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Old 09-06-2016, 07:25 AM   #11
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I think you have to put a fuse in the TRUCK to get it to charge. Most dealers will install it when they wire the truck.

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Old 09-06-2016, 08:00 AM   #12
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The first thing to do is to check for 12vdc at the truck 7 pin plug. Per the diagram above you should find 12vdc when the truck is running on pin 4 of the plug. If you have no voltage there your truck was never setup for the 12vdc aux. Depending upon the brand of truck you may have to install a relay in the fuse box and or a fuse in the fuse box. In a GM truck you may have to connect the wiring harness as it does not come from the factory connected.

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Old 09-06-2016, 01:59 PM   #13
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Same thing happened to me, it was a blown fuse in the truck.
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Old 09-06-2016, 02:18 PM   #14
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My 2000 Ford pickup had a tow package installed. I noticed no charge coming from the correct output on the 7 pin. Went to the little bag that the hardware had come in looking for the wiring diagram and found two small relays in the bag.
The previous owner had never installed the relays for the tow kit.
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Old 09-06-2016, 03:48 PM   #15
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Originally Posted by neal View Post
thanks for the clarification, I will check those items.
First, you need to ensure that the 12 volt line through your 7 pin connector is feeding power to the trailer. If you are here just hooked up with the engine running there might not be sufficient current flow to charge the battery but you should be able to test 12 volts at the trailer when the truck is hooked up and running.

The point of turning on your truck headlights is to put a demand on the truck battery so the truck alternator steps up the current and voltage available for your towed vehicle.

If a new battery has gone dead in your trailer there is probably something wrong with the electrical circuit in your trailer unless it that has taken place over several weeks.

While you're on the open road with the engine it working RPM, at best the feed to the trailer will be a trickle charger. It could take 10 or 12 hours of Road time to make a meaningful difference in the battery charge level

Best tool is a multimeter that is also a AC DC clamp meter. I need to be able to measure the current flow in or out of the battery. If the battery is disconnected, fully charged and allowed to rest for about 12 hours a good battery will read approximately 12.7 volts. Totally disconnected, a week later I would expect a good battery still read 12.6 volts more or less. The same should be true with a battery hooked up to your trailer if you have a true battery cutoff switch. The same problem keeps showing up time and time again of people under estimating the static draw of their trailer either because they don't have a cut off switch or because the cutoff switch doesn't cut everythingoff.

Short answer a good battery should hold its charge if not connected to a load. Enroute the tow vehicle is barely adding a charge to even a good battery so if you start out without a fully charged battery you're going to arrive in Camp without a fully charged battery. Possibly the biggest mistake universally made is that one battery is enough for anything. One battery should be enough to meet your needs from home until you get to the hookups in Camp. If you plan to get dry camping time out of one battery you are used will have to be very very tightly managed.

Finally if you have drawn the battery down to zero chances are very good that the battery is irrevocably damaged. I would wrap it with a big voltage charge and leave it on the charger overnight then tested disconnected voltage 12 hours later and again 3 days after that. It is still sitting around 12.6 or 12.7 you might have some useful life still left in it. Good luck
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Old 09-06-2016, 07:36 PM   #16
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Not sure what truck you have but get out your operators manual and lookup the fuses. Look for the battery charge fuse, or on GM trucks look for a red wire taped back on the wire loom coming up under the fuse box under the hood. It gets attached to the battery stud on the left side of the fuse panel. It goes back to the trailer plug in back. Mfrs. do not install the charge line fuse or hook up the cable.
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Old 09-07-2016, 07:22 AM   #17
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Something almost nobody ever thinks about is the output of the alternator on the truck. If it is a standard one it will have an output of around 150 amps. If you intend to have your
truck power (12V) accessories like electric brakes on a trailer, and extra 15 or so running lights, turn and brakes lights etc. you will have to order your truck with a high output alternator, something like 200 or higher amp output. This is the reason Ford offers a 200 amp as well as duel alternators with a 357 amp combined output, not sure about GM or Ram
but I think they do also. You might think that they would be included with the big tow packages but they usually are not.
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Old 09-07-2016, 11:06 AM   #18
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Caution- thread hijack in progress!
I hear what your saying about extra load but my little ole 04 silverado has
never exhibited any signs of low battery and I've towed thousands of miles.
3 different trailers. Current one has LED marker and tail lights so less load
there but the others were all standard bulbs and never a problem with battery charge. And I run with my lights on all the time, not just at

The ONLY time I had issues was with my 1st trailer which had the 3 way
refer. When that thing was on 12v cooling it sucked power. Truck nearly failed to start one time after a short 30 minute lunch stop with engine off.

Back to the original post we haven't gotten a follow up from the OP but hopefully we've
answered his question!

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Old 09-08-2016, 05:08 PM   #19
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[QUOTE=spock123;1310172]You can turn your lights on your truck and that will put a little more power to your battery

I don't accept that as a blanket true statement for most/all/some vehicles. I have also read the same about selecting tow/haul, but in neither case was there any difference in voltage at the 7-way plugs of my 2016 F-250. It might be so in other vehicles but the only way to know for certain is to check with a voltmeter.
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Old 09-08-2016, 09:07 PM   #20
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I wanted to check out if there was more voltage in tow mode or with the lights on. I have a 2015 dodge ram 3500 with the cummins diesel and the heavy duty alternator. I got the same voltage measures (13.87 volts) at the 7 pin receptacle with nothing on, with tow mode on, with lights on, with tow mode and lights on.

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