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Old 06-24-2014, 03:16 PM   #1
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Truck batteries discharge

Here's the problem:

I hook the hitch up to the AS and my truck batteries seem to discharge. The pig tail is NOT connected...only the hitch ball. It's happened at least 3 times and only takes about an hour for the batteries to need a jump. Truck batteries are in good shape. No problems any other time. Ideas?

Thanks,
Jason



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Old 06-24-2014, 03:57 PM   #2
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Originally Posted by jasonsherman View Post
Here's the problem:

I hook the hitch up to the AS and my truck batteries seem to discharge. The pig tail is NOT connected...only the hitch ball. It's happened at least 3 times and only takes about an hour for the batteries to need a jump. Truck batteries are in good shape. No problems any other time. Ideas?

Thanks,
Jason
Well, the truck batteries WERE in good shape! If you've run them down to the point you need a jump-start at least three times, maybe not so good anymore.

Question: When you hook up and let it sit, is anything turned on inside the trailer? I ask because when you hitch up the trailer, the trailer and tow vehicle all of a sudden share a common ground because of the metal-to-metal contact at the hitch; both ground to their respective frames and if the frames are connected…

Not having the pigtail connected just means that you haven't powered up the trailer taillights or the brakes. Your problem is not in the trailer taillights or electrically-operated brakes; it's somewhere in the rest of your 12v system.

I know that's not much help, but at least it's a start.
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Old 06-24-2014, 04:06 PM   #3
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Thanks. I'm 99% sure nothing's on in the trailer, but will check. If nothing's on in the truck or camper, where's it go? The AS battery doesn't seem to discharge. It sits for weeks without being plugged in and the AS battery is fine. ...strange.


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Old 06-24-2014, 04:26 PM   #4
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if the pigtail is not connected there is NO possible way that the trailer can run down the truck batteries.

The weight of the trailer on the truck might somehow be causing some other load (short) in the truck wiring which only occurs when the weight is on the ball.
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Old 06-24-2014, 05:13 PM   #5
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if the pigtail is not connected there is NO possible way that the trailer can run down the truck batteries.

The weight of the trailer on the truck might somehow be causing some other load (short) in the truck wiring which only occurs when the weight is on the ball.
I agree, I can see no way the trailer can discharge the truck batteries with out a cord plugged in. The common ground would make no difference.
If you leave something on in the trailer the trailer battery would discharge
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Old 06-24-2014, 05:20 PM   #6
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Is there some kind of device or meter I can hook up to the truck batteries to validate my problem?


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Old 06-24-2014, 06:11 PM   #7
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You don't suppose somehow the trailer is wired positive ground? As soon as the ball touches the coupler it could start discharging the truck and trailer batteries.
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Old 06-24-2014, 06:17 PM   #8
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Everything works in the trailer...if reversed, I'd have a few problems with convertor, inverters and such. Right?


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Old 06-24-2014, 06:58 PM   #9
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You don't suppose somehow the trailer is wired positive ground? As soon as the ball touches the coupler it could start discharging the truck and trailer batteries.
That's a long shot that occurred to me also but I can't quite make it work unless the trailer had been redone. More likely something like that could happen with the early 60's

What is the trailer?
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Old 06-24-2014, 06:59 PM   #10
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Old 06-24-2014, 07:43 PM   #11
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Everything works in the trailer...if reversed, I'd have a few problems with convertor, inverters and such. Right?


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It's just barely possible if both the converter and battery were wired reversed it could happen, but it would be a rare thing for everything to work--backwards. Lights would work, water pump would work, furnace fan would run, though little air would come out of the ducts. Easiest way to check for that and confirm it isn't happening is to use a digital volt meter, touch it to the positive terminal of the battery with the + lead touching the positive terminal, and the negative lead touching ground. If you get no reading, try touching the "+" lead to the negative terminal, and repeat the test.
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Old 06-24-2014, 08:03 PM   #12
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Does your tow vehicle have a load sensing system that is possibly turning on? Have you measured the draw? Knowing how big or small the draw is can sometimes narrow down the possibilities.
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Old 06-24-2014, 08:20 PM   #13
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OK, Its late and I'm thinking off the top of my head again. Is the tongue jack down creating a possible ground path? Yea, It's really not likely, but ground issues are the most common problem in automotive wiring. Is the key in the ignition switch? The Euro cars I work on call this the S contact and any number of things are energized. A weak battery could drop below the required cranking voltage. Hence the need for a jump.
If you've got a DVOM you could try a type of voltage drop test. Before connecting the trailer to the ball set the meter to 20 volts DC and connect one lead to the ball and the other to a good clean metal spot on the tongue. If you've got any voltage +or-, over, say a 1/2 volt there is some kind of interaction between the truck and trailer. However I have no idea what that could be.

