Duramax Diesels Forum Truck of the Week
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Old 07-06-2018, 12:47 AM
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Default 13.2v Charging Too Low?

I've had my LML for probably 15k miles now and it's always been charging between 13.2-13.8v. I changed the batteries a few months ago and the voltage stayed the same.

It seems lower than it should be and I don't want to end up stranded one day because I continue to avoid it. The headlights seem pretty low on the output, and I doubt a single volt will really change it, but I guess that's a topic for another day.

The strangest part about the whole thing, and what actually concerns me, is when I disconnect the batteries it's up at 14.4 for the next 3-5 drive cycles. After that it's back to the normal 13.2.

I have cleaned the grounds on the block a the lower drivers side, the strap to the driver head and firewall, and the small ground on the driver body mounting location on the frame. No changes.

Any ideas, or is this normal on the newer trucks? I've tried searching around, but no clear answers.
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Old 07-10-2018, 10:39 PM
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Does anyone know what controls the charging voltage? The alternator seems to have the capacity to charge at a higher voltage, but is restricted for some reason.
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Old 07-10-2018, 11:10 PM
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Where and how did you measure voltage?
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Old 07-10-2018, 11:42 PM
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I have measured the voltage at the battery, OBDII port, and the cigarette lighter with a known good DMM.
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Old 07-11-2018, 01:14 AM
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Check at the alternator also. You could have a bad charging wire. The one on my Lmm would get really hot. I replaced it with 0 guage and my volts read higher at the battery.
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Old 07-11-2018, 01:48 AM
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Are your positive connections good at the pass side battery junction block? Are you measuring voltage on the driver's side battery or pass? The alternator chargeback wire goes directly to the fuse block on n the pass side battery then from there charges the drivers side through the main cable the ties the 2 batteries together that runs along the radiator. I had to replace that cable that runs from drivers to pass. Truck was acting weird wouldn't charge correctly, turned over really slow. I had like a 2 volt drop across that positive cable running along the radiator. Replaced that and all was well.

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Old 07-11-2018, 12:29 PM
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Quote:
Originally Posted by 02greysixer View Post
Are your positive connections good at the pass side battery junction block? Are you measuring voltage on the driver's side battery or pass? The alternator chargeback wire goes directly to the fuse block on n the pass side battery then from there charges the drivers side through the main cable the ties the 2 batteries together that runs along the radiator. I had to replace that cable that runs from drivers to pass. Truck was acting weird wouldn't charge correctly, turned over really slow. I had like a 2 volt drop across that positive cable running along the radiator. Replaced that and all was well.

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The voltage measures the same at both batteries and on the alternator post. The strangest part of all this is that after resetting the system, it seems to function normally for a few drive cycles.
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Old 07-11-2018, 04:08 PM
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The ecm controls the charge voltage. I wouldn't be surprised if they are programmed to drop to a lower voltage to save mpgs or something. 13.2 volts is the minimum needed to put a full charge on a 12v battery. The higher 14+ volts is for equalizing the cells but it doesn't need to be there all the time.

Have you tried turning on the headlights to see if the voltage goes up?

Also most of these alternators will run at 12 or so volts with no exciting current due to its self exciting abilities. If this what's happening then I would make sure the wires from the ecm to the alternator are in good shape with minimal resistance
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Old 07-12-2018, 12:41 AM
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Quote:
Originally Posted by 2004LB7 View Post
The ecm controls the charge voltage. I wouldn't be surprised if they are programmed to drop to a lower voltage to save mpgs or something. 13.2 volts is the minimum needed to put a full charge on a 12v battery. The higher 14+ volts is for equalizing the cells but it doesn't need to be there all the time.

Have you tried turning on the headlights to see if the voltage goes up?

Also most of these alternators will run at 12 or so volts with no exciting current due to its self exciting abilities. If this what's happening then I would make sure the wires from the ecm to the alternator are in good shape with minimal resistance
The MPG thing is why I asked. I was thinking this was a pre-programmed attribute to save fuel and meet the MPG standards. It seemed low, but not something I would put past GM.

