Voltage, normal?

sdeeter19555 · December 27, 2017

If I run around here, voltage is 15.2 to 15.3 VDC. If I flip the headlights on, which isn't really that much of a load as the DRLs are already on and taillights don't draw that much; voltage slowly ticks down to a more realistic 14.4 to 14.3 VDC.

This leads me to believe the 15.2 is a surface voltage, because a minor load shouldn't drop almost a full volt that fast. I have dual batteries, one is original and the other is just six months younger. I'm wondering if I'm looking at a bad battery? Starts fine, even with temps near zero.

Do these have any sort of temperature sensor at the battery to compensate for charging losses in cold weather?

Steve

2012 2500hd 6.0l

redwngr · December 31, 2017

There is a long list of things the computer considers when deciding alternator output.

In the GM 2018 upfiitter guides there is lots of information about the charging modes and idle boost that can be created by the charging system.

I stumbled across the write up -- it is multiple pages of info.

Starts around page 4-17.
File is big - adobe reader says over 1100 pages

https://www.gmupfitter.com/files/med..._2017AUG31.pdf

There are 6 different charging modes it can use.
• Battery Sulfation Mode
• Charge Mode
• Fuel Economy Mode
• Headlamp Mode
• Start Up Mode
• Voltage Reduction Mode

There are also 4 different levels of 'idle boost' that the charging system can use.

Electrical Power Management Description and Operation (Diesel)

The electrical power management is used to monitor and control the charging system and alert the driver of possible problems within the charging system.

The electrical power management system makes the most efficient use of the generator output, improves the battery state‐of‐charge, extends battery life, and manages system electrical loads.

The load shed operation is a means of reducing electrical loads during a low voltage or low battery state‐of‐charge condition.

The idle boost operation is a means of improving generator performance during a low voltage or lowbattery state-of-charge condition.

Idle boost consists of three steps:
idle boost 1, idle boost 2, and idle boost 3
(approximately 750, 900, and 1050 rpm respectively).

Idle boost is activated in incremental steps, idle boost 1 must be active before idle boost 2 can be active.

Each electrical power management function, either idle boost or load shed, is discrete. No two functions are active at the same time. The criteria used by the body control module (BCM) to regulate electrical power management are outlined below:

The alternator output is controlled by the BCM, and there is a charge of the % output it can be asked to run at, and the resulting output voltage from each step.

This is clearly not the charging system control method many of us learned in auto shop.

The engine control module (ECM) controls the generator through the generator turn ON signal circuit.
The ECM monitors the generator performance though the generator field duty cycle signal circuit. The signal is a pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0–100 percent.
Normal duty cycle is between 5–95 percent. Between 0–5 percent and 95– 100 percent are for diagnostic purposes.
The following table shows the commanded duty cycle and output voltage of the generator:
Commanded Duty Cycle Generator Output Voltage
10% 11 V
20% 11.56 V
30% 12.12 V
40% 12.68 V
50% 13.25 V
60% 13.81 V
70% 14.37 V
80% 14.94 V
90% 15.5 V
The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the ECM.
This information is sent to the body control module (BCM).

Information overload (and there is more in the guide), but may explain some of the possible reasons for higher idle levels (other than 'elevated idle' and 'regen idle' and also some of the possible reasons for voltage level variation -- and why it's likely 'normal'

dewfpo · December 31, 2017

Could you post the entire link? The shortcut above does not work for some reason.

Thanks, DEWFPO

sdeeter19555 · January 2, 2018

Well, as an update...

After a week of 15.3vdc readings, with the drop to 14.3vdc with the headlights, started the truck yesterday and 14.0vdc the entire trip home. Start the truck today, below zero, and 13.8 to 14.0 vdc...

Seems like the high voltage was either something in the programming or my alternator is dying at 10k miles...

Steve
2012 2500hd 6.0l