TOW Vehicle Alternator to Trailer Battery wiring (7-WAY)
To charge an RV battery in a travel trailer or a fifth wheel through the seven way connector calls for circuit protection, and one other thing:
A means to prevent the trailer from drawing on the charging system of the tow machine when the engine is not running.
To accomplish this, a self resetting 20 to 30 AMP breaker is placed on the battery positive source (Here I recommend the main hot terminal of the starter relay. It won't corrode as fast). The other side of that breaker is connected as the diagram below shows, to a relay that is triggered by the ignition switch run terminal.
You can in many cases go directly to the ignition module in the column to avoid robbing power from anything critical.
The main purpose of this is to ensure that even if the RV batteries are drained from use, you can still start the tow vehicle to get out of wherever you are with it.
SELF RESETTING BREAKERS
These are designed so that when the internal conductors of them create enough heat from high current, the contacts break. They will remain open until the breaker cools down. These are used typically where high current motors (for example) may be used to run a slideout for a minute or so. Many powered awnings use them too.
They automatically reset after a brief cooling period or time delay.
I'm assuming that the ford superduty system is already wired this way. There are relays for both trailer backup lights and trailer 12 volt battery charge systems.
Normally there is a plug under the dash that the right connector for the brake control goes right into. GM is bad about that - the plug is there, but they stuff it up behind the ducting so far you just can't find the harness connector.
Fords are easy...
Good RV dealers have the adapter for the brake control in stock.
When you absolutely have to trace everything out or do it from scratch though - this should be helpful.
F150's will not have it. They have to be "HARDWIRED".
And what that means is that you will have to run your own wires, or pay someone to do it for you. A flat four-way has no provision for a trailer charging connection or to run electric brakes. Flat four is the normal F150 standard.
The only tasks a flat four accomplishes are running lights, brake, and turn signals.
Yes, I found the brake controller jack on the '99 F350 very much to my liking. Please read my question about backup lights/12v to see if you have any input about using a motorcycle batter as the trailers aux battery/breakaway battery. Do I need (or want) the Tekonsha trickle charger wired inline?
I think I need to go from truck 12/v lead directly to the tekonsha charger and from the charger to the battery. Then take a lead from the battery to the junction box and a lead from the junction box 12v to the breakaway switch and another to the interior lights.
Been in this only since july of 06, but I see a lot of things.
i have not read that post yet, and will be in a much better position to evaluate it after I get to my new tech home.
Yes - i am going to dig into that, but I am not there yet.
Charge managed systems have wrinkles in them that I saw from the beginning. I'm not sure what I think about that from a purely electronic tech point of view.
The more complex it is - the more can fail.
Bluntly - I think I'm going to have to buy a new oscilloscope.
My 01 F150 with tow package only has 12v to that terminal with the key on. I had my old small winch wired into that wire and it only worked with the key in the run position.
I'm assuming that the ford superduty system is already wired this way. There are relays for both trailer backup lights and trailer 12 volt battery charge systems.
Tony
Any truck with the tow package has this allready. No need to wire anything and is 12v hot with key on only.
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05 F-250 CC SRW LB 4x4 6.0 Tow Command Lariat Remote Start All Stock 01 Alfa 40' Toyhouse for all the toys.
I'm doing my part to reduce our dependency on foreign oil and clean up the environment; my truck runs on 100% biodiesel made from soybean oil grown by U.S. farmers. What are you doing?
now this may sound stupid, but would any of this info be different for a 92 Bronco with a 7way plug (wired from factory), because ive been trying to do this and have been told its not possible?? which was strange because my old F250 was wired like that (to charge the battery) so just to recap.... my 7way, backup light terminal is used to charge the battery while the truck is running only, if it is set up as Lxman1 says, nothing else needed, just run the wire to the positive on the trailer battery, otherwise if it is always hot do it the way Greywolf says with the circuit protection. am i clear on this or no?
thanks for your patience
Someone rewired the 92 so that terminal is 12v hot and no back-up lights. Alot of trailers didn't have back-up lights so that was an empty terminal and got used for 12v hot. Why they didn't use the 12v hot for 12v hot I don't know.Back-up lights is the center pin on the 7 way.
Do you have to be in reverse for your trailer battery to charge ?
Or did they just use that terminal.
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05 F-250 CC SRW LB 4x4 6.0 Tow Command Lariat Remote Start All Stock 01 Alfa 40' Toyhouse for all the toys.
I'm doing my part to reduce our dependency on foreign oil and clean up the environment; my truck runs on 100% biodiesel made from soybean oil grown by U.S. farmers. What are you doing?
Any Ford truck with the tow package comes with it pre-wired .
I had a 1968 F-250 camper special and it was pre wired .
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05 F-250 CC SRW LB 4x4 6.0 Tow Command Lariat Remote Start All Stock 01 Alfa 40' Toyhouse for all the toys.
I'm doing my part to reduce our dependency on foreign oil and clean up the environment; my truck runs on 100% biodiesel made from soybean oil grown by U.S. farmers. What are you doing?
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Please read my question about backup lights/12v to see if you have any input about using a motorcycle batter as the trailers aux battery/breakaway battery. Do I need (or want) the Tekonsha trickle charger wired inline? 
01 Alfa 40' Toyhouse for all the toys.
