View Full Version : Trailer wiring question

May. 13, 2012, 04:16 PM
I recently bought a new trailer. It has 12-volt deep-cycle battery to power the interior lights and fans when it's not hooked to the truck. (This is not the same as the breakaway brake system; there's a separate battery for that).

I assumed that the battery would also power the fans/lights when the truck was hooked up but not running, to avoid draining the truck battery, and then recharge from the truck when it runs. But looking at the wiring, it looks like that's not the case.

There's a fusebox on the trailer between the connector cord and the rest of the trailer wiring. Here's a picture of it:


The left side is the wire from the truck; the right side goes to the trailer wiring.

Here's a circuit diagram:


The left/right sides are opposite from the wiring, but it seems pretty simple to understand.

Note that the yellow wire from the truck accessory circuit is not connected to the trailer. There's a jumper between the truck hot/battery wire and the accessory terminal, so it looks like the hot/battery wire would feed both those circuits.

To me, it seems like it would make more sense to disconnect the truck hot/battery wire, and feed both circuits with the accessory feed. When the accessory feed is not hot, they could both draw from the trailer battery. This could avoid draining the truck battery.

On the other hand, I assume the trailer manufacturer had some reason for wiring it they way they did, and didn't just make a dumb mistake, so I'm a little reluctant to just change it myself.

Does anybody have any ideas about why it's wired the way it is, or what might go wrong if I change it?

Tom King
May. 13, 2012, 06:27 PM
Without an automatic converter, like in RV's, it's probably better to be hooked up separate like it is. They did that so they could have some lights when not hooked up to the truck. I guess the biggest concern is keeping the separate battery charged, but that should be easily figured out. I'd get one of the little automatic Battery Tenders to keep plugged in when you are not using the trailer.

May. 13, 2012, 06:40 PM
Ford trucks only power the aux circuit when the key is in the on position. This wiring bypasses that switch so power is always available from the truck.

The 30 amp FUSE looks like a replacement that someone dropped in. The trailer wire doesn't "look" heavy enough for 30 amps. (10 gauge wire)

If your truck powers the aux circuit without the ignition on, I would discard the jumper and connect the yellow aux wire.

As for charging the trailer battery, look into a battery isolator to keep the trailer battery from discharging the truck.

May. 13, 2012, 07:40 PM
The fuses are all original. I bought this trailer new, so everything is just like it came from the factory. I don't know why they put that one 30 amp fuse in.

I'm not even sure what, if anything, is on that circuit. I've got 2 exterior loading lights, interior lights in the trailer and tack room, and fans in the trailer and tack room. I haven't checked to see whether they're all one one circuit, or whether some are on the "hot" circuit and others on the aux.

My truck is a Ford. I haven't actually tested to see if the aux circuit is powered with the key off, but if you say it isn't I'll believe you. But, with the battery on the trailer, I don't need power from the truck when the key is off. In fact, I'd prefer not to have it, because I don't want my truck battery draining when it's not running.

May. 14, 2012, 10:56 AM
Hey Matt - feel free to contact me with anything you might have a question about -
The jump to the 30 amp fuse is because there is power coming in from your trailer battery & truck battery so the fuse is larger ( the jumper wire is a heavier gauge )- then it is jumped to the 20amp fuse that is the black hot wire that runs all your inside lights / fans / load lights - ( the yellow is not used )

Your trailer battery will trickle charge, to help maintain its charge BUT it will only be sending power back from your truck as long as the truck alternator is indicating the truck battery needs a charge -

it would take a very long trip to charge the trailer battery fully if it was low - so, do occasionally use a trickle charger on the trailer battery so it will not get too low or if the trailer has been sitting a while

Thanks Much, Risa

May. 14, 2012, 11:41 AM
I think you just need to send me the trailer and get something less confusing. ;)

May. 14, 2012, 04:50 PM
I don't know the specifics of my trailer wiring, but just for info: I have an accessory battery like yours which I love and use the lights, fans etc frequently. I use the trailer 3-4 times per week and pull with a Ford. I have never NEEDED to charge the accesory battery, but hook up a charger to it about once a year just to be safe.