Complex Electrical Nightmare - BCM? ECM? Short?

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

jeffm333

TYF Newbie
Joined
May 25, 2021
Posts
22
Reaction score
41
Location
Tennessee & Nevada mostly
Posting in "general discussion", wasn't sure if ECM/BCM complex electrical system issues should be in "Audio and Electronics" or somewhere else?


I have found myself in quite the pickle... I've already done quite a bit of troubleshooting and I will try to outline everything pertinent plus the conditions that led me here. 2013 Suburban Z71.

CURRENT WIRING CONDITIONS AND SYMPTOMS:
I have the alternator wire and starter wire disconnected from the starter battery. (the starter wire is not fused, and the alternator cable had a wear spot potentially causing a short to metal engine components - but more on that later) I am only connecting the main positive accessory line to the battery to troubleshoot and keep things simple for now. I do not expect having these disconnected to be the cause of my symptoms, but if anyone feels differently please let me know.

Upon making the connection (between battery positive terminal and the positive lead to vehicles systems via the main under-hood fuse box), numerous accessories immediately power on and remain powered on - the horn, the windshield washer pump (annoyingly but humorously spraying me with windshield washer fluid), A/C blower, high and low beams, DRL, and numerous other systems). Just to limit some of this behavior I pulled the fuses of the washer fluid pump and the headlights and the horn (it was deafening in the garage). The dash display does not power on whatsoever, key in any ignition position does nothing. This feels like some kind of massive short or wire-harness criss-cross; I've detailed below all the things I've done to find such a cause and have failed to find anything wrong. FIRST MAIN QUESTION IN THIS POST: If a BCM/ECM either gets fried or freaked out via being shorted or some other type foolish thing I did to it during the install, would the truck ever exhibit this kind of behavior? (It seems like a ridiculous set of symptoms, but I've never had a bad ECM/BCM with any of my GM's so have no experience in what happens when they fail)


THE MORE COMPLETE STORY:
First off: This is very likely triggered by something I have done while installing a secondary battery under the hood. I've done this same work on my Tahoe and multiple other vehicles over the years; it's pretty straightforward for a tinkering electrical engineer with respectable automotive maintenance experience; I've never had any major issues like this. I generally follow all best-practices methods; batteries disconnected for entire install process; I add negative-terminal cutoff switches to both starter battery and auxiliary battery, I make sure both batteries have fuses and/or in-line circuit breakers, I use a battery isolator/relay to isolate the auxiliary battery, I over-engineer with very heavy gauge wire and all-copper quality ring-terminals professionally crimped, etc. For this install, I can't find a single obvious thing wrong. Not only that, I had all of my secondary battery install completely isolated - all fuses/breakers off, secondary battery ground disconnected. When I connected the starter battery after completing all the work, I left the secondary system completely disconnected; wanted to make sure truck operated normally as expected first. Thus, it is unlikely the actual secondary battery wiring/connections are a direct factor. That being said, I have drilled and mounted bus bars, battery switches/circuit breakers, battery isolator relay, etc. It is feasible to have drilled into an interior fender wall under the hood, and drove a screw right into a wiring harness. I have double-checked all of these screw/mount points, even removed a handful of them, and can find absolutely no evidence of this. They are all in areas I believe to be very safe to drill into, and have had no issues doing so in the past on Gm trucks. Further down I will explain more details of what has been done and what I've looked for and attempted.

When I first connected the starter battery's positive battery terminal, sparks flew more than I expected, but were short lived. Seemed like way more than your typical little arc/spark upon reconnect. Sure enough, I checked the 175A in-line fuse in the harness, and it was blown. (To give people background who aren't familiar with this year GM truck - the positive battery terminal is a fancy little fellow that has multiple connections to it; one is a direct and unfused link to the starter. Then, another branch feeds the 175A fuse, which then has two wires connected to it utilizing the fuse - one for all the accessories and electrical system of the truck, and one is a direct wire to the alternator.) Upon detailed examination of the alternator wire, I did find a bare spot that had worn off - so this could be the culprit causing the short if that bare area touched a valve cover or aluminum head or a fuel rail or whatever. It looked old so it's probably been there a while, and me moving the cable around while working probably helped it find it's way to some bare metal somewhere on the top of the engine. This seems like a likely culprit, but I can't be 100% positive this happened or caused my issues. Thoughts? (I will just replace this alternator wire with a nice beefy 1/0 AWG new cable later; for now I don't need the alternator so it is just disconnected)

