7-way trailer plug

Hello everyone,

My 7-way trailer plug connector at the TV was corroded pretty bad (I don't know why; it's only a couple of years old). Anyway the kit that I ordered came with both the plug for the truck and for the trailer, so I decided to also replace the trailer plug. But I'm a little stumped with the connector plug at the trailer, the positions of the Black and the White wire on the connector are reversed! (the white wire is screwed onto the terminal that is stamped "Black"; and the Black wire is screwed onto the terminal that's stamped "White". What's going on here? is this an error from the factory or the previous owner? Or is this the correct way that hilos came wired form factory? Any help with this wiring question will be greatly appreciated, Thanks.
(1989 hilo - 2589RD).

In trailer wiring, the white wire is normally "ground" or really "negative" (back to the negative terminal of the battery). The black wire is normally +12V or "positive", (from the positive terminal of the battery). Now, the black wire in the trailer should mate up with the black wire on the tow vehicle's plug and the white wire in the trailer should mate up with the white wire in the tow vehicle's plug, when they are connected.

I can't imagine a prior owner reversing this wiring and I doubt the factory wired it wrong. Check how the wire colors match up if the trailer is plugged in to the tow vehicle. Because pictures don't often make it clear if you are looking at the back side of the plug or the front, the wires can appear to be reversed.

- Jack

Jack,
Thank you for your reply and your help with my questions. I am still a little confused about the 7-way trailer plug; I have attached a picture of a trailer plug wiring from a 1990 hilo owners manual, can you please take a look at it and let me know why in parenthesis it says #10 black, 12v. power supply ("White marked Black"); and the White-Ground says in parenthesis (Black marked White). What do these markings mean? Do I put the black wire on the post marked "White" and the white wire on the post marked "Black"? I think this has confused me and so I installed the Black and White wires reversed and on the wrong posts; I think I need to switch these wires back and match them to correspond with the Black wire to "Black" stamped on post", and White wire to "White" stamped on post" on the plug. Please let me know if I'm going about it right. Thank you again for your help.

Angel, your diagram shows the front (or outside) of the trailer plug and it has a raised part that I will call the "top" or, using a "clock" reference, the 12:00 O'clock position. The +12V input to that plug should go to the blade that is at the 10:30-11:00 O'clock position, the one in the diagram that is marked "#10 Black (white marked black). The negative return to the tow vehicle's battery should go to the blade in the 4:30-5:00 O'clock position, the one marked "White-Ground (Black marked White)".

I have no idea why HiLo would have used this confusing terminology in that diagram. It makes no sense to me. Your trailer's black wire should go to the 11:00 O'clock blade plug and the white wire should go to the 5:00 O'clock blade plug.

Here's a fairly clear diagram of both the tow vehicle and trailer plugs looking in to the outside of both that I found: https://www.ajtnt.com/Info/7-Way-Diagram There are many more online.

- Jack

Jack, on the trailer 7-way plug I connected the black wire to the "black" terminal and the white wire to the "white" terminal. I believe the trailer plug should mate up with the truck OK now. But then... I followed the wires into the trailer battery compartment and noticed that the black 12V positive wire ends up connected to what appears to be a grounding post. If that black wire is a positive (+) charge wire, why is it connected to that grounding post on the trailer? Wouldn't this create a short in the circuit? Now I'm very confused! I have attached a photo of that grounding post. The eyelet at the end of the black wire at that post is bigger than the other wires that are connected to the 30 amp fuse/relay that is right below it, so I'm thinking that it would be too big for it to go on that 30 amp fuse/relay. If you can please take a look at the photo and let me know if it makes sense. Sorry for the long explanation, I'm trying not to fry the entire wiring system. Thank you again.

Ugh! That device all those wires are connected to is a 30A Circuit Breaker (C and it provides overall protection for the 12V DC system. The screw that the Black and Brown wires are attached to in the first picture appears to me to be just a mounting screw for the CB, and I don't think it should have ANY wires attached to it. I'd expect all the wires to be connected to the two posts on the upper part of the CB.

I'm going to see if I can find a wiring diagram for your trailer if you're reading this right now, so standby.

OK, I didn't find anything for 1989 models, but we have the owner's manual for 1990 models and I'm pretty sure yours should be wired the same. Here's a link to the one I found and the circuit diagram is on Page 17: https://www.hilotrailerforum.com/f49/owners-manual-1990-25ft-classic-2016/

It shows the confusing trailer plug diagram you have and it has ONE CB in the battery compartment.

