Both batteries bad? - Chevy and GMC Duramax Diesel Forum
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post #1 of 40 (permalink) Old 12-01-2019, 04:49 PM Thread Starter
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Both batteries bad?

Past few days my truck has been startin like the batteries r goin bad but I just figured it was the cold and I wasn’t waitin long enough to let the glow plugs up all the way. Last time I drove it was Friday. Today is Sunday and I tried to start the truck this mornin. DEAD. I drove work truck today and get home earlier and notice my parkin lights r on. But switch is off. WTH. Been a few years since I installed these batteries seems like. They r interstate batteries. But y is my parkin lights on with the batteries dead?

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post #2 of 40 (permalink) Old 12-01-2019, 04:52 PM
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Do you have any LEDs on the grid? (not just parks)

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post #3 of 40 (permalink) Old 12-01-2019, 05:09 PM Thread Starter
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No LEDs. Just factory lights and no accessory lights either. Only thing not factory electronically is that I have an inverter wired to the battery with an in-line fuse (bought the truck that way) it stays turned off.
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post #4 of 40 (permalink) Old 12-01-2019, 05:30 PM
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If you have one bad battery then you have two bad batteries. The way it works is like this:

When a battery dies it typically has a cell short out. This is usually because of a buildup of sulfate at the bottom of the cell but it can be from sulfate between the cells or some other cause. When the cell goes bad the battery becomes a 10.5 V unit. If the other battery is still good and has 12.6V then any time the engine isn't running the good battery will try to charge the bad battery. In other words the good battery will discharge through the bad battery until the voltage is the same in both. 10.5V is a really dead battery, normally. Running a starting battery, like we have in our trucks, down to that level will kill a good battery in short order.

If the good battery is still good, it will be dead whenever it comes time to crank the engine. Only the bad battery will have anything in it but it will be a 10.5V battery so it won't crank very fast and since it is now delivering twice the current it normally would the remaining 5 cells take a beating. Once this situation arises, the only fix is to replace both batteries. The exception would be if they were both fairly new and you caught the situation as soon as it happened. Otherwise you have a couple of anchors.

So what probably happened is that one battery went bad and started taking down the other, hence the bad starting. Now they are both bad.

You don't need glow plugs to get a Duramax to start in weather above 0F. Glow plugs are not your problem. In fact, letting them run may actually make it harder to start with weak batteries because the glow plugs will take a lot out of the batteries even before they get a chance to crank the engine. Not that I'm advocating not running the glow plugs normally. They make the engine run a lot smoother as it warms up and reduce stress on everything, including your ears.

Of course the real problem is the lights that don't shut off. Do you have a function on your truck that keeps the lights on for a minute or so to light the walk into the house?
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post #5 of 40 (permalink) Old 12-01-2019, 05:41 PM
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House walk function lights the DRLs and the backups on the rear, not the parks.
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post #6 of 40 (permalink) Old 12-01-2019, 06:32 PM
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I guess I could have looked it up but I'm lazy that way.

I don't know what they call it these days but it used to be called twilight sentinel on my parent's '65 Sedan De Ville. It looks like it would be "courtesy lamps". If that is so then it involves park/turn lamps, markers, clearance lamps, tail and license lamps. If all of these are on then the BCM is probably commanding the park lamp relay on for some reason.

You could scan the BCM and see what is going on but the likely cause is the door switches being stuck in the door open position. Ford used to have a big problem with this. As far as I know they still do. GM has decided they don't have enough of their own problems and has copied the Ford system to some extent.

I believe on the 2007 LBZ it requires replacing the whole latch mechanism if it is stick in the open position.
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post #7 of 40 (permalink) Old 12-01-2019, 06:39 PM
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Quote:
I guess I could have looked it up but I'm lazy that way.
With the mods I've done to the two (fore/aft), it's quite the light show with the key fob at night.

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post #8 of 40 (permalink) Old 12-01-2019, 07:38 PM
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I don't know about 2007, but the LLY's have a problem with unsealed (on the top) batteries on the passenger side. Look to see if water from the cowl runs down onto the top of the passenger side battery. Water then seeps through the caps on top.

The solutions are to either (1) use batteries that don't have vents/caps on top, or (2) cover the top of the passenger side battery. I bought a battery case at Walmart, cut the top a little to clear the side terminals, plugged the vents it had, and strapped it on, discarded the rest of the battery box).

It turns out the gassers had an extra brace (I think you can see mounting holes) if they had the 2nd battery. The brace would direct water away. Diesels don't have that piece.

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post #9 of 40 (permalink) Old 12-01-2019, 09:04 PM Thread Starter
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Thanks everybody for y’all’s help. And Jake to answer ur question. I think mine does leak water on the pass side onto the batt. Cause I have a rust spot on my driveway kinda under where my pass side batt is. I’ve read somewhere that’s the cause of that. Never knew how to prevent it. Could that affect my batt life also? Btw got back from work tonite and the parkin lights r still on. Makes no sense y since the batt r dead.

*2007 Chevy Greystone Metallic 2500HD 4x4 CCSB LBZ
EFI Live by Idaho Rob DSP5, Transgo jr
Leveled, debagded, demolded
Thunderstruck front replacement bumper
285/75/16 Mickey Thompson ATZ P3
Atwood gneck hitch
Tekonsha Prodigy 3 Brake controller
Stock exhaust cat and muffler deleted with 5" tip
And more to come…
*85 Chevy 2500 custom deluxe sclb 4" lift
Needs a little work here and there…

Last edited by CA88; 12-01-2019 at 09:10 PM.
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post #10 of 40 (permalink) Old 12-02-2019, 12:11 AM
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A battery that is too dead to crank the engine can still run lights.

You probably need to protect the battery from rain runoff because if it dilutes the electrolyte inside the battery it will ruin it but that has nothing to do with your lights staying on.

Try pulling the various lighting relays one at a time to see if that turns the lights off. You have about seven that could cause something like you are seeing but the main suspect is the park lamp relay. If that turns the lights off then try swapping a different relay into the same socket. If the lights come back on then the BCM is likely seeing something it shouldn't. If they don't come back on then you probably have a bad relay.

The rear defogger relay might be the same type of relay and would be a good test piece since you don't need the rear defog to get the truck to run.

Of course it is possible that you have a wire grounding out and keeping the park lamp relay turned on. You could unplug the BCM. It is mounted on the left side of the steering column under the dash. If unplugging it makes the lights go out then you probably have a door switch gone bad. If it doesn't then you probably have a grounded wire.

Last edited by mizterwizard; 12-02-2019 at 12:19 AM.
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