Inconsistent voltage, causing ALP errors

oktavf

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Perfect timing for this issue and my current circumstance. Hope you don't mind me jumping in here but it sounds like there is a fairly good knowledge base and I don't trust what the dealer told me.

I have a 2018 GMC Sierra Denali. I have owned it from late 2019. The voltage gauge always read somewhere just below or just above 14v.

About a month and a half ago it started to drop to as low as 12.3-12.5v. Sometimes stay there sometimes jump back up on its own. I had the battery tested and it had lost a step in holding charge. I expect that for a OEM battery that is 50-48 months old. Replaced the battery. Same thing happened.

Pulled the alternator took it to three parts stores to get it tested. They all said the same thing that they did not have the connector to test it with on their machine. I found that weird since the its 4 years old. I guess they don't update those machines much.

Read a bunch of Chevy forums with many claiming the computer in the truck tells the alternator how much voltage to put out based on load. OK I accept that to a point but I am still skeptical. I can be rolling down the highway, lights, cooled seats, A/C, wipers etc all on and the voltage can go from 14v to 12.3v-12.8v and stay there. (I have a phone charger with a meter on it and I had a ProM both read very close to the dash gauge reading and all read the same).

If I pull over after running that way for a while turn off the truck then turn it back on it jumps to 14.5 to as much at 15v at times. That tells me the computer senses that the battery has been starved and it needs to charge it.

The dealer said all is good, no codes no issues found. The truck is under the bumper to bumper so if they found an issue they would have replaced the alternator or what ever they found defective. They did not replace it. At least they did not charge me for the diagnosis. Its still happening this week.

I don't want to get stuck on a winter night when I go out to the car and find it dead.

I am thinking just go ahead and replace the alternator and see what happens. What do you all think?
Friend had a Arcadia with lots of scary wtf voltage issues in his trip from CT to NC. Dealer checked it no issues. Blah blah eventually they did replace the alternator!
 

TominVirginia

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You could, but I’d really want the dealership to keep it and run it and figure it out. On my Ford, it will vary, but that variance is only whether it’s 14.2V at the beginning of a road trip to 13.8V at the end. It should never ever be at battery voltage when it’s running. Period.

The only way to be 100% sure is to actually run a wire from the battery terminals to a reliable gauge you can monitor. I can’t imagine any vehicle systems running like that, though, for any reason ever. Even stuff going through the computer fed systems like the stereo system should t be reading less than 13.5V when underway.

If you want to pay your own money, you can do what you want, but I’d want the dealer to explain to my why the alternator not charging the battery is OK and to fight for a replacement under warranty. It sounds like you already took it off yourself and put it back on, so you’ve dumped more time into than I would’ve. I wouldn’t trust it at all.
It took two weeks just to get an appt with the dealer and they had it for two days. I would have hoped they could find something. Since its a dealer warranty its my guess that's why they did not replace it.

I hate going there anyway so I think I am going to order a new alternator and pop it on. It only takes 20 min or less so its not too heavy of a lift from a time perspective and rockauto has them a reasonable prices even for OEM.

Thanks all.
 

dn0t

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Hey bud,

Had this problem with my E63. It's unlikely you have a dead battery or bum alternator in your R7 - it is prob related to current distribution from the CAN bus.

The crew at Autocraft NYC installed this filter for my ALP system and all the low voltage errors went away.


Willing to bet for your car it is the same problem.
 

Deacon

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Since its a dealer warranty its my guess that's why they did not replace it.
Then let everyone else know what little the dealer warranty is worth:



It's unlikely you have a dead battery or bum alternator in your R7 - it is prob related to current distribution from the CAN bus.
He refers to his voltage gauge, which means the faulty system probably isn’t just whatever is feeding the mirror or wherever he’s getting power for a detector.

Maybe he can add some clarification to all of this.
 

Rags

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Thats NOT a real Load Tester ...
Thats tests electronics and Dead Cells not weak cells.

Brb with the pic of a Real Load Tester.