But, the diagram shows only 4 wires connected to the CB. You appear to have 6! The diagram shows the black +12V charge wire from the trailer plug connected to the left pole of the CB, along with the blue wire to the refrigerator and another #10 Black wire to the fuse block. The fuse block in turn distributes power to the interior lights, fan, water pump and lift switch. The right hand pole of the CB transports the +12V charging current to the +12V battery pole, via the motor solenoid attachment.

There are NO brown or red wires connected to that CB in the diagram. There ARE red and brown wires coming from the trailer 7-Blade plug, but they should not go through the CB - they are power to the trailer tail lights (turn signals and brakes I think).

You have a 7-Blade Bargman plug on your trailer, and the lower part of the wiring diagram gives blade numbers for the various wires. The Black +12V wire should be on blade #4, and the White negative wire should be on blade #1. This agrees with the plug diagram link I gave you in my last post too.

Can you trace the Red and Brown wires that are on the CB and see what they feed? They could possibly go on the other post that has the single Black wire that probably goes to the motor solenoid. They would get their feed from the battery or the +12V charge line that way through the CB. I don't think you want ANY wires attached to that third "mounting " screw.

If you have a multimeter, it would be good to use it to verify where the electricity is being sent.

The BIG thing you don't want is to have the +12V charging line going to trailer ground and the White wire going to the +12V post of the trailer battery. That would be the same as hooking jumper cables from one car to another incorrectly, and would cause a huge surge of current through the wires.

Possibly, someone with a trailer close to your model year might jump in and show or say how their CB is wired. I hope that will happen.

Oh, I meant to add that I referred to "left" and "right" poles on the CB. It does not matter which pole you label "left" or "right", the CB is not polarized, current can (and does) flow through it in either direction.

AND, looking again at your pictures, the third "mounting" post on the CB may not be screwed into metal? If so, it would NOT ground the +12V feed, but would simply send it on through the Brown wire, to whatever it's connected to. Can you check this out? Maybe the Brown wire brings current back through the Red wire to the CB? If so it is essentially the same as the diagram.

- Jack

Jack,
btw, of the 2 wires you see in the photo at the grounding post; the black wire you see is actually the wire that comes from the 7-way plug "Black" terminal position. Yup I'm confused!! my wiring knowledge doesn't let me understand this! I'm confident and hoping that you can help me with this. I haven't connected the TV for fear of shorting and frying everything.

Jack, this is a link to a post that you helped a hilo owner last year, and his wiring at that grounding post has the same 2 wires that my hilo does, check it out please.
https://www.hilotrailerforum.com/attachment.php?attachmentid=8889&d=1617148912

Jack,
Sorry for all the post replies I'm writing you, I'm just trying to supply as much info from old/previous posts that I found. If it helps, here's another post with a similar problem that you helped a hilo owner with.
https://www.hilotrailerforum.com/f51/1994-towlite-re-wiring-7545/

Angel, I added quite a bit to my previous post while you were posting your last three. DO see if you can see what the Red and Brown wires are connected to. There should be three Black wires at the CB, one of them is the +12V feed from your tow vehicle.

And, I agree, the wiring in the second post you added looks like what you have, although it is pretty hard to see it clearly due to all the dirt and corrosion. So, it looks like HiLo added those Brown and Red wires, but didn't update their wiring diagram.

- Jack

Jack, thank you sooo much for your help, if you look at the 1st photo I posted before I disconnected any wires from the CB or the grounding post, you'll notice that there are only 4 wires connected to the circuit breaker, not 6 wires (1 wire on the left side goes to motor/solenoid/battery, and 3 on the right side). The red and brown wires on the right side of the circuit breaker head towards the back of trailer in the lower section. Then there are the other 2 wires shown in the photo that attach to that ground post, one of them being the 12v + black wire in question from the plug harness.

Yes, the third "mounting" post for the CB is grounded to frame/body. That mounting post holds the circuit breaker in place along with the other 2 grounded wires as well.

Angel, it makes no sense to me to have the +12V input from the tow vehicle going to a ground point. That would keep you from charging your trailer's battery, at a minimum. And yes, I know there are only 4 wires going into the CB itself - the problem is, the wiring diagram says that three of them should be Black, +12V power wires and the other should be the Blue line to the Refrigerator. There is no Red wire in the wiring diagram there.

One of YOUR Black CB wires should go to the motor solenoid. I think it would be the single one on the lower "left" screw. The other Black wire (attached to the post with the Blue wire should go to the fuse block. Do these wires go where I am saying? If so, the third Black wire that is attached to the mounting screw should go to the same post as the Blue-Black combination, according to the wiring diagram.

