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Discussion Starter #1
This past year we purchased a new Sundowner horse trailer. Over the last few months, we have had a few occasions to trailer at night. I checked all of the lights when we got the trailer and they worked perfectly. The first time we used it at night, there were no issues. Since then, we have lights randomly not working. Once it was the little clearance lights that ran around the trailer but the interior lights were working. Once when we left one location, the clearance lights were working but the interior lights were not working but when we got home, I flipped the interior lights on and they worked just fine. Then, last night, no interior lights once again and they never "magically" started working. Checked my truck fuses (2016 Silverado) and they all seem to be fine. I'm really frustrated. Nothing like trying to load a young, inexperienced horse in a trailer late at night when you can't see!
 

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^^^^^^^^^2x^^^^^^^^^
 

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Does your trailer have a battery other than the brake away one for emergency stopping?

The trailer light plug is another thing that can cause trouble. Make sure all are clean and tight.
 

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Discussion Starter #6
Sounds like a electrical ground issue.
Ok. Electrical is not my thing. So, a ground issue in the trailer wiring itself or in the plug that connects to my truck?
 

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Discussion Starter #7
Does your trailer have a battery other than the brake away one for emergency stopping?

The trailer light plug is another thing that can cause trouble. Make sure all are clean and tight.
Nope. No extra battery. I’ll have to look at the plug to make sure it’s clean.
 

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Ok. Electrical is not my thing. So, a ground issue in the trailer wiring itself or in the plug that connects to my truck?
Ground from truck to trailer. Your trailer probably grounds through the trailer ball/trailer hitch. The best ground connection is the truck plug grounded to the truck, and the trailer plug grounded to the trailer. There are 7 points on your electrical plug. One of them is a ground. If grounded this way, you should be able to hook up just the electrical plug only, completely unhooked otherwise and the lights should work.

It also could be the truck end or trailer end of the plug that could be the issue like H-D said. :bigthumb:

Light issues with trailers are a headache, you have to start somewhere and trial and error till you find the issue. Plug and grounds are the first place to start.
 

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Discussion Starter #9
Thanks guys. I’ll start digging around this weekend based on your direction.
 

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plug.jpg

I'm not claiming any real expertise, but I have maintained and had to sort out electrical problems with my own horse trailers for a few decades. And I've done all of it myself, from changing light bulbs to replacing wheel bearings and brakes. Every horse trailer I've had used a connector like the one pictured above, and had the trailer wiring routed pretty much the same. What you have described is an intermittent problem with the trailer interior lights, and with the clearance lights.

I'm assuming that what you mean by clearance lights are the front amber lights and the rear red lights mounted on the roof of the trailer, and also the side marker lights - the amber and the red lights on the sides of the trailer. And by interior lights you mean any tack/dressing room lights, plus overhead light over the horse space, and a switched loading light over the ramp or step-up. Correct so far?

I'm also assuming that all your other lights are working properly; the turn signal lights, brake lights, tail lights, and the back up or reverse lights. All these have a dedicated power wire source coming from the connector between truck and trailer. I would look in particular at the trailer tail lights, since the trailer's clearance lights share power with them. If the tail lights are not working together with the clearance lights, there may be some corrosion in the trailer plug or socket around pin 3. As a starting point, you could spray some contact cleaner on the truck to trailer connector parts. Spray electrical contact cleaner can be found at Walmart, any auto store, Tractor Supply, and such.

That would not explain the interior lights, as they usually draw their power via pin 4. So the next question is whether your trailer interior lights work with the truck engine running, but not with the truck engine shut off? At some point in the Chevy truck model years, power to pin 4 was only supplied to the trailer when the truck was on. I've had two Chevy towing trucks where this was the case. It was a pain loading at night, since you had to crank up the truck to turn on the trailer interior lights. My 2017 truck has power to pin 4 always on, so the interior lights work without the truck switched on.

The wiring to the individual side marker and roof lights, and to the trailer interior lights usually runs in the metal channel between the trailer roof and the lower part of the trailer. These wires are typically laid along an open channel you can feel, and look down into if you stand on a mounting block inside your trailer. There are a lot of wire splices in there where connections can be potentially loose or oxidized so as to make intermittent contact. You will notice when you look that there is only a single wire going into each light fixture or marker light. That is the power wire to the light bulb. The return circuit or ground uses the frame and skin of the trailer rather than individual wires to finally complete the electrical circuit through pin 1 on the connector.

If you'll post whether my assumptions are true so far, I'll be happy to help trouble-shoot further.
 

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Just two quick additional comments.

As you try to sort out your horse trailer wiring problems, first map out where on or in the trailer the problems are located. That can give you a clue about where in the trailer wiring a problem might be found. Trailer manufacturers like to keep the wiring as simple as possible, with the minimum number of splices or connections, and using the least amount of wire possible.

Now, a used trailer might present a whole different challenge, with even the pins in the connector to the truck being switched to a non-standard pattern.

Second, if a light isn't working, the cause likely is one of three things. A bad bulb, lack of power to the bulb, or lack of a grounded connection from the bulb. In the case of interior lights you might throw a bad switch into the picture as well, but that's also a power lack problem.
 