So after this, since I didn't have another fuse and it was late and I wanted to test to see if there was a short somewhere else in case this alternator wire wasn't the only short. I connected a 100A breaker to the positive cable to the trucks accessories (to the main fuse box), then connected it to the positive terminal of the battery... expecting (hoping) the circuit breaker wouldn't trip, nothing scary would happen, and I could put the car in ignition-on state and verify things were working and there was not some other short. Well the circuit did not blow, but this is when all hell broke loose - the horn blares continuously and the washer fluid incident occurred, tons of accessories just power on even though the key is not in the ignition, and is where I am now. (Worthy of note the entire dash cluster is dead and the key changes nothign in any ignition/on/accessory state)

This feels like some kind of massive short or re-wiring issue, where all these components are immediately getting power when they shouldn't. As I stated above, I changed nothing of the trucks electrical system relating to the factory accessories. I'm reasonably experienced at this dual-battery install in GM trucks and have double-checked everywhere I put a screw into any piece of metal and checked behind it as best I could. I haven't messed with the under-hood fuse box or screwed anything into it whatsoever. I can find nothing out of place.

Unlikely to be related - but while the batteries were all disconnected for the wiring work I also had removed all the middle and rear seats along with some rear interior panels; I double checked all the wiring harnesses from the rear seats and around all panels I removed and verified there was nothing shorted or touching the metal frame, etc.

I ran additional accessory wires into the main cabin through the main wiring harness grommet on the drivers side firewall. There is lots of space here; removing the boots/grommets and/or cutting a small hole through them to feed wires is fairly easy and a cleaner/safer solution than drilling new holes into the firewall (IMO), thus I prefer this method. Of course there is always risk when monkeying around near any wiring, so I've quadruple checked this work and can find no cut/frayed wires, and the new wires that are fed through are clean and not touching or cutting into any frame metal. (The wires I added ran through for future modifications and are un-terminated within the vehicle, and either unterminated under the hood or connected with the secondary battery components which are all disconnected and isolated).

I continuity tested every fuse in the interior drivers side fuse block, and in the engine compartment fuse box. Only one blown fuse, labeled "amp", I presume is the radio system amplifier. I can't even be positive if this fuse blew from this issue, or if this was already blown when I got the truck. (Just purchased this truck - had it less than a week; radio worked when I drove it but I didn't really test all the read DVD systems or all the speakers or even crank it up; the radio wasn't a priority for the purchase) Mentioning it just in case it could give clues or be related.

MY MAIN THEORIES:
1) I drilled into a wiring harness or otherwise frayed some system wires. I've checked this over and over and can find no evidence of this. Plus, the level of systems that are not working now, I would have had to drill through a TON of wires to get anything like this behavior. I am seriously discounting this as a possibility. Thoughts?
2) When running the wires through the firewall under the dash, I unplugged all the main wiring harness connections and un-clipped the mount from it's seat to move it out of my way. Could I have criss-crossed one of the connection plugs when I rewired it? This could certainly account for some crazy behavior like this for a multitude of symptoms. This was actually my most suspected culprit - but upon multiple checks and reviews plus comparing color-coding of the connectors to pictures of expected positions online, everything looks completely fine. (Plus, it would be HARD to plug these in wrong - the wiring harness groupings are all near-perfect length to where you'd have to really try to plug one into the wrong socket - and most simply wouldn't reach or fit) In any case, I'm not done verifying here; if anyone has a 2007-2013 suburban/tahoe/silverado and wants to snap a pic or compare to my pics, let me know. If anyone has any ideas of something else I could have done under the dash to cause this, let me know. If anyone has a model year of this truck and is a tech or highly familiar with the electrical systems on these trucks, please chime in or DM me.
3) When the alternator wire shorted against the engine (presumably), did shorting the cars system out a few seconds before the 175A fuse blew cause havoc with the BCM/ECM modules? Could such a thing mess 'em up or fry them outright? Could the alternator cable have shorted to some other sensor wire or caused current to flow into ECM/BCM via some unintended path thus torching one or both of them? This seems possible but unlikely - but honestly I've been lucky to not have any major headaches like this or have an ECM/BCM fail on my so I just don't know what could cause it and what the symptoms would be. Any input here would be great.

