Why only hot with switch on?

I've got a 2011 DCSB SR5 4x4 with the factory tow package. Nicely done from factory, solid hitch and 7-pin electrical, with separate accessory hot lead from light circuits...….

EXCEPT, the accessory lead is only hot when the switch is on. What a PITA, if you want accessory power to your trailer, you have to leave the ignition switch "ON" (and not just in "accessory", but ON). My old GMC dually had the same basic hitch and electrics, and the accessory hot on the trailer plug-in was always hot, so I could use the trailer's interior lights without having the key in the ignition.

I converted my cargo trailer's 4-pin wiring connector to a 7-pin, so I could match the Taco's connector without having to use an adapter; it also allowed me to run a separate hot lead on the trailer to run the interior lights, without having to have the truck's light switch on (the trailer's interior light was wired through the taillight circuit, you had to have the truck's parking lights on at least, to use the interior light on the trailer).

Anyway, I determined that the present wiring arrangement was unacceptable for my needs, so I modified it a little. I ran a strand of 12 gauge wire from the hot connector in the under-hood fuse panel (the one the battery lead attaches to) and routed it underneath the truck along the main wiring bundle, to the trailer wiring connector. I cut and capped the switched hot going to the plug, and wired my single lead to the plug in its place. I have a 15 amp in-line fuse (12 ga. wire has a 17 amp limit). Now, whenever the trailer is plugged into the truck, I have interior lights without having to have the key in the switch and turned on. I can simply pull the fuse to prevent any unauthorized use of my battery power when I'm not pulling a trailer.

 

Most manufacturers do it this way so people don't drain their tow vehicle battery by leaving the RV plugged in while parked for hours to days.

 

I guess that's a good enough reason for the careless owner who wouldn't otherwise appreciate the convenience of power in the trailer without having to keep the ignition switch on, needlessly powering everything in the truck as well, which drains the battery even more quickly.

I don't worry about draining the battery, because I am only powering lights inside the trailer for a couple of hours, and I keep a NOCO jump starter in the truck.

 

Shouldn’t your trailer battery be able to power your lights for a few hours?

 

My trailer is a 5x8 cargo trailer, and not a travel trailer. I do plan on adding a deep-cycle battery with inverter later, because the trailer will be used sometimes as a portable observatory and I need power to operate my telescope mounts, computer, and cameras. But when I first arrive at a site and am still hooked up to the truck, I want to keep the lights powered by the truck battery until I have everything set up.

 

Sounds good if you haven't already upgraded to LED lights, get some to reduce the load. Have you considered a generator?

 

The lights inside the trailer are all LED's. I have a generator, a 3000 watt Honda that's so quiet you can barely hear it 30 feet away, and can carry on a normal-voice conversation while standing next to it. I'll take it along on some weekends, but only to use during the day to recharge batteries. Star parties are fairly quiet affairs, and even a quiet generator like the Honda wouldn't be appreciated (or usually permitted) at 2 or 3 in the morning.

I'd go completely DC if it weren't for having a couple of items with voltage requirements other than 12VDC. My laptop, for instance, runs off 15VDC on its internal battery, and that only lasts for about 3 hours when running a guiding program. It's much easier to keep it plugged in on 120VAC, so I'll use an inverter to run it and my DSLR. Astrophotography requires long periods running the computer and cameras and telescope mounts, usually 5-6 hours at least, with total current drain around 10-12 amps. I sometimes have to use dew heaters to keep the objectives free of moisture, and they require 4-5 amps by themselves. Using the generator would be nice, if I could. I do use it at home, my viewing area is too far from the house to run drop cords. Most of my other equipment is designed for 12VDC; when I run AC power, I use 12V converters for all that, and I have a couple of nice Lithium Iron Phosphate batteries, but again, they have a limit before needing to be recharged that is usually shorter than I need for an all-night session.

 

Most factory 7 pins have everything including an isolation relay to turn off the 12 volt feed from the truck so you can drive the truck and trailer home.

 

Good answer.

 

The 3000 Honda is the quiets generator I know of it it's far quieter than the 2000 but it is heavy! People have a romantic vision of big battery banks and solar but the bottom line if you need heavy power at night the moon ain't going to cut it.