I'm looking to make my trailer light plug into an "isolated" power source to charge my camper battery. When I say "isolated" I mean it's "on" when the truck is running and "off" when the truck is not running. Currently the trailer outlet is always "on", regardless of whether the truck is running or not. Is there an easy way to change this? I was going to follow the steps in this thread http://www.expeditionportal.com/forum/threads/77503-How-to-make-a-cheap-isolated-dual-battery-setup-for-50 , but can't find an available fuse in either of my fuse boxes that is only hot when truck is on. Any ideas?

In the PDC (Power distribution center) under the hood should be a 40 amp fuse for the trailer that is what feed that 12V lead at the trailer plug on the truck. Other than that your going to have to wire in a relay to cut the power on that leg.

Yeah, I found that fuse last night. What I don't know how to do is add a relay to it. Wiring is definitely not my strong suit.

ISX has a wonderful way of explaining the relay hook up. But I'll try.

Looking at a relay.

So in your case you want...

86 going to keyed hot lead in the truck.

85 going to body ground.

87 to the fuse lead.

30 going to the trailer plug.

Also you want a heavy duty 30-40 Amp relay as well...

A sticky on that would be nice. Especially since I'm old school ( or an 'old screw' ) for still using the old continious duty relays on my camper with the points inside.I did place an in cab switch so I can let my truck batteries recuperate first, then switch the camper batteries onto the truck later.An important point is a fuse because if the camper batteries are really down low you can get a huge insurgence of power from the truck batteries to the camper which can cause some problems which the fuse will protect against.

That circuit is already covered by a 40A fuse in the PDC box.

That circuit is already covered by a 40A fuse in the PDC box.

Mike the reason I mentioned the fuse again is because some guys by pass the PDC AND hook the camper batteries directly to the truck batteries. Couple questions: Is it better to pass the PDC and go directly to the batteries for any remote possibility of something happening there? Not sure on this one . I have the camper batteries hooked direct to the truck battery with the old type isolation relay. Mike, on your rv, do you experience your headlight switch getting a little too hot with night time driving like I did? If so, what did you do about that?

Mike, on your rv, do you experience your headlight switch getting a little too hot with night time driving like I did?

No issues. As for being hooked to the truck I left mine alone and just remember to pull the trailer line when camping. Nothing special that way there is no need for wiring mods. Not to mention I feel safer knowing that there is always power getting back to the RV batteries keep them charged all the time I'm rolling so if there was a need for the break-away switch to activate I know the RV batteries have been charged. But with a solenoid / relay in between the power source and the trailer bother me a bit.

I put a relay on mine, except I used a 24v air conditioning contactor in place of a relay. The contactor has quite a bit more capacity than a relay and will last longer.

I put a relay in the 12V line to the RV plug; the relay is controlled by a 5 psi oil pressure switch - so that when the engine is started up and running - the relay is energized and power can flow to the RV batteries. Switch the engine off, and the relay drops out - no more power from the truck flows to the RV. (This keeps me from draining truck batteries while camping - and it is fairly fool-proof!)