Any other ideas or input is welcome. Anyone who wants to make fun of me being sprayed with washer fluid and being deafened by the horn at midnight in the middle of the weak all due to some bonehead things I've likely done and am missing... feel free to indulge; I am likely deserving. Atleast I didn't get electrocuted at the same time.
 

B-train

Full Access Member
Joined
Sep 12, 2022
Posts
2,029
Reaction score
3,426
Holy cow! What a train wreck. The horn and washer fluid was comical......LOL

I have a theory and suggestion:

1. Theory: The unintended arc welding gave the BCM power on stuff it should have power on. A lot of ECMs switch the ground on and off for various controls, so backfeeding a ground circuit could definitely let smoke out of stuff.

2. Suggestion: check ohm resistance on your positive cables to ground and see if you can work through the system to find a short if there is one. This may prove futile if things melted in the BCM. But, you could omit circuit connections to test wiring.

Send us pics of the install and areas of work to see if something stands out as a
No-no or boo-boo
 
OP
OP
jeffm333

jeffm333

TYF Newbie
Joined
May 25, 2021
Posts
22
Reaction score
41
Location
Tennessee & Nevada mostly
INCREDIBLE train wreck indeed. With no instrument cluster and no ignition response with the key, driving it to a shop or dealer for help isn't even an easy option.

I'll send a few pics of crude sketches/wiring-diagrams of the fundamental design of the system, give some background on my modifications. (at this point the secondary battery system is mostly moot - I need to figure out the issue with the primary electrical system of the vehicle and get it working before connecting any of this work)

I'll head down to take some pics of the install and work area here shortly.

Basic Wiring Diagram of factory wiring - before 2nd battery install. (if anyone has input on things missing or wrong here, let me know)
2013-Suburban-FactoryBasicWiring.jpeg

Basic wiring diagram of system post-auxiliary battery install: (This isn't the best picture for visualizing everything, but again if anyone sees any blatant issues let me know. I've used this precise type of solution for 2 and 3 battery systems in the past with no issues and awesome results) [edit: I noticed this sketch is missing the connection from the 175A Factory fuse to the alternator]
2013-Suburban-SecondBatt-BasicWiring.jpeg


Worthy of noting is the hall-effect current sensor which is around both the engine-ground and frame-ground connections off the negative terminal of the battery. My understanding of how this affects adding additional batteries to the charging system, is you need to be sure this hall effect sensor is capturing all the current that the alternator is directing to the multi battery system. The ECM monitors this, and changes the alternator's charging profile parameters depending on what it sees. If you charge a second battery but only measure the ground current on the starter battery, the ECM will see this as a discrepancy of alternator output, and worst case give you some warning lights/codes or put the vehicle into "limp mode"... best case it will likely not maintain appropriate voltage to adequately charge the secondary battery, leading to dead/depleted auxiliary batteries and reduced battery life. Most install guidelines that are aware of this issue, simply suggest you ground your auxiliary battery(or batteries) directly to the negative terminal of the factory starter battery and NOT directly to the frame/body/engine. This is sensible, but I don't like all this additional current having to go through the arguably insufficient size/gauged factory ground wire. Thus, I use all 1/0 gauge wire for the secondary battery connections, run the factory battery ground to a new heavy-duty 300A rated ground bus, and replace the engine ground wire with heavy 1/0 gauge wire. (You can argue that 1/0 is overkill for the application, and that argument has merit, but worthy of note is I like to over-engineer... plus by the time I am done with mods for this vehicle, I will have one or two additional 100Ah LiFePO4 batteries, DC2DC converter, solar, solar controller, 2kw inverter/charger set up as a "house battery" auxiliary system and it will also be connected to the engine charging system. (albeit a higher output alternator will replace the factory 160A alternator).
 
