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Old 09-07-2020, 12:11 PM   #1
HammerToe
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No Aux '+12V Charging ' at 1 O'Clock Truck Connector

A follow-up to an earlier post I made a few days ago: had to replace a broken 7 wire RV connector on my truck (2005 GMC Sierra 1500 HD) (broke it backing into a stump too low to see). After I got the new connector all wired up and tried to test it with a six LED tester that I had purchased, I discovered that I had no +12V at the/THIS ‘1 PM’ connector to ground. (I was pretty sure it’d been there with the old connector, even though it was smashed; I seem to definitely recall being able to measure +12 V from the 1 PM connector and the 7 PM connector. Anyway, spent a lot of time trying to figure out what the problem was and fix it, without success.
Here’s my current situation:
• the under hood fuse at stud #2 is okay.
• there is +12 V available on one of the fuse socket pins (and no voltage on the other side as you might expect).
• BUT as far as I can tell - I tried to measure for continuity between the ‘non-hot’ pin in this fuse socket all the way to the back of the truck to the ‘red wire’ connected to the 1 PM pin in the connector using a long, made up patch cable.
• RESULTS: NO CONTINUITY.
This I do not understand. That’s a great big wire around 12gauge I think and it’s hard to believe I broke something/highly unlikely. I suppose I could try further to check all the wiring back to the front of the truck and I was advised to try to inspect the actual ‘stud #2’ BUT that’s a fair amount of work pulling stuff out to get underneath the fuse panel on this truck. I managed to sort of peak under the engine side of the fuse panel box and saw what looked like a bundle of wires containing all the colors that looked like they would/COULD belong to the bundle going to the back of the truck but that’s as far as I went.
Anyway, to the point - here’s what I plan on doing: when I started out, I thought the +12 V Aux+ Charging connector was really, really important… But I’ve been told that it kind of sort of isn’t really.
I was told that basically, all it does is feed ‘juice’ back to the trailer it’s being towed in order to keep the onboard house batteries charged so that IN TURN, in the extremely unlikely event (‘already’) that your trailer might come loose, health guarantee that there will be ‘juice’ in that battery to activate the trailers breaks when the trailer disconnect switch is ‘pulled’.
If that’s the case, then...: While I do intend to make some phone calls and try and see if I can get everything fixed before I am scheduled to leave for a month’s camping in seven days, I’m just ‘thinking’/trying to come up with a ‘contingency strategy’ JUST IN CASE the repairs cannot be completed in time, what with everything that’s going on in the world…
If that’s truly all that line is used for, well heck: I’m only going to be traveling a little more than 100 miles to get to my campsite and as long as I ensure that the onboard battery is fully charged up before I leave (which I intend to do), it should be quite safe. (First, the risk of disconnect itself is EXTREMELY LOW to put it mildly AND the chance that the battery would be ‘that low’/become depleted enough assuming it’s all charged up at the start is virtually zero.
So my question: bottom line. Any thoughts on this?
(I REALLY can't understand 'what might've happened' to cause me to have 'no continuity' (around zero resistance) between the 'non-hot' terminal in that fuse socket AND the wire at the back which connects up to that 1 PM terminal. Again, that's about a 12 gauge wire and not likely to have become broken or anything. I WAS TOLD THAT THAT WIRE GOES DIRECTLY FROM THE #2 STUD (which I was also told was 'down downstream' FROM that 30 amp fuse... So if it ain't broke, then it either has to be a connector or something that may not be doing its job (again, I thought it is supposed to go direct) or maybe there's a relay or something? ALL OTHER CIRCUITS/TERMINALS in the new connector on the truck seem to work perfectly per the LED tester: turn signals, backup lights, running lights, BRAKE LIGHTS (the 'wire of which' it seems to me must 'carry the signal' to activate the trailer brakes as far as I can tell because I can see no other way it can be done). All LED lights on the tester light up when attempts are made to test things except for the +12 V positive LED).
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Old 09-07-2020, 01:06 PM   #2
chuckster57
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I prefer a test light for these kind of issues. Clip it on a good ground and start testing.

