Expert help needed on Voltage Gauge ALWAYS above 14

[PhilB]

I am looking for some expert opinions from some of the techs on here or others that may know what is causing my problem.  I am very familiar with how the GM charging system works and the voltage gauge will fluctuate.

 

Back story:

My voltage gauge has been reading above 14 volts since I noticed it last Oct but could have started sooner.  My truck has turned into a weekend grocery getter since Covid shut down everything and I work from home full time.  I have put only 4k miles on my truck in the last 12 months so since I am not driving it as much, I may not have noticed it.  In Oct when I noticed this I thought that maybe I wasn't driving it enough and the battery was getting low.  In Nov I noticed that my autostop wasn't working either.  I eventually took the truck in to the dealer in Dec to have both issues checked out.  They said that the battery had a bad cell which was causing the autostop to not function, and that may be why the volt meter was always above 14.  So a new battery was installed and that fixed the autostop issue, but the volt meter still reads above 14, like close to the next mark to the right.  I took it back again in Dec and they said that it was all within specs and gave me a copy of the GM Charging System Description and Operation.  In Feb this year, during the Polar Vortex, the volt meter crept to the other side of that right tick mark.  I estimate that it was over 15V.  Now that Spring is almost here, the volt meter needle is between 14 and the next tick mark.  I put my ODBii reader on it the other day when the ambient temps were in the 50s and the voltage was 14.6 consistently.  

 

Redwngr posted on another topic the following which reads exactly like my GM handout the dealer gave me.  This is part of it:

Charging System Operation
The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 6 modes of operation and they include:
• Battery Sulfation Mode
• Charge Mode
• Fuel Economy Mode
• Headlamp Mode
• Start Up Mode
• Voltage Reduction Mode
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). The signal is PWM signal of 128 Hz with a duty cycle of 0–
100 percent. Normal duty cycle is between 5–99 percent. Between 0–5 percent and 100 percent are for diagnostic purposes.

Battery Sulfation Mode
The BCM will enter this mode when the interpreted generator output voltage is less than 13.2 V for 45 minutes. When this condition exists the BCM will enter Charge Mode for 2–3 minutes. The BCM will then determine which mode to enter depending on voltage requirements.

Charge Mode
The BCM will enter Charge Mode when ever one of the following conditions are met.
• The wipers are ON for more than 3 seconds.
• GMLAN (Climate Control Voltage Boost Mode Request) is true, as sensed by the HVAC control head. High speed cooling fan, rear defogger and HVAC high speed blower operation can cause the BCM to enter the Charge Mode.
• The estimated battery temperature is less than 0°C (32°F).
• Battery State of Charge is less than 80 percent.
• Vehicle speed is greater than 145 km/h (90 mph)
• Current sensor fault exists.
• System voltage was determined to be below 12.56 V
When any one of these conditions is met, the system will set targeted generator output voltage to a charging voltage between 13.9–15.5 V, depending on the battery state of charge and estimated battery temperature.

 

Based on the Commanded Duty Cycle, I am running in the 70%-80% range.  For the bulleted items above, I can eliminate wipers (not raining), HVAC is off, not driving over 90mph.  If I am driving 30 - 40 minutes, I would expect the Battery State of Charge to be at 100%.  The system voltage should be above 12.56v.  Battery temp should be above 32 since the ambient temps are in the 50's.  

 

I have kind of narrowed it down to the battery current sensor through the process of elimination.  How can I test to see if the battery current sensor is bad?  Or could it be that the ECM is stuck and needs to be rebooted or reprogrammed?  (FWIW, I had recall N192210560 Engine Recalibration Torque done on 8/30/20 so the ECM was reprogrammed then).

 

I am thinking of dropping the truck off at the dealer and let them keep it until it is fixed.  This is not normal.   

 

Sorry for the long post.  

[aliveguy5]

If you have a trickle charger, I'd suggest putting that on overnight to see if the truck acts different. With such short drives it may be trying to handle both the voltage load the truck is asking for and keeping it charged. Leaving with a full battery should let the truck focus on other tasks.

[diyer2]

 As mentioned put a charger on it. Drive it to get your groceries and check if the voltage is different.

Try this, check the voltage at the battery posts with a DVM after it sits before you start it. If the battery is below 12.5 volts it needs charged. 

[newdude]

Any aftermarket devices installed on the truck?  Range Pulsar, insurance drive recorder in the OBD, a dash cam, etc?  Anything at all not factory installed?

[PhilB]

Thanks for the replies and suggestions.  I should have stated that my grocery trips usually consist of a 40+ mile round trip as I live in a rural area and the wife likes to shop at a particular store.  I drove yesterday and the truck has been sitting for over 24 hours in the garage.  I just checked the battery voltage and it read 12.6V.  I do not have anything extra plugged in.

 

I have noticed that when I first start the truck, the needle will be below 14 on the gauge.  Then within a mile of driving, it will slowly crawl up to the the high range.  After a restart from the auto-stop, the needle will go right back to where it was before the auto-stop.