Note: the air holes to release the hot smoke.
This will TRULY test your batts.
Weak cells, partial spikes,etc


This is ....
Exactly. A capacitance tester is NOT a load tester. Not a "different type of load tester". It's simply not a load tester. The two perform two very different functions. Both can agree on a bad battery, but they don't always agree. One type of test can find faults the other cannot.
Post automatically merged:

Perfect timing for this issue and my current circumstance. Hope you don't mind me jumping in here but it sounds like there is a fairly good knowledge base and I don't trust what the dealer told me.

I have a 2018 GMC Sierra Denali. I have owned it from late 2019. The voltage gauge always read somewhere just below or just above 14v.

About a month and a half ago it started to drop to as low as 12.3-12.5v. Sometimes stay there sometimes jump back up on its own. I had the battery tested and it had lost a step in holding charge. I expect that for a OEM battery that is 50-48 months old. Replaced the battery. Same thing happened.

Pulled the alternator took it to three parts stores to get it tested. They all said the same thing that they did not have the connector to test it with on their machine. I found that weird since the its 4 years old. I guess they don't update those machines much.

Read a bunch of Chevy forums with many claiming the computer in the truck tells the alternator how much voltage to put out based on load. OK I accept that to a point but I am still skeptical. I can be rolling down the highway, lights, cooled seats, A/C, wipers etc all on and the voltage can go from 14v to 12.3v-12.8v and stay there. (I have a phone charger with a meter on it and I had a ProM both read very close to the dash gauge reading and all read the same).

If I pull over after running that way for a while turn off the truck then turn it back on it jumps to 14.5 to as much at 15v at times. That tells me the computer senses that the battery has been starved and it needs to charge it.

The dealer said all is good, no codes no issues found. The truck is under the bumper to bumper so if they found an issue they would have replaced the alternator or what ever they found defective. They did not replace it. At least they did not charge me for the diagnosis. Its still happening this week.

I don't want to get stuck on a winter night when I go out to the car and find it dead.

I am thinking just go ahead and replace the alternator and see what happens. What do you all think?
Your Sierra has an alternator that is controlled by the BCM. When the battery SOC is at a desired point, the voltage command for the alternator will switch to economy mode to prevent overcharging the battery and to save on fuel consumption. It's working as designed.
You could, but I’d really want the dealership to keep it and run it and figure it out. On my Ford, it will vary, but that variance is only whether it’s 14.2V at the beginning of a road trip to 13.8V at the end. It should never ever be at battery voltage when it’s running. Period.

The only way to be 100% sure is to actually run a wire from the battery terminals to a reliable gauge you can monitor. I can’t imagine any vehicle systems running like that, though, for any reason ever. Even stuff going through the computer fed systems like the stereo system should t be reading less than 13.5V when underway.

If you want to pay your own money, you can do what you want, but I’d want the dealer to explain to my why the alternator not charging the battery is OK and to fight for a replacement under warranty. It sounds like you already took it off yourself and put it back on, so you’ve dumped more time into than I would’ve. I wouldn’t trust it at all.
Well, from everything he's said, his truck is working properly...so there is that. Also, the truck would definitely set codes if the voltage wasn't what it was expecting. Since the BCM is in control of the alternator output, when the desired voltage and actual voltage doesn't match within a set amount of time, a code is set. If the problem persists, the BCM will go into load shedding where it shuts down non critical groups of systems one at a time to try to bring the actual voltage to where it desires.
 
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cincinnatiman

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i seen this in other cars one dodge charger if you put scaner on it while car is off last 3 digs move.
and reason for this system in part remans on . but voltage probem is do to the ground these cars put togather with plasic pins
and they expec power run throw them. i know this if one cell gose out on battery your get computer errors some people will take advanage of this . is there way fix ? all they had do is take copper tape ran to back car to front to gound the car to motor block it is that simple but they rather deal with recalls and blowing up batterys
and ault going out becuase there location to the cats i meen child could did bedder
 

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