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That's a good point about the truck shutting off power to your inside lights. Everything is computer controlled especially with Chevys. For instance you can usually turn the vehicle off but the radio (excuse me infotainment system) will keep playing until you open the door.

This is why I asked about the additional battery. I hooked up a friends car trailer with one so the inside lights would work without the truck being hooked up at all. The power wire/truck or a battery tender kept it charged. This battery also ran a Warn 3,500 lbs winch to load non running cars. So we needed the battery anyway.

They make a special wire brush for cleaning them plugs. If you trailer a lot it might be worth picking one up. Put some dielectric grease in the connections once they are clean.

Lastly the cover to the hitch plug holds the trailer plug in. So make sure it is latching correctly. Once hooked up the plug shouldn't wiggle around.
 

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Ok. Electrical is not my thing. So, a ground issue in the trailer wiring itself or in the plug that connects to my truck?
Ground from truck to trailer. Your trailer probably grounds through the trailer ball/trailer hitch. The best ground connection is the truck plug grounded to the truck, and the trailer plug grounded to the trailer. There are 7 points on your electrical plug. One of them is a ground. If grounded this way, you should be able to hook up just the electrical plug only, completely unhooked otherwise and the lights should work.

It also could be the truck end or trailer end of the plug that could be the issue like H-D said. :bigthumb:

Light issues with trailers are a headache, you have to start somewhere and trial and error till you find the issue. Plug and grounds are the first place to start.
If you lose ALL lights together, and they come back all together, the I would agree that it's a grounding issue from trailer to truck. I would even go a step further and say that you're getting ground through the hitch instead of the harness (which indicates a problem with the "main ground" of the trailer itself, or where the ground from the truck actually attaches to the trailer).

Wiring on a trailer usually works like this:

- The plug carries ground from the truck and it gets attached directly to the trailer frame
- Individual lights on the trailer are also attached directly to the frame, allowing them to each have ground individually
- +12v is carried in series from one light to the next, front to back, but often can bypass a bad bulb (not like old Christmas lights where a burnt bulb killed the string)

Check to be sure that your trailer frame is actually grounded through the plug using a multi-meter that can test continuity. Put the truck and trailer near each other but don't connect the hitch. Plug in the wiring connector. Put one probe on the frame of the trailer and one on the frame of the truck (exposed metal areas that will work to test continuity) and see what happens. If you can't get a solid continuity this way no matter what you do, start at the main wire harness and work back to find where it's connected to the trailer and be sure that you're actually carrying ground to the trailer properly.
 

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After thinking over your light problems for a bit, I am ready to bet that the source of the problem is the simplest thing to fix. My working diagnosis is that there is just enough oxidation on the electrical contacts inside the trailer part of the trailer to truck connector that the conduction of electricity to the lights involved has become inconsistent.

And if this fix doesn't work, you will only be out around $8.00, which is the price of a spray can of electrical contact cleaner at Home Depot. It's in the electrical section of the store.

contactcleaner.jpg

Just spray a liberal amount into the end of the trailer wiring connector, and let it drip off and dry. Same on the prongs in the connector on your truck. It won't harm your hands or the plastic parts of the setup. When you then hook up the trailer plug, wiggle the connector around a bit to help rub off any oxidation remaining on the brass blades and sockets.

Once upon a time, when I used to hack from my barn to the hunt, instead of trailering, my trailer sat for over a year. After that time there was so much oxidation or corrosion that none of the pins made any contact, and I had to resort to scraping the brass off with a screwdriver tip. If you use a trailer regularly, plugging and unplugging will usually keep the connectors relatively clean, but in your case I think there's just a bit of corrosion affecting two of the connectors.
 

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Discussion Starter #15
View attachment 490746

I'm not claiming any real expertise, but I have maintained and had to sort out electrical problems with my own horse trailers for a few decades. And I've done all of it myself, from changing light bulbs to replacing wheel bearings and brakes. Every horse trailer I've had used a connector like the one pictured above, and had the trailer wiring routed pretty much the same. What you have described is an intermittent problem with the trailer interior lights, and with the clearance lights.

I'm assuming that what you mean by clearance lights are the front amber lights and the rear red lights mounted on the roof of the trailer, and also the side marker lights - the amber and the red lights on the sides of the trailer. And by interior lights you mean any tack/dressing room lights, plus overhead light over the horse space, and a switched loading light over the ramp or step-up. Correct so far?

I'm also assuming that all your other lights are working properly; the turn signal lights, brake lights, tail lights, and the back up or reverse lights. All these have a dedicated power wire source coming from the connector between truck and trailer. I would look in particular at the trailer tail lights, since the trailer's clearance lights share power with them. If the tail lights are not working together with the clearance lights, there may be some corrosion in the trailer plug or socket around pin 3. As a starting point, you could spray some contact cleaner on the truck to trailer connector parts. Spray electrical contact cleaner can be found at Walmart, any auto store, Tractor Supply, and such.