So do you have 7 wires in a bundle at the bumper? You said you smashed the 7 way plug, and did it have the factory wiring harness that “clipped” onto the back side? I’m trying to get “up to speed” so we can figure this out.
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Old 09-07-2020, 01:14 PM   #3
notanlines
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Henry, I'm going to ask a question that you probably can't answer. Are you ABSOLUTELY sure you had voltage at the 1:00 position on your trucks plug? Those years GMC/Chevy's commonly had a red wire bundled and attached to either the bottom of the brake master cylinder, steering box, somewhere near, that had to be unraveled by the owner and connected underneath the fuse box location you reference. If that hasn't been done then you have no power at that position on the 7-pin plug. I'm just asking because you always start at the things you know for certain can't be wrong but often are.
Edit to include this video for those who aren't sure what I'm talking about:
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Old 09-07-2020, 01:21 PM   #4
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Quote:
Originally Posted by notanlines View Post
Henry, I'm going to ask a question that you probably can't answer. Are you ABSOLUTELY sure you had voltage at the 1:00 position on your trucks plug? Those years GMC/Chevy's commonly had a red wire bundled and attached to either the bottom of the brake master cylinder, steering box, somewhere near, that had to be unraveled by the owner and connected underneath the fuse box location you reference. If that hasn't been done then you have no power at that position on the 7-pin plug. I'm just asking because you always start at the things you know for certain can't be wrong but often are.
There are 2 studs and 2 wires tucked down underneath. One is power for the brake control and the other is the charge line. I agree, are they BOTH connected?
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Old 09-07-2020, 11:43 PM   #5
HammerToe
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Chucket57: in response to your responses/questions:

Yes, I have ‘seven wires in a bundle at the bumper’. Not exactly sure what you mean by “did it have the factoring wiring harness that ‘clipped’ onto the backside”.

The wiring harness and connector were OEM. Seven wires in total were connected into the OEM seven way trailer connector. As far as I know, everything has been working fine since 2008 when I had a brake controller installed.

(There was in eighth wire - some kind of plug or something on the end looking like it was intended to plug into something BUT IT WAS NEVER ‘USED’. It was just clipped and unused to the old RV seven wire OEM connector).

I am virtually 100% certain (I have a Physics degree - we types never say we’re 100% certain, but I do recall measuring +12 V there TWICE with a voltmeter before I cut the wires and removed the broken OEM connector).

I cannot easily access as far as I can tell the actual ‘physical studs’ as they appear to be hidden underneath the shroud which surrounds the engine compartment fuse block. On some vehicles, they are just lying out there in the open and easy to access but not on mine.

NoTanLines:

I am ‘absolutely certain’ I had/could measure +12 V at the 1 PM pin position with the OEM connector before I removed it. As stated above, everything was OEM with the exception of a brake controller that was installed in 2008 and I did have +12 V there before I removed the connector.

THANKS FOR THE VIDEO THOUGH - watched it and it enabled me to get a much better sense of what ‘people were talking about’.

I will double check both the Stud#1 fuse this morning. I’m pretty sure it’s okay but I will double check it. It’s been there since OEM day

Again, I can see no way to actually ACCESS the ‘physical studs’ easily on my vehicle. They are displayed as ‘dotted outlines’ in my owner’s manual and I think I may need to remove the shroud around the fuse panel in order to see them/get to them. (Or else they are located someplace physically in a manner not consistent with the way they are portrayed as ‘dotted outlines’ in the owner’s manual).

This morning I’m going to call the people who installed my brake controller and everything back in 2008. They are ‘pros’ - specialize in doing trailer hitch and brake controller installs and have been in business for decades. I suspect/am hopeful that if I simply describe my problem to them they may be able to give me a ‘probable diagnosis’ over the phone specific to my model and year of truck… I.e. if I simply ask them: “What possible reason could there be for my not being able to measure CONTINUITY between the stud number two fuse and the 12 V line at the back of the truck?” might be sufficient for them to give me a diagnosis.

I’ll let you know what I find. This is been very irritating and a better understanding of what the problem turns out to be might be helpful to others in the future as well.

Thanks for your questions and suggestions. Will keep you posted on what I find.
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Old 09-08-2020, 05:21 AM   #6
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Two thoughts, when you "checked continuity between the load side of the fuse socket and the 7 way connector, you said no joy. Are you certain there's no relay in-between those two points?

Secondly, when you "measured 12 volts at the 7 way" and then didn't. Were you using the same ground for the meter? Was that ground the ground in the 7 way connector or the frame?

Just thinking out loud.

Edit: Third question, when you replaced the receptacle did you check all positions for 12 volts or just the one pin? That would rule out a mis wired connection.
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Old 09-08-2020, 06:02 AM   #7
ChuckS
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Pic of feed wire at stud terminal from fuse box .... I’d measure from that feed point to ground to see if you actually have 12 volts

Pic from Etrailer for 2005 Sierra
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