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Fog diode mod causing BCM SHORT and no warranty coverage

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it would be simple enough to come up with a low cost driver module to do the heavy lifting of driving the extra relay. Maybe something like a 2N2222, config it like a saturated switch, power and ground, an enclosure...


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This. When I first saw this mod, I knew it wasn't something I was willing to do to my truck. To me, it was too simple an approach given none of us know the BCM circuitry, or how hardened it is from things like reverse-recovery effect of the diode. Others can do what they want to their truck, more power to them, and at the same time I don't fault GM for voiding any BCM or electrical warranty for a mod like this.


When I looked at doing such a mod, I decided tapping the high beam circuit to an old fashion mechanical relay to drive the fogs was a much more isolated approach. Open relay means no current going back into the BCM when lowbeams and foglights are on. Back current from the relay when energized to the BCM circuit for the foglights could be isolated with a Zener diode. A 2N2222 transistor acting as a biased switch could definitely act as a nice stand-in for my relay approach. And there wouldn't be the annoying click from the relay. :)

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I had the mod for 2 years on my 2014..no issues. Now I have had it 6 months on the 2016...no issues. But after reading this I'm thinking about removing.


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It was a freak deal, and the diode didn't have anything to do with it. They just chose to use it as an excuse. The relays are ground switched, not power.


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I agree...it was a cop out from the dealer.

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Two cents:


This whole situation is analogous to a person who develops lung cancer......doctor's first question is always: "Do you or did you ever smoke cigarettes?" If you did.....case closed, you did this to yourself....that's the cause and excuse for the condition despite the fact that humans (like the vehicle's BCM) are subject to millions of operations, interactions and environmental extremes over it's lifetime, any one of which could have been the trigger for a partial or catastrophic failure.


Whether someone's smoking really did trigger the cancer or someone's screwing around with their electrical system actually caused the eventual BCM failure is irrelevant. The dealer will use that same justification for any external screwing around with the electrical system: It was not necessary for the vehicle to operate as delivered........you did it for your own purposes and assumed the risk despite a specific worded disclaimer in the warranty.......So, it's likely you caused it yourself and you will pay the price. And there you are with no way to prove differently!


As far as expecting coverage under the warranty......reminiscent of TV Baretta's catch phrase for advice: "If you can't do the time, don't do the crime".

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Stuffing a diode into a connector isn't professional workmanship. All I'd need to see myself to think whoever did that kind of install was clueless. Opens up the suggestion maybe he poked it into a different hole by mistake and maybe even another until he got it to work.

Not saying that's what happened, but seriously - you'd expect a tech to see that kind of mod and think .. 'cool !' ?

Other ways to do what you want - eg disconnect the fogs and run through a relay triggered by low beam lights. Power supplied by a fused connection to battery. Run everything in wire loom and use proper connectors and wiring practices. If you don't have the experience to do it right, then dont be surprised when the dealer cries foul.

Edited by 15silver
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Yep. This is why I won't do anything but drive mine. I even have my dealer do oil changes. This way, if ever a problem I can tell them your the only ones who have touched it. Good luck!


Exactly and ditto.


Doesn't matter if what you did actually caused the failure or not... it's on you to prove it didn't, your word against theirs.


Nobody but the dealer touches my truck for as long as it's under warranty (and it'll be under warranty a long time). Everything recorded and written down. If something fails, it's either going to be the warranty problem or the dealer's problem.

Edited by OmniGLH
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I believe there have been hundreds that have done this mod with 0 issues. My truck being one of them. Also have all 4 footwell lights that come on with the dome, love it.

curious to see how this turns out. Subscribed.

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I don't wonder, because it is not a cheap kludge like jamming a $1 diode to cause a one way short in the wiring harness and pray it doesn't screw something else up. Worth the extra $99 or so for piece of mind and designed protection for a $50K vehicle.


