Battery/ Alt Gauge

cxkali

Original poster
Member
Jan 20, 2012
125
So my question is about the battery/alt gauge mine has caught my eye the last few weeks and mine is right on 14 most of the time but bounces up and down is this normal? .. also ive been watching videos of tb's and voy's on youtube and all of them are well over the 14 mark on the gauge.
 

Boricua SS

Member
Nov 20, 2011
3,080
Ohio
What is the range that it bounces between? I put a voltmeter in my gauge pod, and I've seen it as low as 12.2V-12.6V (10.9V-11.9V when the bass hits on my subs), all the way to 15.1V at startup in the morning...
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
1) Since the gauge is pretty inaccurate, what's your handheld voltmeter say when you measure at the battery? We sort of expect all DIY mechanics to own a meter, and even a $4 Harbor Freight cheapie will do.

2) The gauge is not reading voltage at the dashboard, but it's driven by a data message from the PCM. So engine compartment wiring issues can have an affect on the gauge. Check the ground from the engine to the driver's side fender, and wiring to the battery terminals, for looseness or corrosion.
 

cxkali

Original poster
Member
Jan 20, 2012
125
Boricua SS said:
What is the range that it bounces between? I put a voltmeter in my gauge pod, and I've seen it as low as 12.2V-12.6V (10.9V-11.9V when the bass hits on my subs), all the way to 15.1V at startup in the morning...


it bounces between 12-14 while driving withe the ac on i only have an aftermarket head unit so i dont have the extra draw from an amp or anything. went to autozone and they hooked up there handheld unit and said everything checked out fine.
 

cxkali

Original poster
Member
Jan 20, 2012
125
the roadie said:
1) Since the gauge is pretty inaccurate, what's your handheld voltmeter say when you measure at the battery? We sort of expect all DIY mechanics to own a meter, and even a $4 Harbor Freight cheapie will do.

2) The gauge is not reading voltage at the dashboard, but it's driven by a data message from the PCM. So engine compartment wiring issues can have an affect on the gauge. Check the ground from the engine to the driver's side fender, and wiring to the battery terminals, for looseness or corrosion.



will do this before work today and see what i find.
 

Moots12

Member
Sep 7, 2012
111
I just had a problem with my battery ad alternator and I installed a aftermarket volt gauge and it usually pins on 13 while the dashboard moves from 12-14. And I have a problem with the large draw which I read a bunch of places is normal.
 

vipergg

Member
Dec 7, 2011
191
If you are saying it just goes between like 12 and 14 at times thats normal, even says so in the owners manual . If its "jumping" around then that's different. The computer adjusts the output as the vehicle load varies.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
vipergg said:
The computer adjusts the output as the vehicle load varies.
Well.......to be precise, the voltage regulator in the alternator adjusts the voltage as it drops due to an added external load. The PCM does two things in our architecture:

1) The PCM has one control wire that is either on or off. It doesn't vary with load. Its purpose is to hold off (or suppress) the alternator from immediately starting to work when it's extremely cold out, the battery is already low from being cold, and it goes even lower when cranking a cold engine with cold oil and cold fuel being sprayed into cold cylinders. If the alternator threw its mechanical load onto the serpentine belt to instantly recharge the depleted battery, GM found it didn't allow the engine RPM to stabilize nicely. So this control wire delays the alternator from starting right up, and you can see that in the dashboard voltmeter. When it's wicked cold, it will stay at 11-12V for up to 30 seconds, THEN go up to the 14-15 the alternator puts out normally.

2) There is another wire FROM the alternator to the PCM, telling the PCM how hard the alternator is working. (It's a sense wire on the field, for those who know how alternators work). The PCM uses that as a measure of the load put on the alternator. Doesn't control anything, just sees if the alternator is putting out all it can. At 100% duty cycle, if the PCM sees the voltage is STILL going down, it concludes the alternator has a fault (like a dead diode), and can't keep up with the demand. Now this is cool - the PCM has a feature never discussed in the Owner's Manual, but it is in the shop manual (that if you own, you MUST read cover-to-cover). The PCM can load-shed! It tries to keep enough voltage in the system to try to let you limp home. It disables things like the power windows, then the entertainment system, then the HVAC and blower, then the interior lights. It disables the exterior lights last, as a safety issue. It tries to get you home or to a place of safety. You'd think they would tout this in the Owner's Manual, but no.