Last edited:
OP
OP
jeffm333

jeffm333

TYF Newbie
Joined
May 25, 2021
Posts
22
Reaction score
41
Location
Tennessee & Nevada mostly
As requested, some images with annotations from the install. Some things are not fully connected in the images, and a number of finishing-tasks won't be completed until the truck is working and the auxiliary battery system tests successfully (Things like completing wire-ties and organizing routes of all wires ensuring nothing is in the path of moving hood springs/latches or any other problem-obstacles; the wire harness hole boot and other items will be re-taped with wire harness tape, the damaged alternator cable will be replaced, etc.)

Here is the factory alternator cable between the main starter battery and the alternator, showing where I found the chunk of insulation missing and bare wire exposed. I'm suspecting this is what caused the 175A fuse to blow, and most likely contributed to BCM being shorted/fried, along with whatever other damage I'll be working to find and fix:
SuburbanAuxBattInstall-01-Frayed Cable - Alt-MainBatt-Annotate.jpeg

SuburbanAuxBattInstall-02-Frayed Cable - Alt-MainBatt-zoom.-Annotate.jpeg


main starter battery overhead view (things are not fully connected in the image since I am still troubleshooting)
SuburbanAuxBattInstall-03-MainBatt-overhead-Annotate.jpeg

new negative terminal cutoff switch I added to the main battery. If you've never used one of these for your main battery... once you do, you'll never want to be without it. So convenient for disconnecting battery for any install work, or for isolating things for electrical testing, for killing connection to the vehicle before removing/changing battery, or for disconnecting battery if leaving the vehicle sit for an extended period of time.
SuburbanAuxBattInstall-04-MainBattCutoffSwitch-Annotate.jpeg

New ground-bus bar installed on inside engine compartment wall behind the battery:
SuburbanAuxBattInstall-05-GrounBusBar-BehindMainBatt-Annotate.jpeg
 
OP
OP
jeffm333

jeffm333

TYF Newbie
Joined
May 25, 2021
Posts
22
Reaction score
41
Location
Tennessee & Nevada mostly
Continuing with images....

Hall effect current sensor feeding ECM information (which must be preserved in the auxilliary battery setup, and all battery-charging current needs to flow through the same ground as discussed earlier):
SuburbanAuxBattInstall-06-HallEffectCurrentSensor-Annotate.jpeg

New 1/0 AWG engine ground connection:
SuburbanAuxBattInstall-07-NewEngineGroundConnection-Annotate.jpeg

On the drivers side of the engine compartment where the auxiliary battery is installed, there is another 300Amp-rated negative/ground bus bar. (Yes, it's colored red instead of black.. but it is all grounds... these are pricey so the package of two is what I needed. In retrospect maybe I should just paint it black?
SuburbanAuxBattInstall-08-GroundBusBarForAuxBatteryComponents-Annotate.jpeg

Auxiliary Battery isolator/relay (mounted next to the ground bus bar high on the firewall):
SuburbanAuxBattInstall-09-AuxBatteryIsolationRelay-Annotate.jpeg

Overhead view of the back right corner of the engine compartment where most of the auxiliary battery components are mounted. It's a bit tight and tricky here, as you must be sure to leave the area where the hood spring and bracket can move down in their natural rest position when the hood closes. If you don't you run the risk of the hood spring components resting on or fraying your wiring over time. I still need to do some wire-routing cleanup and wire-tying here to ensure this is perfectly clear; but all the mounted components were measured and tested for clearance already.
SuburbanAuxBattInstall-010-AuxBatteryComponentsOverhead-Annotate.jpeg
 
