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Old 07-16-2007, 07:52 PM   #1
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GMC Diesel and charging batteries while towing

Here is a question for your Duramax Diesel guys out there.

We just completed a nice 3 week road trip. The only glitch was that our tow vehicle didn't seem to be charging the batteries on the trailer. We had a generator to plug into to charge the batteries while we were out, so it wasn't a big deal.

We have a 2006 GMC Sierra with the Duramax Diesel. I am just wondering if there is fuse in line or a relay that prevents the trailer from draining the truck batteries when not running. What should I be looking for before taking it to the dealer?

Thanks,

Sam
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Old 07-16-2007, 08:02 PM   #2
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Hi Sam -- I happen to have a Duramax with the HD alternator and dual batteries (snowplow package). Other than that you could probably figure that any alternator is matched to little more than the onboard demand of the tow vehicle. It has been said that up to 200 miles need to be driven between campsites for significant recharge to have occurred. Performance may be poor if more accessories are turned on; ie, air conditioner, headlights.

It would be difficult under most circumstances to expect more given the significant distance between the alternator and the coach batteries. Are your's located forward in the trailer?

Yes, there's supposed to be an isolator to prevent drawing down the truck batteries. Some members have been brought to their knees come morning. Until I test it thoroughly I still undo the umbilical on the few instances I have stayed hitched overnight.
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Old 07-16-2007, 09:40 PM   #3
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it's a simple test to see if you have power to the 7 pin connector--pieman
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Old 07-16-2007, 10:17 PM   #4
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Quote:
Originally Posted by Mike Lewis
it's a simple test to see if you have power to the 7 pin connector--pieman
I did that. No 12 volts on, I think pin 7 (or 4) I forget, but, I did follow the pin out for the connector.

I realize too that it would take a few hours of running to get a significant charge on the batteries. We were running anywhere from 4 to 7 hours of drive time between locations.

I also decided to put a VOM accros the batteries after everything was hooked up and engine running on the truck. I saw no change in the voltage.

Fortunately it's not a problem that would be a show stopper.

Thanks,

Sam
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Old 07-16-2007, 10:23 PM   #5
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The line to the trailer may be fused in the TV. For some reason not all auto makers put the fuse in. Check your fuse box.
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Old 07-16-2007, 10:39 PM   #6
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Oh Richard -- thank you for tickling some grey cells!

Sam, do you see the fuse in this location just over the top of the driver side quarter panel? http://www.airforums.com/forums/f439...art-15454.html -- pictures at post 11.
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Old 07-16-2007, 10:50 PM   #7
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GMC
decided to be cheap(er) after 2003 and decided not to include the fuse that runs the 12 volt current to the 7 pin. I replaced the dummy with one I purchased at Napa Auto. I think I figured out how/which one via the diesel page.
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Old 07-16-2007, 10:59 PM   #8
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Quote:
Originally Posted by rjd
GMC
decided to be cheap(er) after 2003 and decided not to include the fuse that runs the 12 volt current to the 7 pin. I replaced the dummy with one I purchased at Napa Auto. I think I figured out how/which one via the diesel page.
I was just poking around the Duramax forum and the same question was posted. The answer was under the hood on the drivers side, Stud 1. Sure enough, no fuse....

Now for the funny part. When I bought the truck, and installed the trailer brake system, there was a green fuse taped to the wiring harness. I looked at that and wondered what it was for. 1 year later, I NOW know what it was for.

Now to the auto parts store to buy a new fuse. (who knows where the original one is, probably in a land fill by now.)

Sam
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Old 07-16-2007, 11:06 PM   #9
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Quote:
Originally Posted by CanoeStream
Oh Richard -- thank you for tickling some grey cells!

Sam, do you see the fuse in this location just over the top of the driver side quarter panel? http://www.airforums.com/forums/f439...art-15454.html -- pictures at post 11.
Found your picture. Yup, there be a red dummy fuse in that spot on my truck right now.....

Thanks guys for your help.

Sam
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Old 07-16-2007, 11:07 PM   #10
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Congrats -- my post 11 photo in the thread above came about because I still had the fuse and had our GMC dealer's diesel mechanic show me exactly where it went.
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Old 07-17-2007, 11:43 AM   #11
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Yup - same thing happened on my first trip with my Duramax. Checked this forum, looked at the wiring harness that came with my truck (but didn't use) and, sure enough, there was the 40amp fuse taped to the harness, complete with instructions. Who'd'a thunk.

Why in the world GM, in its wisdom, doesn't just install the fuse on vehicles with the tow package (!) is beyond me.

Glad you found the right answer.

Pat
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Old 07-17-2007, 02:47 PM   #12
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for those of you with the snow plow prep package on a gm vehicle, here is a thread on how to use the second battery and the factory relay.

http://www.airforums.com/forums/f449...ucks-3770.html

the relay prevents discharge of the starting battery, those of you with diesels may or may not find it helpful.

john
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Old 09-17-2007, 11:27 AM   #13
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Same thing here but with an added bonus: I had the dealership install our brake controller on our HD Duramax ('04) and today I went out to check:
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Old 09-17-2007, 02:52 PM   #14
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I'd say interesting... But not! Not interesting. Didn't they know what they were doing? At least it's an easy fix that anybody can do themselves after picking up the proper automotive fuse.
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