And in a million posts, I've never read once that this feature has ever been actuated and had somebody ask about it. Sigh.......Useless knowledge.

garfieldmice.jpg
 

vipergg

Member
Dec 7, 2011
191
the roadie said:
Well.......to be precise, the voltage regulator in the alternator adjusts the voltage as it drops due to an added external load. The PCM does two things in our architecture:

1) The PCM has one control wire that is either on or off. It doesn't vary with load. Its purpose is to hold off (or suppress) the alternator from immediately starting to work when it's extremely cold out, the battery is already low from being cold, and it goes even lower when cranking a cold engine with cold oil and cold fuel being sprayed into cold cylinders. If the alternator threw its mechanical load onto the serpentine belt to instantly recharge the depleted battery, GM found it didn't allow the engine RPM to stabilize nicely. So this control wire delays the alternator from starting right up, and you can see that in the dashboard voltmeter. When it's wicked cold, it will stay at 11-12V for up to 30 seconds, THEN go up to the 14-15 the alternator puts out normally.

2) There is another wire FROM the alternator to the PCM, telling the PCM how hard the alternator is working. (It's a sense wire on the field, for those who know how alternators work). The PCM uses that as a measure of the load put on the alternator. Doesn't control anything, just sees if the alternator is putting out all it can. At 100% duty cycle, if the PCM sees the voltage is STILL going down, it concludes the alternator has a fault (like a dead diode), and can't keep up with the demand. Now this is cool - the PCM has a feature never discussed in the Owner's Manual, but it is in the shop manual (that if you own, you MUST read cover-to-cover). The PCM can load-shed! It tries to keep enough voltage in the system to try to let you limp home. It disables things like the power windows, then the entertainment system, then the HVAC and blower, then the interior lights. It disables the exterior lights last, as a safety issue. It tries to get you home or to a place of safety. You'd think they would tout this in the Owner's Manual, but no.

And in a million posts, I've never read once that this feature has ever been actuated and had somebody ask about it. Sigh.......Useless knowledge.

garfieldmice.jpg



Wow , great post Roadie. You know these cars inside and out...:yes:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
vipergg said:
Wow , great post Roadie. You know these cars inside and out...:yes:
Why thank yew. Invested years of research, reading, and practice so I can help others and pass it on.

gmcman said:
Fixed it for you....:biggrin:
Damn. I knew I needed to edit a few jpgs. Thx for the reminder.
 

Matt

Member
Dec 2, 2011
4,022
I had the same issue and was scratching my head about it. Checked everything with my meter and have had torque on to monitor it. I cleaned all the ground points on the vehicle and it helped a little bit but the issue still remained.

It turned out that although the negative battery terminal was on tight-ish, it wasn't tight enough. When I put a spanner on it, it felt loose, so I tightened it up as much as I dared and haven't had a problem since.
 

smervin

Member
Mar 24, 2017
1
USA
The PCM can load-shed! It tries to keep enough voltage in the system to try to let you limp home. It disables things like the power windows, then the entertainment system, then the HVAC and blower, then the interior lights. It disables the exterior lights last, as a safety issue. It tries to get you home or to a place of safety. You'd think they would tout this in the Owner's Manual, but no.
]
Not sure if you're still on here, but I feel like I've been experiencing this load shed feature. 2016 Tahoe LT, and the seat heaters turn off if the temperature outside is under 32 degrees. they turn off a lot until the engine is super warm. On days when the temp outside is like 40 degrees they stay on normal. Have had no luck at dealer to even identify that it is happening. Would love any help in this area...
 

budwich

Member
Jun 16, 2013
2,038
kanata
well your first bet would be to post some "measureable observations" on the voltages that you are seeing during "normal" and your "I think its shedding" scenarios. IF you (and we) are lucky, you might even include some current measurement (with a clamp meter..... again if "we" are lucky). Go from there. Further, If your negative cable has a "box" attached to it (voltage controller), you can try unplugging that (cable... 6 pin I think), and the voltage should stay basically "pegged" at ~13.6 or there abouts.
 

Mooseman

Moderator
Dec 4, 2011
25,307
Ottawa, ON
And seeing it's a 2016, it's under warranty. Complain to the dealer. It could be a programming issue and an update to the BCM might fix it. Doesn't sound like anything it should be doing.
 
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