?



The first answer is a simple ohm meter.  By the fact that you're asking, while I'm quite sure you know how to use one, I'm guessing that electrical calculations like counting filiments  and estimating electrical loads in your head is probably not on your list of things you enjoy....  So here's the (slightly) longer method.  It uses a battery, masking tape, and a couple of scraps of wire.  You could put a fuse in line if you wish, but you really can't "kill" anything (including yourself) if you make your first couple of "contacts" quickly with a brushing motion.  You'll know right off if you get a "lighting some lights" spark, or a dead short "you're doing it wrong" spark.  This will "break down" the trailer circuits into individual pieces so that you may eliminate 5038 of those 5040 possible combinations.  I say it's "longer", but it probably will take you as long to read this as it will to actually do this.....

Never rely on a color code.  While all the plugs are wired the same (usually, go with that standard.....), there is NO official standard for this.  That means manufacturers (and any previous DIY or even professional repairs) have no color standard to go by.  There are some "more common" colors for functions, but there are several common systems, and NO guarantee that any standard is followed.  Use the actual function.  It'll save your sanity.

First, find the GROUND wire on the trailer.  Follow that one to the damaged plug.  Attach that to your battery negative.  

Second, Functionally verify the ground.  Find another wire and quick touch, then attach it to the battery positive.  Go and see what lights up.  Don't do anything with that yet.  If nothing lights up, it might be your brake wire.  Ignore a null result FOR NOW.  Try a different one until you get lights.  Even just one light.  Leave it hooked up.  Now, find a SECOND wire (again, skip over any null results), and while still lighting the first function, light another function.

Option one-  You get two discrete lighting functions that work as they should.  You can carry on.  Option two, you get lighting results, but they are not discrete functions, you get odd combinations of things.  That will indicate that you've misidentified the ground wire.

Third, remove the two wires from the battery positive.  Use the tape to make a flag on the battery negative wire, and mark it as such.  Attach (one at a time now), one additional wire, holding out for a lighting function, ignoring null results.  Observe the function, make a flag, and label the wire.  You now have two wires identified.  5040 possibilities just got reduced to 120 possibilities remaining.  Now repeat until you find the remaining two lighting wires.  (They are ground, tail, left turn, right turn.  There is no "brake light" wire.)  Once you've identified those four wires, 5040 has become 6 possibilities.  

After this step, you will have four wires identified.  Ground, tail lights, left turn, and right turn.  (The turn signals will burn steady, they won't blink).  Also note, the turn signal and the brake lights are the same two wires.  In other words, the brake lights are the left and right turn signal burning steady at the same time.  So that function sorts it's self out.

When those are sorted, the remaining wires....  Could be only one depending on the age of the trailer, probably two, and possibly three.  Those functions,  brakes, auxillary power (to charge a breakaway battery), and reverse lights.  Being a commercially available harness instead of a hand made one (eveidenced by the molded plug), you MIGHT have extra wires that are unused.  That'll mess up your day.....  But you can't hurt anything...  Power them one at a time with your battery, but put the battery on the trailer.  Attempt to move the trailer.  One wire will lock the brakes.  That is your brake wire.  By the nature of electric brakes, it takes some degree of wheel travel to activate the brakes, so go a couple of feet each way before you give up, so you might want to attach it to a tow vehicle for this.  Just don't plug in the partial harness yet.  

So now, worst case, you've got two wires left.  Two possibilities.  One to charge the battery, and one for the reverse lights.  You could brute force guess at this point, or if you're missing wires (less than seven), process of elimination might tell you what feature is left based on what you have.

Reverse (which you've found if you have that feature, but probabbly not), and battery charge/auxiliary power.  Now you need the meter.  Or a light bulb.  Or the sketchy way, raking a wire lightly and quickly across a connection to see if it acts "shorted".   One of those two wires goes straight to the positive terminal of the breakaway battery.  The remaining one (assuming you probably don't have the feature) is a dead end wire that "could" power reverse lights. 


As with most things in DC electrical systems, the key to make it work is to break it down into pieces.  Working in a systematic way will get you to where you need to be.


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## chatter chatter cut cut (Apr 16, 2022)

mmcmdl said:


> Got everything ready and had a terrible thunderstorm . Figured out there is only 5040 different combinations with wiring the plug . How bad could it be ?   Usually I have a 50/50 shot at wiring and see magic smoke appear before my eyes .


find a friend . my buddy spent 3 days trying to wire his . with him up front working switches and turn signals and me with really cheap meter we had it sorted in about 10 min.


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## matthewsx (Apr 16, 2022)

mmcmdl said:


> OK , going to head up to Tractor Supply and try to do this . Got to get this trailer road ready once again . Firewood for the campers may take off quickly !


Nice looking rig there


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## mmcmdl (May 1, 2022)

2 weeks later . It is wired but not tested .


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## Cadillac (May 1, 2022)

Right on schedule


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