Converter 12V fuse blowing only after towing

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The WFCO manual won't help identify what is on the #5 fuse - that was determined by the trailer builder. And maybe some subsequent owner mods. I've seen mods to illuminate the trailer marker lights while parked on shore power and similar seemingly innocuous wiring changes. Nor can we tell at this point whether there is a short or just an intermittent overload of the #5 circuit. It's even possible that the trailer power is backfeeding into the tow vehicle when connected. It shouldn't, but it can happen. Connecting the trailer plug to the vehicle completes a circuit between the trailer battery & tow vehicle to enable charging while underway. Power is supposed to flow into the battery from the tow vehicle, but the path is there to go either way unless a diode is present.

I think I would start isolating the trailer towing circuits from the trailer "house" circuits in hopes of finding where the interaction with the #5 fuse it. That won't help if the problem is an intermittent short in the house wiring that is triggered by movement, but you have to start somewhere. It's pure guesswork (or wire tracing) until you can narrow things down.

Another approach would be to split out the loads on the #5 fuse, temporarily putting each on its own fuse. Maybe wire a small fuse bus to the #5 fuse and connect the various feeds to separate fuses on the new sub-bus. But that assumes you can find those separate branches (pump, CO, etc).
 
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The WFCO manual won't help identify what is on the #5 fuse - that was determined by the trailer builder.
The #5 is for a control panel that turns on lights in the RV (with rocker switches), toilet pump and fan. I have not checked if it's tied into any specific action from the tow vehicle. I will try and do that. I
You may want to separate the different items that are supplied. It would seem that none of the items you have listed would be turned on while traveling, unless you leave the pump on all of the time? I would doubt that anything related to the tow vehicle would be supplied by that fuse. Most 12V fuse panels that I have worked with have a common point in the supply to the fuses that connects to the battery, and the converter and the only way that the tow vehicle ever supplies anything on that circuit board is via the aux power lead from pin 4 of the 7 pin plug. That lead usually connects to the positive terminal of the battery and lifting the connection between the tow vehicle and the battery would eliminate the tow vehicle as a cause. It sounds to me like it is something that is triggered by vibrations of travel and not related at all to the tow vehicle.
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Hi, First, thank you for the help. I’m on my last night before I get home. So I replaced the fuse and began to jiggle all the 12V lines including my 7 pin on the vehicle and where it enters the junction box on the trailer. Well now the circuit is dead ( no fuse blown) and the little LED red light is not lit. All other 12V circuits in the trailer are all still good and the batteries are fully charged. Only the one is dead. I guess the wire is disconnected somewhere?? Other suggestions? I’ll have to troubleshoot further when I get home. I’ll keep you abreast.
 

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Power to the fuses is usually a common bus. So I would guess you have a loose/broken connection somewhere after the fuse. Possibly at the distribution center itself.

Kevin
 
Power to the fuses is usually a common bus. So I would guess you have a loose/broken connection somewhere after the fuse. Possibly at the distribution center itself.

Kevin
Hi, Electricity is not my strong suit. I checked continuity across the fuse and there is none, while there is at all other fuses. I also checked the input side of the fuse with a 12v lamp tester and it lit so there is 12v at the fuse.

Does that lead me to an issue with wiring on the output side to the devices/loads. I’m guessing on the output side since it failed after a jingled the wires. Tks
 
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Hi, Electricity is not my strong suit. I checked continuity across the fuse and there is none, while there is at all other fuses. I also checked the input side of the fuse with a 12v lamp tester and it lit so there is 12v at the fuse.

Does that lead me to an issue with wiring on the output side to the devices/loads. I’m guessing on the output side since it failed after a jingled the wires. Tks
If you have no continuity across the fuse tangs while out of the fuse box, then the fuse is blown. You still have to find out where it’s shorted. Obvious culprits are loose dangling connections, but also where wire bundles bend around sharp corners.

Kevin
 
I checked continuity across the fuse and there is none, while there is at all other fuses.
You do realize that to check continuity of a fuse you must measure across it with the ohm scale of the meter and 0 ohms means it is good or has continuity while a high reading means the fuse is open. (the service tech in me said to ask :giggle: ) The other way is to check voltage with a meter and look for no voltage loss from one end of the fuse to the other. A good fuse will read the same voltage at both ends when measured to ground.
I also checked the input side of the fuse with a 12v lamp tester and it lit so there is 12v at the fuse.
The test lamp says that there is some voltage there but that could be 12V or it might be only 1V. While the light is good for some things, it is not a good troubleshooting tool.
I’m guessing on the output side since it failed after a jingled the wires.
If wiggling the wire harness caused the fuse to be blown or open, then you have some type of short in those wires somewhere and need to start tracing circuits. In order for it to blow the fuse the problem has to be on the output of the fuse as excessive current through a fuse is what will cause it to blow.
Electricity is not my strong suit.
You may need to get help or buy a real, digital volt/ohm meter and learn to use it. To understand the 12V part of your RV, I suggest that you read The 12V Side of Life, by Mark Nemeth.
 
Hi, I believe I found the problem. The 12V wire coming from the fuse is crimped to 4 wires. The connection for the wire was loose. Having 4 wires tied into the single crimp connection was a point of failure. I have some crimps, but the biggest I have is still too small to get all four wires into the metal barrel of the crimp. I'm looking for a bigger crimp. Thank you for the help.
 

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Hi, I believe I found the problem.
If that ties into the fuse 5, I'd be certain that is the issue. None the less, it needs corrected. A wire nut might work just as well. I like taking electrical tape over the wire nut. Keep us posted.
 
If that ties into the fuse 5, I'd be certain that is the issue.
Hi, Yes it ties into circuit 5. On my last day of camping this week, I jiggled some of the wires and it went from intermittent (mostly while driving ) to compete lack of power on that circuit. When I inspected today, the wires were seperated from the crimp connector. So I'm assuming the wires were loose. The only thing that makes me unsure is, if it was loose while driving, why would that blow the fuse? I'm done for the year camping as of this week. So it will have to wait until next spring to see if the problem persits after I repair the connect. Thank you.
 
This is 2nd issue by my memory on this site with quick connection methods like this causing issue. Here it was just a fuse, but the last one involved frying wires, I believe a wire nut came loose. It is definitely worth a look through the entire wiring system, armed with a soldering iron and plenty of solder
 
The various quick-connect devices work pretty well but make sloppy workmanship all too easy. Hurrying assembly line workers probably don't take the time to check that each wire is firmly in place, nor do they tape the wires together to reduce the effect of vibration and a shifting body/chassis on the road.

A wire nut is no better if not properly sized for the number & gauge of wires to be connected and then carefully installed. And well-taped, because any twist-on device is susceptible to loosening under vibration and Rvs get a lot of that.

Screws on wiring blocks can loosen too. A dab of rubber cement or silicone sealer on top of the screw can prevent that. Or thread locker (Loctite, etc) if you can get a bit onto the threads.
 
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