OP
OP
jeffm333

jeffm333

TYF Newbie
Joined
May 25, 2021
Posts
22
Reaction score
41
Location
Tennessee & Nevada mostly
Continuing... Post #3 with images:

Zoomed shot of the 200A circuit breaker between the alternator and the auxiliary battery isolator and other components. This protects the alternator and the factory electrical system components in the event there is some unexpected short in any of the auxiliary battery systems. This also enables me to just pop the breaker, and completely disconnect all auxiliary systems and components from the vehicles primary electrical system. Handy for testing/troubleshooting, and a nice extra safety feature to keep things isolated when installing new components/accessories on the auxiliary system. (I built a prototype mount for this out of a metal bracket and small block of wood; this is probably fine as is, but I generally try to fabricate something from aluminum or other metal brackets. Long-term I will have a friend use this prototype as a model to fabricate something simple out of aluminum or stainless next time I visit him.)
SuburbanAuxBattInstall-11-ZoomOfCircuitBreakerBetweenAlternatorAndAuxBatteryComponents-Annotate.jpeg

This is a Hall effect sensor monitoring current flow to/from the auxiliary battery, and it feeds a battery monitoring display that will be mounted in the interior. For these auxiliary battery systems, I just use a cheaper monitor/display, as I don't need it to be very precise. I just want a basic idea of how charged the secondary battery is, and be able to monitor the amount of current the battery is pulling from the alternator when charging. (or how much it is discharging under load) In addition to using the ignition-on 12v signal to engage the battery isolator and charge the secondary battery, I also have it switched on the inside of the vehicle; this way I have full control over when the battery is charging, and have the ability to put the car into "jump" mode by the flip of a switch in the event my main starter battery fails to start the vehicle. I can visually see the current draw during charging, and manually disconnect the charging once adequately charged. Having this display also helps keep me notified of a dying/poor secondary battery, and help notify me of anything out of the ordinary with the system. (this would be useful on the main starter battery too; at some point I'll likely add one for it as well)
SuburbanAuxBattInstall-12-MyHallEffectSensorForMonitoringAuxBatteryCurrent-Annotate.jpeg

Overhead view of where the auxiliary battery is mounte. Shows the auxiliary battery ground disconnect switch. This is a yellow optima battery I had sitting and being unused for about a year... (kudos to optima - it was at ~12.2V still, and I trickle charged it overnight and she charged right up to 12.7-12.8. for what optima charges, I'd expect no less. :) ) the switch was from the install in a previous vehicle; I prefer the marine style switches and don't truly need this one since I have a circuit breaker I can use to cut off this battery, but it was already mounted so I kept it.
SuburbanAuxBattInstall-13-AuxBatteryCornerOverhead-Annotate.jpeg

Zoom shot of the auxiliary battery 200A circuit breaker and disconnect switch:
SuburbanAuxBattInstall-14-AuxBatteryCircuitBreaker-Annotate.jpeg


Here is a view back towards the engine showing the alternator, and the disconnected alternator wiring. (Currently disconnected for testing... the factory connection to the starter battery is frayed and needs to be replaced still. I have the 1/0 AWG cable that feeds the aucilliary system fully disconnected for now as I am troubleshooting the main electrical system issues:
SuburbanAuxBattInstall-16-AlternatorConnectionsForAltAndMainBatteries-Annotate.jpeg
 
OP
OP
jeffm333

jeffm333

TYF Newbie
Joined
May 25, 2021
Posts
22
Reaction score
41
Location
Tennessee & Nevada mostly
This thread looks more like a "how to" for installing a secondary battery than a plea for input on a potentially fried BCM/ECM or other weird short.... for any moderators reading about my disaster, if this would also be useful in a "how to" section somewhere, feel free to copy there, or let me know and maybe I can just create a new post with these images and a writeup.

BACK TO THE PRIMARY ISSUE: Dead instrument cluster and ignition switch, and an obscene number of components being powered on immediately upon battery connection...