Purchased their fog wiring harness for my Camaro......uses all GM OEM electrical connectors which plug in series with the existing lamp connectors. As I see how it operates, the harness contains the it's own fuse and relay(s) which switch using the power from the upstream headlamp side, isolating it from the downstream BCM side of the GM headlamp relay, and a separate fused power tap to fire the its relay(s) instead of increasing electrical draw on the existing GM circuitry. Simply plug harness connectors in-line with existing headlight/fog light connectors, looks like an extension of the OEM harness and not readily apparent to other than the installer........but works great and maintains existing OEM operation of the (off with) DRLs.


Fogs go on automatically with low only, or low and high beams depending upon the version of harness. I bought their more complicated harness which contains 2 instead of 1 relay because in my State it is illegal to have fogs on with highs and an instant flunk for State inspection (can't do that with diodes!).

Edited by Thomcat
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Well. The repair could be done a lot cheaper than what the dealer is quoting These modules can be had for under $100. Part number 13587704 shows as fitting the 2015 Sierra. I'm sure there is someone on this forum that can program the VIN number for you. eBay or some place like GM Parts Direct is where I'd go. I think even another dealer would probably program it for you. Just my opinion.

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The first issue is the puddle/domelight's. I first noticed the issue when the puddle lights were flickering when the unlock button was pushed or the door was opened. Also, the interior dome lights would not come on when the door was opened. I've pushed the overhead switch to the on position and sometimes they would flash on quickly and shut off, but most of the time not come on at all. The dome lights come on when the individual buttons are pushed, but not at all with the overhead switch. Anyone know where the fuse is that controls the overhead switch?


My second issue is with the left turn signal. On the DIC I got a turn indicator failure for the left rear. The indications are with the blinker activation, the mirror/dash blinker lights will flash normal for about three seconds and then pulse quickly, followed by DIC message. *edit* Brake lights (left) also don't activate.


I took the truck in today and got it diagnosed. The diode mod, performed exactly like the steps in this forum, somehow shorted out my BCM. I can't believe that would have done it, considering several other things that BCM controls work without issue. Well, new BCM and programming repair will cost one $675 dollars, all because I forgot to take out the damn diode before going to the dealership. Hard life lessons.


sidenote: I also performed the footwell lights modification and I'm certain this somehow factored into the equation.


I just removed the bcm and there are no chared areas or visible broken solders. If the bcm shorted out, wouldn't I be able to see something on the circuit board? Can any trained eyes spot any areas of interest where I can snap a closer photo to post.


With everyone who has had zero issues with the diode mod, I find it hard to believe this is the case. I think they saw the mod and blamed it all on that right away.


Should I even take it as far as to say per the magnuson warranty act, in order for them to decline warranty coverage, they have to prove the diode caused the failures?


Financially, what could that lead to in further costs for me, or will they not want to hassle and potentially just fix it? The entire BCM looks unaffected


Why don't you just remove the diode and go to a different dealer

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The next option is the Chevy dealer closest to me. Won't they be able to see the previous service record from the gmc dealer somewhere in the system? I'm unfamiliar and I don't feel like paying another $$$ for diagnosis fees only for them to find the previous write ups. I considered it.

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The next option is the Chevy dealer closest to me. Won't they be able to see the previous service record from the gmc dealer somewhere in the system? I'm unfamiliar and I don't feel like paying another $$$ for diagnosis fees only for them to find the previous write ups. I considered it.



Yes, that notation will be with you forever.

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Well. The repair could be done a lot cheaper than what the dealer is quoting These modules can be had for under $100. Part number 13587704 shows as fitting the 2015 Sierra. I'm sure there is someone on this forum that can program the VIN number for you. eBay or some place like GM Parts Direct is where I'd go. I think even another dealer would probably program it for you. Just my opinion.


Wrong, no one can program the BCM other than the dealer and maybe White Industries. There is no 3rd party software out there that can touch the BCM on any of GM's platforms. While you can get the BCM probably cheaper thru someone else and maybe even get another dealer to program it, truck isnt driveable with the BCM out of the truck.

Edited by TJay74
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