Help with electrical melt down

[jheimann]

I am new to camping having purchased a KZ Sportsman camper over the winter. I towed it home without issue with my pickup. I hooked it up this spring to take it for our first weekend adventure and as soon as I plugged the trailer into the truck the engine bogged down. Minutes later the 7 way connector started smoking. I quickly unplugged it and noticed the 12V accessory pin was completely melted away. The 30 Amp breaker in the truck never tripped. The battery on the camper was fully charged before connecting it to the truck (having been on shore power for days previous to our hooking it up to go). I used a Volt meter to test the trailer and truck and see that both seem fine. I have 12V at the trailer plug (or what's left of it) and can use a jumper wire at the plug to run all the lights (tail lights, left and right brake/blinker) from the RV battery and all works as expected. The truck still produces 12V at the accessory pin. The battery on the camper appears no worse for the wear and remains fully charged.

What happened? I can't figure it out. I will replace the connector on the trailer (since its melted away) and the connector on the truck for good measure, but until I know what caused the problem, I am afraid to connect it together again. I would have thought it was a bad connection causing heat build up/melting, but that would not have bogged down the engine on the truck when I connected it.

Also should I put an inline fuse (smaller than the 30A breaker in the truck) to protect the wiring since it obviously can't handle the 30 amps?

Any help you kind folks can provide would be greatly appreciated!

 

[tlmgcamp]

Even a 30 amp fuse should have popped with something that is bad enough to BBQ parts. I am all about saving a buck as the next guy, but something with that kind of result could threaten the tow vehicle electronics which could cost a fortune if a DIY trial-and-error attempt goes wrong, so calling in a professional might be in order.

That said, you got it home from the dealership without issue. Assuming there were no modifications to the trailer or tow vehicle since then, a logical culprit could be critters chewing through wires and those wires grounding out. If you disconnect all power and use a multimeter to see if one of the legs of the trailer side of the 7 pin connector is showing continuity to the trailer's ground that might help track it down the circuit you are looking for.

 

[Larry N.]

My first thought would be that one side or the other is wired wrong, so be extra careful with the connections when you replace them. Or perhaps something is shorted but only when the two are connected to each other, which still may be mis-wiring.

But note that circuit breakers are generally for AC (shore power), not for DC (battery power), so you need to be careful not to confuse the two electrical systems. And that one in the truck, are you sure it's not a fuse?

But if you have a multimeter (voltage, especially) you can likely do some checking at various points prior to connecting them.

Good luck...

 

[tlmgcamp]

Follow up...one of the pins in the connector is a ground and should show connectivity. Which pin does what can be found on line

 

[Kirk]

There is little doubt that something must have shorted the 12V pin to ground in order to draw enough current to melt the pin. I would start by measuring the trailer's 12V pin to chassis ground as I suspect that something has created a short there. Since it was OK when you brought your Sportsman home, I would tend to agree that it was probably a mouse but it could be most anything. As a temporary thing you could leave the trailer side of pin 4 disconnected in order to use the trailer. I would also install a much smaller fuse inline with that pin. You should never see 30a from that supply so I'd put in a 15a or at most 20a fuse.

 

[Jim18655]

Was the trailer on the hitch the second time? I've seen some plugs melt due to the ground wire not being large enough to carry the load. when the trailer is on the hich the metal to metal contact helps carry the load.

 

[Gary RV_Wizard]

One exception to the above comments: if the trailer brake "breakaway" switch was engaged (it's cord pulled), the amp flow to the trailer brakes is enough to fry a cheap trailer connector or associated wiring but still won't trip a 30A breaker. 20A-25A therough and undersized or corroded connector or wire is enough to melt a lot of things.

 

[tlmgcamp]

One exception to the above comments: if the trailer brake "breakaway" switch was engaged (it's cord pulled), the amp flow to the trailer brakes is enough to fry a cheap trailer connector or associated wiring but still won't trip a 30A breaker. 20A-25A therough and undersized or corroded connector or wire is enough to melt a lot of things.

Following this chain of thought,, OP towed it home over the winter in New Hampshire. Maybe road salt got into breakaway switch

 

[Mark_K5LXP]

Wouldn't power for the breakaway brakes come from the trailer battery? One data point I would consider is that if there are fault currents that melt things and breakers or fuses don't pop, then the fault lies before the breaker/fuse. Could be a "short" but also intentional or unintentional wiring that's bypassing the circuit protection. If a visual doesn't reveal anything there are simple techniques for tracing low resistance faults.

It's assumed that the circumstantial observations was an overcurrent event but it could also be "normal" current and a compromise connection. So before one goes down a rabbit hole tracing wiring, check some voltages and currents and isolate any faults.

Mark B.
Albuquerque, NM

 

[jheimann]

Following this chain of thought,, OP towed it home over the winter in New Hampshire. Maybe road salt got into breakaway switch

The breakaway switch was not tripped, and that pin was not melted. Not sure it would be the breakaway switch. I was thinking it could be a compromised (corroded) connection causing excessive heat build up, but the engine clearly bogged down (under load) as soon as I plugged in the trailer. I will be checking for chewed wires tonight as well as doing some more indepth testing with my volt meter, and post back. Thanks for the help here folks, I truly appreciate it!

