5th Wheel 7 pin electrical issue

[Kies277]

I have a short on the 7 pin connector wiring. I took apart the junction box and disconnected the 7 pin connector and found that the connector is fine, it is in the trailer wiring (2015 Crossroads Sunset Trail 5th wheel). I have continuity between ground and the brake lights, ground and running lights, ground and trailer brakes.

After checking all the wiring I could, I found no damage. I suspected they may be pinched in the nose cone so I cut the running light wire in the trunk where the batteries are to isolate the section of wire between the junction box and there and now I'm baffled. I got continuity between both sides of the cut wire. Also, both sides of the cut wire have continuity with ground.

All the wires in the junction box are disconnected. Anyone have any ideas?

 

[Kirk]

I have continuity between ground and the brake lights, ground and running lights, ground and trailer brakes.

That is as it should be since the return side of the circuit that operates all of those is chassis ground. With each of those items there is one side connected to a source of 12V and the other is connected to chassis ground.

What was the problem before you began to work on the electrical wiring? What symptoms did you have?

 

[Kirk]

 

[Kies277]

I was blowing a fuse on the vehicle side and none of my running or tail lights were working. I initially checked continuity on the trailer side 7 pin connector and found continuity between the ground and running lights pins.

 

[Kirk]

I initially checked continuity on the trailer side 7 pin connector and found continuity between the ground and running lights pins.

The only way to check that is to remove all of the light bulbs. Doing that will open the circuit to the chassis ground that has to be there for the lights to work. Once you have done that you can check but you will need to separate each light's wiring from the rest to know which light has the problem.

 

[Kirk]

I was blowing a fuse on the vehicle side and none of my running or tail lights were working.

What fuse was opening? If it was the one that supplies 12V to pin #4 that has nothing to do with the running lights. The lights not working does make that suspect but which fuse is also important. Somewhere on the trailer there will be a junction box that ties the individual lights together.

 

[Kies277]

It is marked trailer parking lights on my truck, it supplies 12 volts to pin 3.

 

[Kirk]

That does narrow it down so you are on the right track. Do you know where the different lights connect together before they go to pin #3? And have you removed the light bulbs? If it were me, I would find and separate the individual lights as much as possible next.

Just so that you know, I am a retired electrical service technician and have many years of RV experience.

 

[Kies277]

I do not know where they all connect together. I did not remove all the light bulbs, there are a bunch of them and they are all sealed in so I was hesitant to do that.

 

[8Muddypaws]

I had a similar problem back when we had a fifth wheel. The person who installed the 7 pin connector in the bed of the truck bent the wires at such a tight angle it shorted out about half the time I applied the brakes. Also the central pin (backup lights) was loose and sometimes shorted two other pins even when the trailer was connected to the manufacturer installed connector in the tag tow receiver.

 

[Contributor]

There should be a junction box about 6-8 ft from the 7 pin connector in the trailer. Start there. The trailer wires go out from the trailer J box and disappear into bundles an throughout the trailer. Sometimes the bouncing around rubs the insulation and causes shorts to ground.

 

[Kirk]

I did not remove all the light bulbs, there are a bunch of them and they are all sealed in so I was hesitant to do that.

With all of the bulbs in place you will read a very low resistance to chassis ground so it is nearly impossible to know where the problem is. You could do as Pedro suggested and then just test to see if the fuse blows if you have plenty of fuses and find that less work than taking out the bulbs.

 

[Ex-Calif]

With all of the bulbs in place you will read a very low resistance to chassis ground so it is nearly impossible to know where the problem is. You could do as Pedro suggested and then just test to see if the fuse blows if you have plenty of fuses and find that less work than taking out the bulbs.

Trust Kirk - He is legit and he is totally on the right track. With bulbs installed and all wires lead to ground "via" the light bulb.

If opening the covers and removing bulbs seems daunting it may be easier to locate where the "fixture" ground wire attaches and disconnect the grounds.

It took me a minute to understand Pedro's recommendation but it is a good one.

Either at the J-box pull all the wires and connect them one at a time until you find the one that blows the fuse.

If it were me I would simply make a pin/alligator jumper and connect from the truck socket to the trailer socket pins one at a time until the fuse blew.

Edit to add - the downfall of the J-box method is you discover which circuit is at fault but not which leg of the circuit.

The downfall with the remove grounds method, If you reattach them one at a time and check for fuse blowing, you find out which fixture but not which circuit i.e. combined tail/brake and turn light fixtures. However at that point with the correct box figured out you may only need to take apart one box.

Big Caveat! - I have seen bulbs "explode" and the broken bulb grounds itself. I personally would be pulling fixture covers to check bulbs before I start disassembling J-boxes.

But that is probably more of a "gut" call than anything. The important thing is to troubleshoot logically and sequentially as you attempt to rule things out.

 

[Kies277]

I will pull all the bulbs. With all the bulbs out, there should be no continuity, correct? Then just replace bulbs one at a time and see which one pops the fuse?

 

[Kirk]

With all the bulbs out, there should be no continuity, correct?

Correct.

Then just replace bulbs one at a time and see which one pops the fuse?

Wrong. What you need to do is to separate the leads to the individual lights so that you can measure the resistance to ground. The good ones will measure infinity or near that and the bad one very low resistance to ground or O ohms.

If there is a short and you connect everything even without any bulbs in it will still cause the fuse to blow since you didn't remove the problem.

 

[Ex-Calif]

What Kirk is saying is that for example - all the running light wires are connected together from the j-box. If you check the lower left forward running light and there is continuity to ground - the actually grounding point could be near the right rear upper running light.

The meter (or blown fuse) method will isolate the circuit. Then the painful process of isolating the leg of the problem. Brake/Tail/Reverse lights are easy compared to the running lights.

 

[Tulecreeper]

What Kirk is saying is that for example - all the running light wires are connected together from the j-box. If you check the lower left forward running light and there is continuity to ground - the actually grounding point could be near the right rear upper running light.

The meter (or blown fuse) method will isolate the circuit. Then the painful process of isolating the leg of the problem. Brake/Tail/Reverse lights are easy compared to the running lights.

At which point it's sometimes easier just to pull it and start over.

 

[Kies277]

Ok, I got it figured out. Thank you guys for all your help, especially Kirk. I pulled all the fixtures (what fun) and started testing each one. When I got to the left rear taillight, I saw that the running light wire had low voltage. I looked underneath the Fiver and saw that happens to be where a 4 pin trailer connector was spliced in. I don't use that connector so I took it out. That was the ground issue. All lights are working beautifully.

Since I have all the fixtures off, I am replacing them with LED sealed fixtures. (Make lemonade, right?) I changed out the 3 fuses it blew in my truck and everything seems right in the world now.

The bonus is, I got to learn a bunch about continuity and resistance.

 

[Kirk]

Since I have all the fixtures off, I am replacing them with LED sealed fixtures.

Probably a good decision.