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Old 07-04-2013, 12:19 PM   #1
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Cookeville , Tennessee
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Trailer using 12 volt from tow vehicle

We have a 1971 Airstream Ambassador, we used it all last year as bought. This year we gave it a minor cosmetic upgrade and a new battery, with a 12 volt cut-off switch to the battery.

The trailer has a intelli-power 45 amp battery charger/converter, which was there last year, so this is not different.

Anyway, the trailer is now pulling 12 volt from the tow vehicle battery when connected, regardless if the 12 volt is cut off from the trailer battery. (We left the 8 track playing and ran our battery down in the tow vehicle).

It did not do this last year and I want to stop it from using the truck battery, how do i do that?

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Old 07-04-2013, 12:27 PM   #2
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Unplug the tow vehicle? Or disconnect the wire in the harness from the tow vehicle.


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Old 07-04-2013, 12:30 PM   #3
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The easiest answer is to unplug the trailer umbilical from the tow vehicle when you are parked or stopped...

The more complicated option is to consider a battery isolation switch in the tow vehicle, which includes diodes that allow the tow vehicle alternator to charge the trailer battery (the way they were designed) but can block current draw.. You may need hitch shop to wire that into truck...

In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...
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Old 07-04-2013, 12:37 PM   #4
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Ha, yeah i realize I can just unplug the trailer from the tow vehicle, LOL.

I was just wondering if there was a simple solution to keep the truck out of the 12 volt loop (interior 12 volt trailer) all together. Which wire in the trailer wiring harness is the one in question?
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Old 07-04-2013, 03:05 PM   #5
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Usually, the hot wire that runs from the TV battery to the trailer connector next to your hitch has a relay installed in the engine compartment. Most of the time, it's located very near the TV battery. The one our Airstream dealer installed in HAL (our old 1978 Chevy crew-cab pickup) had the input to the relay going to the positive terminal on the TV battery, the output from the relay ran to the back of the TV and terminated at the hot lead in the trailer connector, and the solenoid actuator lead was spliced to one of accessory wires that is only hot when the TV ignition switch is on (sorry, don't recall exactly which accessory that was).

Anyway, the hot lead that goes to the trailer only carries +12 volts DC when the TV ignition is ON. When the TV ignition switch is OFF, the solenoid is deactivated and the hot lead is dead. That way, your Airstream cannot draw off of your TV starter battery when the engine isn't running.

See example of relay at the link below. However, please note that this is not necessarily the solenoid that was installed in HAL. This one just looks similar to the one I watched the Airstream service techs install, and the description sounds like it would work. Tekonsha 7000 12 Volt 3 Terminal Battery Isolator: Automotive
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Old 07-04-2013, 03:21 PM   #6
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Originally Posted by L911 View Post
I was just wondering if there was a simple solution to keep the truck out of the 12 volt loop (interior 12 volt trailer) all together.
Are you sure you want to do this? The way it's set up, when you are in "travel mode," the tv's alternator will charge the trailer battery bank while you are underway. For long days on the road, you could get maybe 100 amp hours into your battery bank that way (seems to me that I recall that the charge line on mine is fused at 10 amps). Ten amps times ten hours = 100 amp hours (neglecting losses). Not important if you're staying places with shore power, but if you're boondocking, that's quite a bit of extra charge available at the end of the day. And as noted, a battery isolator will prevent the tv battery from getting discharged ... or you could just unplug the trailer's umbilical at the end of the day, which is pretty much always a good idea.
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Old 07-04-2013, 10:46 PM   #7
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Well, I was more curious than anything, because the trailer didnt do it last Im assuming I "fixed" a loose connection or something.

Also, its pulling power from my tow vehicle...with the tow vehicle NOT running. So if we stop for lunch or to play somewhere, if i dont disconnect the trailer harness (and leave the radio on or something) it drains my battery. So this is un-acceptable.
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Old 07-04-2013, 11:06 PM   #8
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You mentioned a cutoff switch... you may want to make sure that it does NOT cutoff power to the trailer breaks if accidently left in the open position. Keep in mind that your trailer brakes need the trailer battery if the trailer ever breaks-away from the Tow vehicle.
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Old 07-05-2013, 03:57 AM   #9
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Another solution

I asked about this a couple of weeks ago, bought one, but have not yet installed it in my truck. This is made by the same people who make the solar controller in my AS. This device is solid state and only allows the TV to charge the trailer batteries when the alternator is running. Even turning on the TV key will not produce high enough voltage to charge so it is impossible to run down the TV battery.
The installation is going to be very easy; I do not need to look for any ignition switch enabled 12 v. On my Silverado the 12v feed to the trailer is on the front of the fuse box. I simply have to take that wire off, connect one wire from this to device to it and out the other wire from this device on the post, and a ground. More elegant than a relay.
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Old 07-05-2013, 06:50 AM   #10
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Just put a switch on the wire for constant +12. This is different from the wire for brakes, lights, etc., so you shouldn't have any other issues from doing it. Heck your tow vehicle may even have a fuse for that circuit you can just pull.

A better, cooler option would be the relay that Condoluminum mentioned - then it would only be powered when, say, your tow vehicle's ignition was in the "Run" position, guaranteeing it won't run down your battery when you're not traveling but recharging while you are. Newer trucks have this in it already - I know my '06 F-250 only powers that circuit when it's running, so it's safe for me to leave it plugged in.
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Old 07-05-2013, 08:22 AM   #11
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You need a battery isolator mounted from your TV alternator. If you have one it's not working correctly. Been there.....Done that.
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Old 07-05-2013, 08:59 AM   #12
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i think you can use a blocking diode. hopefully one of our resident electricians will chime in with details. something along the lines of 50a/50 piv?
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Old 07-26-2013, 01:10 PM   #13
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My F150EcoBoost with the HD Tow Package has a 30 Amp fuse for the trailer 12VDC feed, and only sends 12VDC when the ignition is in the on position. I installed a Solar 30Amp Blocking diode between the truck and Travel Trailer. Works great, EXCEPT for 1 little issue. The blocking diode makes the trailer look like it is not there to the truck's electrical system, which will block any 12VDC power surges from the trailer. This will protect the trucks electrical system. I have a solar panel on the roof. Here is the issue... if your trailer's voltage is dropping while you are towing, the trucks charging system will not recognize the drop (blocking diode doing its job) so it will not step up the charging amps from the truck. Before I installed the blocking diode, the truck would generate between 13 and 14 volts (after engine was running a while and the truck battery was charged). After the diode was installed the voltage at the trailer from the truck was what ever the truck's electrical system needed. This was usually between 12 and 12.6 VDC. Not going to help the trailers battery. There are a couple ways of automatically detecting the trucks voltage using 2 blocking diodes and an automotive relay, but I would not recommend someone without electrical experience wire it in. Easy way is a battery switch between the truck and trailer.
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Old 07-26-2013, 01:57 PM   #14
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Your trailer should not be using so much power that it can drain the tow vehicle battery, I left my trailer parked and plugged in to the truck for 2 weeks and the truck started just fine. Remember the truck has a battery and the trailer has at least one battery, maybe more, so you should be able to play the radio for a Long time without draining the batteries.

Look for additional drains on the trailer battery. Also, if the trailer battery is bad, it will draw down the truck battery to the same level.

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12 volt

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