 

[Kirk]

I quickly unplugged it and noticed the 12V accessory pin was completely melted away.

The breakaway switch was not tripped, and that pin was not melted.

As to the break away switch, if it were tripped for a long time it would completely drain the RV battery. Since you were connected to shore power and your RV battery was fully charged it is pretty safe to assume that the break away was not the problem. But as I read your first post it did melt a pin in the plug? What did I misunderstand?

 

[jheimann]

As to the break away switch, if it were tripped for a long time it would completely drain the RV battery. Since you were connected to shore power and your RV battery was fully charged it is pretty safe to assume that the break away was not the problem. But as I read your first post it did melt a pin in the plug? What did I misunderstand?

Kirk, yes the 12V accessory pin on the trailer plug was completely melted away. I meant to say the brake pin was not melted. Actually no other pins on the plug appear to be melted, just the 12V accessory pin. I suspect the ground pin was not melted because the trailer was connected to the truck at the time so the hitch provided plenty of grounding.

I started looking at the wiring closely last night and did not see any evidence of mice (no chewed wires), no pinched or scorched wires anywhere that I could see between the trailer plug and the battery. Still a mystery. Parts should arrive from Amazon today and I will replace them this weekend. Still no clue what happened or why.

 

[Ex-Calif]

What is the state of charge on the trailer batteries?

If you have dead trailer batteries and a 120amp alternator on the truck it's possible that the trailer was trying to draw that much current.

As I think about this for the first time I am wondering what, if anything, limits the current through the 7-pin plug.

Alternately, there could be a short on the trailer side.

And finally, it's my experience that the load wire from the alternator is not fused to my knowledge.

 

[jheimann]

What is the state of charge on the trailer batteries?

If you have dead trailer batteries and a 120amp alternator on the truck it's possible that the trailer was trying to draw that much current.

As I think about this for the first time I am wondering what, if anything, limits the current through the 7-pin plug.

Alternately, there could be a short on the trailer side.

And finally, it's my experience that the load wire from the alternator is not fused to my knowledge.

The trailer battery was fully charged when it was connected to the truck. After the melt down, I checked the battery status again to make sure no damage was done, and it still said fully charged (and all the lights in the trailer were nice and bright). I can see a 30A circuit breaker in line between the trailer connector on the truck and the battery on the truck. The 7 pin connector is not factory installed and the prior owner of the truck ran a 10AWG wire from the battery directly to the 7 pin connector, with a 30A circuit breaker mounted to the firewall in between (so its hot all the time). When the trailer is disconnected from the truck, I have 12V at the plug (proper polarity) and can jumper the plug to run the running lights and brake/blinker lights from the trailer battery with no issue. The battery has been off shore power since this incident and it does not appear to be draining much at all (only things running in the trailer is the CO and Gas detectors). Still a mystery that leaves me scratching my head.

 

[Gary RV_Wizard]

Wouldn't power for the breakaway brakes come from the trailer battery?

Only if it actually broke away. If the trailer plug is still connected to the truck, current is sucked from that source as well. As it in normal operation (when towing).

 

[tlmgcamp]

OP is using term "circuit breaker" instead of "fuse". I am familiar with circuit breakers for 110v but not for 12v. Not sure if piggybacking on another's thread with my own inquiry is proper forum etiquette, so please forgive any unintentional fo paux.

 

[Kirk]

Still a mystery that leaves me scratching my head.

Have you measured the resistance (ohms) between the 12V supply pin of the trailer plug (pin #4) to the trailer chassis? It should read some value in the K ohm range. I think that you are dealing with a short of some kind.

 

[jheimann]

Have you measured the resistance (ohms) between the 12V supply pin of the trailer plug (pin #4) to the trailer chassis? It should read some value in the K ohm range. I think that you are dealing with a short of some kind.

I disconnected the battery and measured the resistance from the 12V accessory cable to ground and it was 1.5M ohms. Same was measured at the positive battery cable to ground. While I grounded the positive cable before measuring resistance, there might still have been some residual power somewhere which could have thrown off the meter. I measured the current draw from the battery to the trailer itself and found it was 712mA (0.712A) which should not have burned up my trailer connector or loaded down my truck engine when connected.

 

[Kirk]

I believe that I already know the answer, but since it has not been mentioned, have you made sure that you do not have a polarity mismatch between then trailer and truck?

 

[tlmgcamp]

I am trying to understand the whole circuit breaker v fuse thing. I found this on an Airstream forum:

When a fault developed, a fuse blows and cuts the power off completely until it is replaced (and the fault is found). The automatic resetting circuit breakers I think you are referring to keep trying to reset themselves and in doing so, keep sending power to the fault, over and over and over. Eventually something fails, like the wire insulation and then you have a potential fire.

Automatic resetting 12 volt circuit breakers are not a good solution at all for the circuits. There are a very few situations that they are useful, and even then, one can question that they be used.

I cannot recommend them.

Again, trying to understand cb's myself