[flynscott]

Replaced rear fusebox after trunk water damaged it. Replaced alternator due to no charging (same water event). Car runs and starts mostly fine

Battery shows 12.5V car-turned off, and 14.4V when on and charging. However, voltmeter on panel pegged at 9 regardless. Charging system service message. Grems include: no light in shift handle display, windows roll down not up, first push on start button no response, second push ACC mode, then third push it starts, interior trim lights don't work.

Dealer says:

"Found an open in the body harness to the bcm causing the BCM to read battery voltage as 35.3V recommend replacing body harness
Additional parts and labor may be required.

Parts:
Body harness
Coolant
Refrigerant 1.2 R134a
$5,056.14"

Also says harness is unavailable for order which is inconsistent with above recommendation.

SO, 5 grand is a non-starter even if I COULD get a harness. Going to go pick up car and trouble shoot more.

Thoughts
1. If accept open-circuit wire, could I just add a jumper wire? (I'll have to get into diagram to see if anything taps into it along the way)(they will have to ID the wire for me)
2. If I don't accept analysis, thinking this:
- what/where is source of voltage signal?
- could a bad AGM battery be causing this? replaced 6months ago Autozone and they test it good now. Has run dead a few times due to different issue.
- bad grounds?
- bad BCM? ECM?
3. If BCM thinks 35.5V, wouldn't it disengage alternator so that I would not see 14.4v when running and batt eventually go dead? Wouldn't the voltmeter peg high rather than low? or is this just an indication of an invalid signal?

Anyway, REALLY would like to avoid 5grand to start with - prob will end up more... and this is after a 4grand tranny rebuild due to the dreaded torque-converter shudder ... sheesh! (shoulda bought that corolla .. lol!)

Any insights and ideas to get me started on troubleshooting greatly appreciated.

2016 2LT 120k miles

***Update 1***
Dealer rescinds estimate due to harness not order-able/out of production. Asks if wants tech to run the wire for $200/hr and how much do I want to dedicate to the goose-chase. I am not liking my odds... prob take it home and poke at it a while longer.

***Update 2***
Discussed with tech. He ran a jumper to confirm the open-circuit wire he found - it solved the only two codes listed and that relate to charging system. Windows, although they have power, will still not roll-up, interior light trim still no workie, three press to start, no light in shifter. Says to run a permanent jumper prob 5-6 hrs@$200/hr to solve that one issue so $1000+. The other issues are still a mystery, but agreed that they are tied into BCM, so BCM replacement $220part/$200 labor. SO, having them do the BCM, and then I'll run the jumper at home. Hoping that solves all issues. If not, perhaps I"ll discover other wire issues when running jumper..

[ctrlz]

Originally Posted by flynscott

Dealer says:

"Found an open in the body harness to the bcm causing the BCM to read battery voltage as 35.3V recommend replacing body harness
Additional parts and labor may be required.

Parts:
Body harness
Coolant
Refrigerant 1.2 R134a
$5,056.14"

there is a lot to cover in your description.
I do not understand how an OPEN circuit would cause a module to overreport voltage. Since you probably paid for dealer diagnostics, I would try to get the specifics of what they diagnosed.
It does sound like you are getting an active charging cycle if you are reading 14.4 V at the battery. I have posted other threads with lots of details on the power management. I am pretty certain a BCM reports battery voltage to the ECM and there is one wire from the ECM that controls the alternator duty cycle. I think the controls go 0-100% in 10% steps. So your digital meter reading 14.4 V charging may not exactly mean CONSTANT 14.4 V. it could read that at 80% duty cycle depending on the meter. Not such a big deal, as it's great to know it IS charging.

What is with the AC recharge? I'll assume that's unrelated to the water damage or dealer trying to upsell some service.

[CalgaryZL1]

This is a long shot but do you have after market led side markers? If so when the harness gets moisture/water in it, it can cause havoc. If you haven’t changed side markers, disregard comment.

[flynscott]

ctrlz,
Yeah, the 35V doesn't make intuitive sense. If the circuit is open or broken, I would expect 0V. Or possibly the BCM invalidates the signal and chooses a default - but still, 35V as a default? I am also suspicious of his contact point to measure resistance in that wire - poor contact point = infinity resistance (open). I'm also interested in the source for the voltmeter and/or wire to the BCM. So far they have stiff-armed getting the tech on the phone so going in person in a couple days. AC recharge? No idea. Perhaps have to open AC system to get at harness??

[flynscott]

Calgary,
The only aftermarket component is a sub-woofer amp also in trunk that had about 2-3 inches of water. Will disconnect the amp once I have car back - cuz, why not. I have read that add-ons can cause some strange effects in this beast.

[CalgaryZL1]

Originally Posted by flynscott

Calgary,
The only aftermarket component is a sub-woofer amp also in trunk that had about 2-3 inches of water. Will disconnect the amp once I have car back - cuz, why not. I have read that add-ons can cause some strange effects in this beast.

If you are literal about 2 to 3 inches in the trunk, good luck brother. That’s a can of whoop ass.

[flynscott]

Originally Posted by CalgaryZL1

If you are literal about 2 to 3 inches in the trunk, good luck brother. That’s a can of whoop ass.

No doubt. He called me while driving as various systems started cutting out (water sloshing about). If you haven't the had need to see it, car has a low lying tub below trunk mat where a spare would normally be. It is about 6 inches deep with no drain. Here is where Chevy decided to put the fuse box and Bose Amp. I was surprised there was not more visible damage, just one area of scoring/melting of box. We have since drilled a couple of drain holes. After some investigating, we think that water came from two sources: seepage through top seal of trunk obstructed by pine needles, and possibly sunroof drain hoses. Sunroof drain leaks/clogs normally present in interior of car, but was told the headliner is fiberglass that would allow drain gutter overflow due to a clog to flow back and on-top of hard headliner and into trunk. Sux. Recommend preemptive drain creation.

Side note: after draining, rained overnight and then froze. Ended up with another inch of ice in trunk we had to thaw and drain before starting work. Doh!

[flynscott]

***Update 1***
Dealer rescinds estimate due to harness not order-able/out of production. Asks if wants tech to run the wire for $200/hr and how much do I want to dedicate to the goose-chase. I am not liking my odds... prob take it home and poke at it a while longer.

***Update 2***
Discussed with tech. He ran a jumper to confirm the open-circuit wire he found - it solved the only two codes listed and that relate to charging system. Windows, although they have power, will still not roll-up, interior light trim still no workie, three press to start, no light in shifter. Says to run a permanent jumper prob 5-6 hrs@$200/hr to solve that one issue so $1000+. The other issues are still a mystery, but agreed that they are tied into BCM, so BCM replacement $220part/$200 labor. SO, having them do the BCM, and then I'll run the jumper at home. Hoping that solves all issues. If not, perhaps I"ll discover other wire issues when running jumper..

[ctrlz]

If you have parts with the original stickers, you might be able to find junkyard pulls on ebay.