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Old 12-26-2014, 02:22 PM   #21
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REVERSAL / RETRACTION!!

One of the worst problems on public forums is the definitive statement of opinions as if they were facts. Secondly, we are often merely wrong when we post things as facts. I do that myself although I'd rather think I didn't. I did that very thing on this thread.

My son is certified as a World Class GM Master Technician. When I told him my opinion on this subject (meaning that the truck's charging circuit is dead when the key is OFF), he said, "I think that's wrong." He went home and researched the topic from a GM perspective - meaning he couldn't speak for the other two of the Big Three or the other entries in the RV towing business. Here's what he wrote me.
"Positive cable off battery powers the HOT AT ALL TIMES input to the under-hood fuse block. The input to the 30A fuse is tied to this buss bar inside the UBEC. The output from the fuse goes straight to the 7-wire plug at the bumper with no relays, modules, switches or any other means for it to be remotely or automatically disconnected from the battery."

When members ask questions on this forum regarding a TV's (in)ability to tow a particular trailer, someone or more people will recommend that the OP weigh his particular rig to get real measurements/real numbers. I decided that might work for my 2014 GMC (please note I am not projecting this information on another manufacturer's units nor on older or differently configured GM units). I have a gas engine and a single battery on the truck.

Here are the voltage readings I found this morning from my 7-wire electrical socket measuring from tab marked - to tab marked + :
12.46 – key off,
12.19 – key set to ACC mode,
12.14 – key on; NOT running,
14.81 – key on and running; T/H OFF.

With my key OFF, I have 12.46 volts ON at the trailer plug/socket.

Even after all this, my operating procedure will not change since I always have the trailer plugged into 110v AC when it's parked up by the garage before a trip. Therefore, I don't worry that the trailer cord is plugged into the truck because I ASSume the 110v - converter link will keep my trailer battery up so my truck battery couldn't be "drawn down" anyway. I doubt if I'll have any problems leaving the trailer connected to the truck for 90 minutes while I eat a steak while I'm out on the road.

Thanks to my son, I'm now more educated about how my own truck works. Your mileage may vary. The assumptions I gleaned from poorly conducted experiments and experiences were WRONG. I'm sorry. Consider metering your own voltage!
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Old 12-26-2014, 03:38 PM   #22
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Wow, very surprising that GM wouldn't have this switched on a current generation truck! I know for a fact Ford's are switched and have been for a long time to prevent the exact thing this thread was written to try and avoid. I've read on the forums that GM and Ram were the same but I guess you're right. Don't believe it unless you know for sure! Can anyone confirm which way the new Rams are?
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Old 12-26-2014, 04:55 PM   #23
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I just checked my 2011 F350. The 7-wire connector only gets power when the ignition is on. There is no power when the ignition switch is in either the ACC or OFF positions.


I read several blogs where some Chevy trucks also had no voltage to the 7-wire connector unless the ignition was on but other Chevy trucks had constant power to the connector. I didn't notice the years of these trucks.


I didn't see any way to correct this problem on those trucks with constant power to the connector.
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Old 12-26-2014, 05:26 PM   #24
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Sorry to highjack somemore, but i think we had a little of the key on or off discussion a couple weeks ago on another thread about adding fuses to get some trucks to have power back on the plug.
I also said that my gm truck is hot all the time at the plug, and i believe eppenydad said his ram had power with the truck off at his plug.
You guys have to realize that most people use these trucks to tow more than just campers. I know alot of horse and stock trailers have flood lamps, and interior lights on them that run off the truck battery since they dont have a "house" battery. Some guys might not have their power wire hooked up to their junction box.

To the op's question, yep your fine with your tv and shore hooked up at the same time.
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Old 12-26-2014, 05:28 PM   #25
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So, us Ford guys are alone in this one eh? Thanks for the info MT.... and you're right about the other uses. Sometimes, I'd prefer it hot all the time for my sled trailer as it's interior and flood lights run off the charge line.
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Old 12-26-2014, 05:51 PM   #26
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Originally Posted by TGR View Post
I didn't see any way to correct this problem on those trucks with constant power to the connector.
I don't view mine as an error which needs to be corrected. I really just want to know "just the facts, ma'am."
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Old 12-26-2014, 06:06 PM   #27
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As others have mentioned, all modern trucks from the big three disconnect the charge line in your seven way RV plug from the trucks battery when the key is off so it would be impossible to run down your trucks battery through trailer 12V usage. So, by shutting off the key, you are essentially disconnected from the trailer other than the driving light circuits. ( exterior parking, turning, and brake)
So... I guess I've learned this is not true unless my 2014 truck is not what one would call modern OR GMC is not part of the big three.

Do we have any measured voltage readings/facts from Toyota owners? Power on/off with switch and what year model?
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Old 12-26-2014, 06:13 PM   #28
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So... I guess I've learned this is not true unless my 2014 truck is not what one would call modern OR GMC is not part of the big three.

Do we have any measured voltage readings/facts from Toyota owners? Power on/off with switch and what year model?
I think I already stood corrected on that one but hey, if it makes you feel better, rub away... I'm tough, I can take it. LOL.
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Old 12-26-2014, 06:18 PM   #29
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I think I already stood corrected on that one but hey, if it makes you feel better, rub away... I'm tough, I can take it. LOL.
I'm also rubbing on myself for a post or two of my own AND getting outside the so-called big three. I'm beginning to wonder if we're not still too general... as in the possibility that a one ton in a particular line MAY BE unlike a half ton, etc.
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Old 12-26-2014, 06:22 PM   #30
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I'm also rubbing on myself for a post or two of my own AND getting outside the so-called big three. I'm beginning to wonder if we're not still too general... as in the possibility that a one ton in a particular line MAY BE unlike a half ton, etc.
That's a fine point. Never even considered that any manufacturer would wire a 1500 differently than a 2500/3500 but I guess it's another possibility.
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Old 12-26-2014, 06:55 PM   #31
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Here is the wiring diagram for mine an it shows the relay
Click image for larger version

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All of my past trucks Ford and chevy had some way to isolate the batteries from the trailer. If they didn't I would add them no use in killing my wiring by backfeeding to the truck also with todays computer systems you could toast them fast. On a buddy's truck if you didn't tell the computer it had a tow package it wouldn't work the black box at the rear of the truck for the trailer lights to work.
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Old 01-05-2015, 09:59 PM   #32
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Leave a sticky note on steering wheel - "UNPLUG CORD"

Definitely, I have ripped my power cord connector off more than once in 40 years of Rv’ing.
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