Good Luck,
Tom
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Old 06-25-2014, 07:40 AM   #14
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Good idea...I will test using the method above. Here's something else: last time I used the electric jack, it has a loose wire. I had to wiggle the wire to get a connection to get the jack working. Do you think this could be the cause of my problems? Or, just an unrelated problem.
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Old 06-25-2014, 09:46 AM   #15
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When I connect a volt meter, I get between .05 and .09 volts. The red line is touching the ball of the truck and the black is touching inside the AS coupler.

I'm going to try this on another camper in a bit and I'll keep u posted.


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Old 06-25-2014, 11:14 AM   #16
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The above test will tell you nothing since the tow vehicle and the trailer are not sharing a common ground point.

What you need to measure is the draw on the battery. I would suggest first setting your meter to the 20A range. Then, on your meter move the red test lead to the socket just above where it currently is (to the one marked "20A"). Then disconnect the negative batter cable on the tow vehicle....keeping the negative battery cable disconnected, you need to put your meter "in series" between the negative post and the negative battery terminal. Connect the black meter lead to the negative battery cable and the red lead to the battery cable you earlier removed from the battery post. With your meter setup in series with the battery you are now measuring the actual draw on the battery. Now connect your trailer and monitor what happens to the battery draw.

One note of caution while doing this.....be sure to close and keep closed all the door, keep the keys out of the ignition and make sure nothing is turned on in the tow vehicle. It's not hard to draw over 20A and blow the fuse in your meter if you're not careful.

Good luck!
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Old 06-25-2014, 11:16 AM   #17
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Here's the problem:

I hook the hitch up to the AS and my truck batteries seem to discharge. The pig tail is NOT connected...only the hitch ball. It's happened at least 3 times and only takes about an hour for the batteries to need a jump. Truck batteries are in good shape. No problems any other time. Ideas?
The problem is in your truck, not your trailer. It doesn't matter whether the trailer is positive or negative ground. Most of the theories presented upthread don't make engineering sense. There's one (or possibly more) of four things going on:

1) Your truck has some sort of load leveling or ride control system that doesn't work quite the way it should, and the weight of the trailer is turning some part of it on, causing the battery drain.

2) Your truck has existing problems with its battery or charging system, and when you drive around with the trailer, the slight extra load of the trailer brakes, lights, and charge line is enough to knock over the house of cards. When you park the battery's already low, and an hour later the truck won't start.

3) You're in the habit of leaving something on in the truck when you park with the trailer and you don't realize it. Perhaps you leave a door open that results in the dome light staying on, or you run the stereo or some other load, or leave some other light on when parked.

4) There is some sort of intermittent wiring fault in your truck and the added weight of the trailer on the hitch stretches the frame just enough that the problem shows up. I believe the chances of this are extremely remote.

The way to find these kinds of problems is to 1) test the battery and measure its state of charge before parking to rule out problems there, and 2) connect an ammeter to one of the battery leads and monitor the current draw while parked. If an hour is enough to create a problem with a good, fully charged battery, you should see at least a 10 amp draw. Then pull fuses in the truck fuse box until the draw disappears, at which point you've found the circuit that is creating a problem.
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Old 06-25-2014, 11:17 AM   #18
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Connect the red test lead to the negative battery cable and the black lead to the NEGATIVE BATTERY POST.

sorry for the typo....
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Old 06-25-2014, 11:18 AM   #19
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Well said jammer.
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Old 06-25-2014, 01:05 PM   #20
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I see you have a ninety one ford diesel. Us there a secondary trailer hook up wire under the hitch that might be being pinched when weight is applied? My truck has a regular rv hookup and a blade type hook up underneath..can it be shorting? Jim
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