In continued troubleshooting, got out the good ole endoscope "snake camera", and meticulously used it to search behind every piece of metal I mounted a component to in order to 100% verify I didn't drill or screw anything into any wiring. I was able to verify all of these mount points, drill holes, and screws were completely free of absolutely any wiring. I also meticulously looked over all the wire bunches coming through the firewall into the main body wiring harness junction block, and could find no broken or frayed wires or any damage of any kind. So at this point, I've effectively eliminated this as a possible cause.

I also did the "positive cable to negative cable" for about 30 minutes to do the "BCM/ECM/PCM/TCM" reset. Unfortunately this didn't magically fix my issue.

This pushes me to believe that shorting the starter battery to some metal on the engine through that annoying fray spot on the alternator wire is my primary cause... and some components have indeed been damaged or effectively destroyed completely. The BCM is obviously a primary likely candidate, but what about the ECM, PCM, TCM, and any other potentially-sensitive electrical compnents that wouldn't particularly like suddenly seeing a +12.6V short to ground, possibly resulting in a reverse-polarity situation and reverse current flow through the device before the 175A factory-fuse popped? Does anyone have any input/experience on this happening? Presuming it is a thing - any input on which devices are most sensitive to damage? Any procedures for testing those components?

Unfortunately the OBDII port appears full dead; I get no power-up at all with my standard scan tool on the OBDII port; regardless of the position of the key in the ignition. So no clues here or any codes to help diagnosis.

1) Is there a way to test the BCM and ECM for functionality if I uninstall it from the vehicle?
2) Are there any fuses I'm missing that could have popped and thus some of the issue is that there simply is no power to some of the components? (I find it odd that the instrument cluster and the ignition switch are 100% dead, but other things are powering up (albeit completely erroneously)... are there any fuses I'm missing? (I have continuity tested every single individual fuse in the main engine compartment fuse box, in the interior fuse block on the drivers side of the dash, and the individual 25A fuses in the main body wiring harness junction block under the dash on the drivers side.)
3) Are there any how-to threads for BCM/ECM replacement and reprogramming procedures anyone can point me to?
4) Any recommendations on research materials for learning more details about which vehicle systems are controlled by which control modules, in order to troubleshoot which may be faulty? i.e. for example, what control module(s) manage the scan tool port? getting that up and running first is maybe the place to start?

Thanks for getting this far - I know this is one lonnnng post and on a headache-generating topic.
 

exp500

Full Access Member
Joined
May 14, 2017
Posts
1,820
Reaction score
1,657
Have you tested the starter solenoid? And the actuation wiring for it in case you are backfeeding. 175 Amps is a BIG short. Should be easy to find with ohmmeter to ground.
Have you compared your 2 battery install diagram against a Factory 2 battery system?
 

smb3

TYF Newbie
Joined
May 17, 2023
Posts
5
Reaction score
4
My gut says you may have some SERIOUS relay related problems. AFAIK, the BCM does not directly control things like the A/C blower motor, or windshield wiper fluid pumps.
 

thingraylinetah

Full Access Member
Joined
Sep 20, 2021
Posts
292
Reaction score
330
Location
Crossville Tennessee
Posting in "general discussion", wasn't sure if ECM/BCM complex electrical system issues should be in "Audio and Electronics" or somewhere else?


I have found myself in quite the pickle... I've already done quite a bit of troubleshooting and I will try to outline everything pertinent plus the conditions that led me here. 2013 Suburban Z71.

CURRENT WIRING CONDITIONS AND SYMPTOMS:
I have the alternator wire and starter wire disconnected from the starter battery. (the starter wire is not fused, and the alternator cable had a wear spot potentially causing a short to metal engine components - but more on that later) I am only connecting the main positive accessory line to the battery to troubleshoot and keep things simple for now. I do not expect having these disconnected to be the cause of my symptoms, but if anyone feels differently please let me know.