That would not explain the interior lights, as they usually draw their power via pin 4. So the next question is whether your trailer interior lights work with the truck engine running, but not with the truck engine shut off? At some point in the Chevy truck model years, power to pin 4 was only supplied to the trailer when the truck was on. I've had two Chevy towing trucks where this was the case. It was a pain loading at night, since you had to crank up the truck to turn on the trailer interior lights. My 2017 truck has power to pin 4 always on, so the interior lights work without the truck switched on.

The wiring to the individual side marker and roof lights, and to the trailer interior lights usually runs in the metal channel between the trailer roof and the lower part of the trailer. These wires are typically laid along an open channel you can feel, and look down into if you stand on a mounting block inside your trailer. There are a lot of wire splices in there where connections can be potentially loose or oxidized so as to make intermittent contact. You will notice when you look that there is only a single wire going into each light fixture or marker light. That is the power wire to the light bulb. The return circuit or ground uses the frame and skin of the trailer rather than individual wires to finally complete the electrical circuit through pin 1 on the connector.

If you'll post whether my assumptions are true so far, I'll be happy to help trouble-shoot further.
Yes, completely correct regarding which lights I am calling clearance lights versus interior lights.

Regarding Chevy truck running and interior lights: The first time I had difficulty with the interior lights not working the truck was off. After I got my boy loaded and home and was backing trailer into drop-off location, I happened to check again with truck running and interior lights worked. So, on that day, you described my situation perfectly but I had not associated it with truck running or not until you mentioned it. Last week though, loading at night and the interior lights were again not working. We turned on the truck to see if that corrected the issue and it did not. Got home and double checked to see if issue was intermittent but with truck running still had no interior lights. Now, I’m curious about something that may be totally unrelated. I’m thinking about turning on Bluetooth for music after truck is already running and it won’t work. But, if Bluetooth is already on and then I start truck, it always works. I really don’t know squat about electrical but it seems like competition for a circuit or a lock-out of some sort when that happens. Wonder if this is similar circumstance. If truck is running before electric hookup attached or after it is attached does that make a difference?! Darn. Now I’m really curious. I have an appointment this am but am going to check that out and pick up contact cleaner on the way back home.
 

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Discussion Starter #16
After thinking over your light problems for a bit, I am ready to bet that the source of the problem is the simplest thing to fix. My working diagnosis is that there is just enough oxidation on the electrical contacts inside the trailer part of the trailer to truck connector that the conduction of electricity to the lights involved has become inconsistent.

And if this fix doesn't work, you will only be out around $8.00, which is the price of a spray can of electrical contact cleaner at Home Depot. It's in the electrical section of the store.

View attachment 490826

Just spray a liberal amount into the end of the trailer wiring connector, and let it drip off and dry. Same on the prongs in the connector on your truck. It won't harm your hands or the plastic parts of the setup. When you then hook up the trailer plug, wiggle the connector around a bit to help rub off any oxidation remaining on the brass blades and sockets.

Once upon a time, when I used to hack from my barn to the hunt, instead of trailering, my trailer sat for over a year. After that time there was so much oxidation or corrosion that none of the pins made any contact, and I had to resort to scraping the brass off with a screwdriver tip. If you use a trailer regularly, plugging and unplugging will usually keep the connectors relatively clean, but in your case I think there's just a bit of corrosion affecting two of the connectors.
Hmm. And I know this trailer sat on the lot for a year before we purchased it. You’re on to something, Doc.
 

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If you want to at least eliminate the truck side as a possible problem there are cheap testers available at Amazon and most auto parts stores. With something like you are going through it is a process of elimination. If you can verify that the truck side is OK then you can concentrate on the trailer.

https://www.amazon.com/58270-7-Way-Blade-Connector-Tester/dp/B001EOWQY4

BB00300F-4516-4B0C-B725-E344314314FB.jpeg
 

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Discussion Starter #18
If you want to at least eliminate the truck side as a possible problem there are cheap testers available at Amazon and most auto parts stores. With something like you are going through it is a process of elimination. If you can verify that the truck side is OK then you can concentrate on the trailer.

https://www.amazon.com/58270-7-Way-Blade-Connector-Tester/dp/B001EOWQY4

View attachment 491378
I need one of those things in my trailer emergency kit. Thank you for the idea, Coaltrain.
 

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Discussion Starter #19
This week has shaped up to be nutty busy. I purchased the contact cleaner but haven’t had a minute to hook up the truck and see what the result is. I’m hopeful we will find time on Thursday since we both have the day off at least.
 

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This week has shaped up to be nutty busy. I purchased the contact cleaner but haven’t had a minute to hook up the truck and see what the result is. I’m hopeful we will find time on Thursday since we both have the day off at least.
I just remembered something from a past trailer where there were issues with the lights....

On the plug on the trailer wiring, make sure that the contacts are position properly and will "hold" the blades from the truck tightly. You might need to slip a small flat blade down into the various clips to make sure they will contact the blades properly.

I had a brand new trailer that the ground connector need to be "tightened" like this. Took me 5 seconds and I never had an issue again.
 
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