Again, I have no idea what the Red and Brown wires are for, and it makes absolutely NO sense to me that a Brown and Black wire are grounded.

I'm not able to offer any better advice until you, or someone else, can let me know what the Red and Brown wires serve. I've looked through owner's manuals through 1997, and the wiring diagrams in all of them do NOT show the Red and Brown wires. This looks like a change that HiLo made that was not documented. Possibly, J&R in Ohio might be able to shed some light on this, but I'm not able to with what I have access to.

If you disconnect the Brown/Black combination only, leaving the others attached, you should be able to plug your trailer into the tow vehicle. Then, with the engine on the tow vehicle running, you could test the loose Black wire for voltage with a multimeter. It should read battery voltage, and if it does, it should NOT be grounded. This test would not damage anything.

The Red wire might be one that delivers power to the lift switch. It does in my trailer, but that is not shown in the wiring diagram for yours. If you disconnect the Red wire and the lift system does not operate, you will have found what it's for, and, it can stay connect to the CB.

This just leaves the Brown wire, if the Red is part of the lift system. My trailer has a Orange/Black stripe wire that goes to the breakaway switch to apply trailer breaks in case the trailer becomes disconnected, It is connected to the same post as the Refrigerator wire. Again, it would not be grounded if it is part of that system.

- Jack

Later on this afternoon I will take a closer look at these wires and use my multi-meter to test and trace them. I will post a couple of pictures showing the "White" wire on the 7-way plug/harness in the battery compartment that is not grounded like it normally should be; instead it is used for 12 volt power to the breakaway switch and other sources.

I don't like the white wire being used that way at all. I don't see how it even gets any power to deliver. It comes from the negative post of your Tow Vehicle's battery.

- Jack

Jack, remember that my original post question/concern was that the white and the black wires at the plug were switched and I wanted to know why. I guess maybe certain years hilo were wired purposely this way?? That kinda explains the black being used as ground and the white for power. The connections appear to be original and not modified. I removed the electrical tape from the white wire connection so you can get a better view of what's going on. On the white wire connection/junction one end goes to the breakaway switch and the other end to the CB. Here's a picture.

Jack, here is the kicker...everything electrical worked OK with the Black and the White wires being switched/reversed at the plug. I don't know why it was done that way, I'm kinda tempted to rewire the harness the correct way but then again, if everything is working OK I may have to live with them 2 wires reversed at the 7-way plug. I just have to remember that they are reversed at the trailer plug if I ever needed to replace that plug again. What do you think? Thanks.

I don't like what I'm seeing in the first picture. It appears the white wire terminates in a ring or similar connector that is spliced onto the black wire (very near the center of the picture). That is a horribly amateurish connection and I can't believe HiLo ever did that kind of hookup. It also makes no sense to me.

Again, if you look at the text for Bargman plugs in the lower part of the wiring diagram, it shows the White Negative wire coming from blade #1 and the +12V Black Wire coming from blade #4. Use your multimeter and see if blade #4 is hot and blade #1 is negative at the tow vehicle. I bet they are. Your trailer plug would have to match this. The only way it would be different is if the trailer is wired for Positive ground. I don't think HiLo ever did this.

Without having your trailer physically in front of me, I'm not able to do more than give opinions based on the wiring diagram and what I know of standard uses for wire colors, i.e. Black = +12V, White = Neg.

- Jack

Angel said:

Jack, here is the kicker...everything electrical worked OK with the Black and the White wires being switched/reversed at the plug. I don't know why it was done that way, I'm kinda tempted to rewire the harness the correct way but then again, if everything is working OK I may have to live with them 2 wires reversed at the 7-way plug. I just have to remember that they are reversed at the trailer plug if I ever needed to replace that plug again. What do you think? Thanks.

Click to expand...

OK, I don't think I ever understood this before. If that's the case, then I think you are right in just connecting everything the way it was before you got the new plug. I don't like it, and it does not agree with the wiring diagram that HiLo provided for trailers in that era, but I can't argue with something that works.

Do it that way, have someone start your tow vehicle and then you plug the trailer in, as you watch for smoke or anything else unusual. If that happens, unplug it quick!

- Jack

Replacing TV plug and Trailer plug.

These items wear out due to being dragged around and bad weather. Here in the north USA we get lots of snow and ice. I remember at one point DH carried a spare in the glove compartment. Our HiLo dealer sells at least five hundred of these parts a year.