So I have this ongoing mystery with my 97 Ram 1500 trailer wiring harness.

The truck came with the towing package which includes the 7-pin plug. This should enable the truck alternator to recharge the batteries in my trailer. However, twice now the wire from the power distribution box in the engine compartment to the rear 7-pin plug has melted. There is no short in the wire or plug and the fuse has never blown. I have looked in detail at this circuit and here is what I've found. Dodge installed a 40 amp fuse in the power distribution box for this circuit and ran a #12 wire to the 7-pin plug by the trailer hitch (#12 wire can't carry 40 amps for 25 feet!). The trailer umbilical cord has a #10 wire on this lead. Obviously, the wire was overloaded and hence melted. Effectively, it became the fuse. My question is: "What regulates the current flow from the alternator down this wire". Its a 120 amp alternator and there is not current control device anywhere in the circuitry of the truck or trailer, at least that I can find. If the batteries on the trailer (2 X 6 volt golf cart batteries) are low they can easily suck up more than 40 amps. Something has to give and since Dodge decided to put a bigger fuse than wire, apparently its the wire. Any help would be appreciated.

 

the 12 v source is NOT meant to run charge batteries or pull 40 amps. as you said the wire is not big enough for anything more than about 10-15 amps for running some lights or powering a relay. whoever wired it up this way was not thinking clearly.

as far as charging batteries, you'd have to match the size of the big wire on the back of your alternator in order to carry any real load.

your question about current regulation - i don't think there is any.

look for some kind of trailer/rv remote charging kit. it will likely need a big wire directly from the battery (maybe a ground also) or possible from a charging switch of some kind.

 

The OP is correct, your 7-pin connector should charge your trailer battery.

Have you tried unpluging one of the batteries and charging just one at a time?

 

All the RV literature and the Dodge manual say the circuit is for recharging the RV battery. The trailer manufacturer (old and reputable) wired the circuit to charge the battery. So, I have to assume this is its purpose. The first time the wire melted, I was running a single deep cycle battery that came with the trailer. The second time was after I upgraded to 2 X 6 VDC golf cart batteries. I am perplexed by Dodge's use of small #12 wire after a 40 amp fuse. The fuse is original and the size is marked on the lid of the power distribution box as well as the owners manual. As I said in the title: 'It's a mystery'.

 

ok, i'll fess up to being wrong - but only by guessing that the 12v center wire goes to a charging regulator of some kind that should limit amperage draw.

but - even this requires a 10 gauge or large wire to handle 40 amps.
http://atkinsonelectronics.com/produ...s/TBCM-40A.pdf

 

Thanks for the update. I talked to my RV dealer and the manufacturer of the trailer. Both were stymied. I call the Dodge dealer where I bought the truck. They, of course, wanted me to bring the truck in for servicing. At $80/hour, this isn't going to happen. Dodge Canada's website wasn't any help. They just tell you to see your dealer. Gee, maybe Dodge just didn't get it right back in '97