I pulled apart the passenger fuse block, cleaned the drivers fuse block feed terminal, and re-seated the whole drives fuse block assembly. At this point I'm willing to try anything and it's been asked a few times, so I want to rule it out. So far it's stayed above 14v longer than before. Ill have to give it a few days since a reset has always made it run at a higher voltage for a period of time.

Edit: I have tried turning on all different types of accessories to see if I can cause a voltage change, no luck. It doesn't ever drop below the current voltage, which doesn't make me think the alternator is weak. Just an ECM control issue or parameter. I can run the AC full blast, cooled/heated seats, hold all the windows up, headlights with fog lights, wipers, ect, no dip in the current voltage. It seems higher ambient temperatures cause a lower voltage, but I don't have enough time monitoring it to say for sure.
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File Type: jpg Pass Fuse Block.jpg (75.7 KB, 25 views)
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Last edited by snowman22; 07-12-2018 at 12:47 AM.
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Old 07-12-2018, 09:24 AM
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At this point I like I would try a different alternator lol.

Have you disconnected battery's over night and see if one drops volts? I know there new just throwing out an idea
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Old 07-12-2018, 09:25 AM
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Quote:
Originally Posted by 2004LB7 View Post
The ecm controls the charge voltage. I wouldn't be surprised if they are programmed to drop to a lower voltage to save mpgs or something. 13.2 volts is the minimum needed to put a full charge on a 12v battery. The higher 14+ volts is for equalizing the cells but it doesn't need to be there all the time.

Have you tried turning on the headlights to see if the voltage goes up?

Also most of these alternators will run at 12 or so volts with no exciting current due to its self exciting abilities. If this what's happening then I would make sure the wires from the ecm to the alternator are in good shape with minimal resistance
Was this something that started for lml? I know earlier 1500's had it, but 8 lug trucks didn't get it, at least not early on. Wasn't sure if they started it for lml or not as I know the alternator type changed for them vs earlier trucks.
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Old 07-12-2018, 10:47 AM
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I know that the ECM has control over the duty cycle of the regulator in the alternator. This is pretty obvious from looking at the wiring and schematic diagrams. How much control it exerts over it I don't know. That is way I said I wouldn't be surprised if it did.

My LB7 seamed to put out a constant duty cycle and a steady voltage at the alternator but my LMM seams to vary this far more. I haven't had a chance to look over an LML but I know it has been creeping into more and more vehicles.

Alternator self exciting for a nominal voltage is quite normal so it will not normally drop below that if the alternator is in good shape
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Old 07-12-2018, 12:29 PM
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Gm upfitter documents seem to indicate that my earlier post was not far off. Lots of good info on charging voltage and diagnosis

*https://www.gmupfitter.com/files/med...abs_100713.pdf

Starts at A-18
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Old 07-12-2018, 02:45 PM
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Quote:
Originally Posted by 2004LB7 View Post
Gm upfitter documents seem to indicate that my earlier post was not far off. Lots of good info on charging voltage and diagnosis

*https://www.gmupfitter.com/files/med...abs_100713.pdf

Starts at A-18
A good read for sure, but my truck doesn't follow that pattern too well. It doesn't change in tow/haul or when the head lights are one.

Unfortunately I'm back to 13.4v today...not really sure what to do next, or if I should even be concerned.

Maybe GM has decided to be more aggressive in the 15" and up with the algorithm for lowering the voltage to decrease fuel consumption.
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Old 07-12-2018, 03:14 PM
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Quote:
Originally Posted by snowman22 View Post
A good read for sure, but my truck doesn't follow that pattern too well. It doesn't change in tow/haul or when the head lights are one.

Unfortunately I'm back to 13.4v today...not really sure what to do next, or if I should even be concerned.

Maybe GM has decided to be more aggressive in the 15" and up with the algorithm for lowering the voltage to decrease fuel consumption.
I honestly wouldnt worry about it. turn EVERYTHING on (headlights, fogs, interior lights, a/c on high, stereo up, and anything else you can kick on that draws power), then re-test at the batteries. if its still 13.2 or higher, i wouldnt worry. if it drops, id look at the alternator and have it tested. batteries are set around 12.6v so as long as you are above this voltage, in theory you wont have an issue with batteries draining.

If i remember tomorrow, ill check my dads 15.5 LML and see what his is at.
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