Upon making the connection (between battery positive terminal and the positive lead to vehicles systems via the main under-hood fuse box), numerous accessories immediately power on and remain powered on - the horn, the windshield washer pump (annoyingly but humorously spraying me with windshield washer fluid), A/C blower, high and low beams, DRL, and numerous other systems). Just to limit some of this behavior I pulled the fuses of the washer fluid pump and the headlights and the horn (it was deafening in the garage). The dash display does not power on whatsoever, key in any ignition position does nothing. This feels like some kind of massive short or wire-harness criss-cross; I've detailed below all the things I've done to find such a cause and have failed to find anything wrong. FIRST MAIN QUESTION IN THIS POST: If a BCM/ECM either gets fried or freaked out via being shorted or some other type foolish thing I did to it during the install, would the truck ever exhibit this kind of behavior? (It seems like a ridiculous set of symptoms, but I've never had a bad ECM/BCM with any of my GM's so have no experience in what happens when they fail)


THE MORE COMPLETE STORY:
First off: This is very likely triggered by something I have done while installing a secondary battery under the hood. I've done this same work on my Tahoe and multiple other vehicles over the years; it's pretty straightforward for a tinkering electrical engineer with respectable automotive maintenance experience; I've never had any major issues like this. I generally follow all best-practices methods; batteries disconnected for entire install process; I add negative-terminal cutoff switches to both starter battery and auxiliary battery, I make sure both batteries have fuses and/or in-line circuit breakers, I use a battery isolator/relay to isolate the auxiliary battery, I over-engineer with very heavy gauge wire and all-copper quality ring-terminals professionally crimped, etc. For this install, I can't find a single obvious thing wrong. Not only that, I had all of my secondary battery install completely isolated - all fuses/breakers off, secondary battery ground disconnected. When I connected the starter battery after completing all the work, I left the secondary system completely disconnected; wanted to make sure truck operated normally as expected first. Thus, it is unlikely the actual secondary battery wiring/connections are a direct factor. That being said, I have drilled and mounted bus bars, battery switches/circuit breakers, battery isolator relay, etc. It is feasible to have drilled into an interior fender wall under the hood, and drove a screw right into a wiring harness. I have double-checked all of these screw/mount points, even removed a handful of them, and can find absolutely no evidence of this. They are all in areas I believe to be very safe to drill into, and have had no issues doing so in the past on Gm trucks. Further down I will explain more details of what has been done and what I've looked for and attempted.

When I first connected the starter battery's positive battery terminal, sparks flew more than I expected, but were short lived. Seemed like way more than your typical little arc/spark upon reconnect. Sure enough, I checked the 175A in-line fuse in the harness, and it was blown. (To give people background who aren't familiar with this year GM truck - the positive battery terminal is a fancy little fellow that has multiple connections to it; one is a direct and unfused link to the starter. Then, another branch feeds the 175A fuse, which then has two wires connected to it utilizing the fuse - one for all the accessories and electrical system of the truck, and one is a direct wire to the alternator.) Upon detailed examination of the alternator wire, I did find a bare spot that had worn off - so this could be the culprit causing the short if that bare area touched a valve cover or aluminum head or a fuel rail or whatever. It looked old so it's probably been there a while, and me moving the cable around while working probably helped it find it's way to some bare metal somewhere on the top of the engine. This seems like a likely culprit, but I can't be 100% positive this happened or caused my issues. Thoughts? (I will just replace this alternator wire with a nice beefy 1/0 AWG new cable later; for now I don't need the alternator so it is just disconnected)

So after this, since I didn't have another fuse and it was late and I wanted to test to see if there was a short somewhere else in case this alternator wire wasn't the only short. I connected a 100A breaker to the positive cable to the trucks accessories (to the main fuse box), then connected it to the positive terminal of the battery... expecting (hoping) the circuit breaker wouldn't trip, nothing scary would happen, and I could put the car in ignition-on state and verify things were working and there was not some other short. Well the circuit did not blow, but this is when all hell broke loose - the horn blares continuously and the washer fluid incident occurred, tons of accessories just power on even though the key is not in the ignition, and is where I am now. (Worthy of note the entire dash cluster is dead and the key changes nothign in any ignition/on/accessory state)

This feels like some kind of massive short or re-wiring issue, where all these components are immediately getting power when they shouldn't. As I stated above, I changed nothing of the trucks electrical system relating to the factory accessories. I'm reasonably experienced at this dual-battery install in GM trucks and have double-checked everywhere I put a screw into any piece of metal and checked behind it as best I could. I haven't messed with the under-hood fuse box or screwed anything into it whatsoever. I can find nothing out of place.

Unlikely to be related - but while the batteries were all disconnected for the wiring work I also had removed all the middle and rear seats along with some rear interior panels; I double checked all the wiring harnesses from the rear seats and around all panels I removed and verified there was nothing shorted or touching the metal frame, etc.

I ran additional accessory wires into the main cabin through the main wiring harness grommet on the drivers side firewall. There is lots of space here; removing the boots/grommets and/or cutting a small hole through them to feed wires is fairly easy and a cleaner/safer solution than drilling new holes into the firewall (IMO), thus I prefer this method. Of course there is always risk when monkeying around near any wiring, so I've quadruple checked this work and can find no cut/frayed wires, and the new wires that are fed through are clean and not touching or cutting into any frame metal. (The wires I added ran through for future modifications and are un-terminated within the vehicle, and either unterminated under the hood or connected with the secondary battery components which are all disconnected and isolated).

I continuity tested every fuse in the interior drivers side fuse block, and in the engine compartment fuse box. Only one blown fuse, labeled "amp", I presume is the radio system amplifier. I can't even be positive if this fuse blew from this issue, or if this was already blown when I got the truck. (Just purchased this truck - had it less than a week; radio worked when I drove it but I didn't really test all the read DVD systems or all the speakers or even crank it up; the radio wasn't a priority for the purchase) Mentioning it just in case it could give clues or be related.

MY MAIN THEORIES:
1) I drilled into a wiring harness or otherwise frayed some system wires. I've checked this over and over and can find no evidence of this. Plus, the level of systems that are not working now, I would have had to drill through a TON of wires to get anything like this behavior. I am seriously discounting this as a possibility. Thoughts?
2) When running the wires through the firewall under the dash, I unplugged all the main wiring harness connections and un-clipped the mount from it's seat to move it out of my way. Could I have criss-crossed one of the connection plugs when I rewired it? This could certainly account for some crazy behavior like this for a multitude of symptoms. This was actually my most suspected culprit - but upon multiple checks and reviews plus comparing color-coding of the connectors to pictures of expected positions online, everything looks completely fine. (Plus, it would be HARD to plug these in wrong - the wiring harness groupings are all near-perfect length to where you'd have to really try to plug one into the wrong socket - and most simply wouldn't reach or fit) In any case, I'm not done verifying here; if anyone has a 2007-2013 suburban/tahoe/silverado and wants to snap a pic or compare to my pics, let me know. If anyone has any ideas of something else I could have done under the dash to cause this, let me know. If anyone has a model year of this truck and is a tech or highly familiar with the electrical systems on these trucks, please chime in or DM me.
3) When the alternator wire shorted against the engine (presumably), did shorting the cars system out a few seconds before the 175A fuse blew cause havoc with the BCM/ECM modules? Could such a thing mess 'em up or fry them outright? Could the alternator cable have shorted to some other sensor wire or caused current to flow into ECM/BCM via some unintended path thus torching one or both of them? This seems possible but unlikely - but honestly I've been lucky to not have any major headaches like this or have an ECM/BCM fail on my so I just don't know what could cause it and what the symptoms would be. Any input here would be great.

Any other ideas or input is welcome. Anyone who wants to make fun of me being sprayed with washer fluid and being deafened by the horn at midnight in the middle of the weak all due to some bonehead things I've likely done and am missing... feel free to indulge; I am likely deserving. Atleast I didn't get electrocuted at the same time.
Dadgum what a novel
 

Forum statistics

Threads
130,120
Posts
1,826,705
Members
93,598
Latest member
04 Z71 learning

Latest posts

Top