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Discussion Starter #1
On my drive to work yesterday, I noticed my battery voltages drop to 12.4-12.5v at highway speeds (70mph). I monitored them for about 20mins and it continued to hold under 13v with the occasional spike to 13+v. Only accessory that was on was auto climate control.

Drive home last night, aprox. 30min at highway speeds, and voltage held above 14v the entire time.

Drive to work today and all is well until car had ran for about 15min and then hit highway speeds. Voltage dropped from 14+v to a steady 13.6-13.7v. Today no auto climate control, so maybe the reason voltage was above 13v instead of below.

i figured out that when I let off the gas and let it coast, it would go back to 14.4-14.5v. instant I hit the gas pedal, it would decline to a steady 13.6-13.7v.

Serpentine looks fine and is tight. No squeezing or funny sounds from belt or alternator.

Is this a common thing for these alternators when they start to go or is this more likely a belt slipping? Anything else this could be?
 

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2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
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This is normal. The ECM controls field excitation to the alternator to provide charging when needed and greater economy when possible. When the engine is unloaded, the alternator will often be commanded to a higher state of charge (thus higher voltage output) and under higher engine load the alternator will be commanded to put out less power. Under heavy electrical load, the alternator will be commanded to increase output regardless of the impact upon economy. The AC compressor is shut out at initial full throttle applicatoin to improve acceleration and although I have never monitored voltage output at full throttle I wouldn't be surprised if excitation is also dropped to it to allow maximum power at full throttle.

So all of this is entirely normal behavior.

Some of the recent model GM engines also have variable displacement oil pumps as another means of increasing operating efficiency; this shows you the extremes that are used to garner a little more economy for the all important CAFE standard. My 2016 Corvette Z06 has this variable displacement pump and a few minutes after startup, the oil pressure will cycle from a normal to fairly high reading three times in rapid succession as the ECM runs a test on the oil pump control system. That prompted a lot of calls to GM dealers with customers wondering about the erratic oil pressure reading under steady state engine operation.

A lot is going on with modern engines to meet efficiency and emissions standards and although the manufacturers try to make these as unnoticeable to the customer as possible sometimes they are obvious. My 2018 GMC diesel is quite annoying if it goes into an exhaust particulate filter cleaning while not on the open road and part of the cleaning program increases the idle speed significantly to keep the filter hot so that cleaning can quickly resume once the light changes. It takes a lot more brake to slow/hold the pickup when in regen so that emissions system is far from completely transparent in normal operation.

Rodger
 

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2013 ATS 3.6 Premium RWD
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How old is your battery? This seems normal, but if you're 5 years or more I would just change it. I'll try to pay attention on my trip home today to see what an optimal situation looks like (new battery and alternator within the past 4000 miles). When my battery was getting old (after 6 years), it would charge at very high voltages (between 14.5V to 15V) but I never experienced any power/starting issues. About a month after the new battery went in, my alternator died - this is probably due to the weak battery demanding lots of alternator usage and prematurely wearing it out. When this happened I was notified by a "Power Saver Mode" (something like that) message on my DIC, and then going to the voltage screen showed no charging behavior. As for the belt, I haven't heard or noticed any effects of it slipping and am still on the factory unit (7 years, 93k miles).
 

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Discussion Starter #4
Mine is a 2017 and battery checked out fine via a battery tester.

Does everyone else have volts suddenly drop to the 12s while at highway speeds?

Rsingl- Thank you for the insight. Only thing I'll say to your post is that my ATS absolutely positively does not disengage the ac clutch at full throttle. I'm tuned and own HP Tuners and have logged the AC clutch multiple times. It stays engaged at full throttle. I recall someone else in another thread saying that there's does disengage...so perhaps different trims have a slightly diff oem ecm tuning? Regardless, I'm going to use HPT to command the ac clutch to disengage at 85% throttle at around 4k rpm. And re-engage sometime after.
 

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Mine drops to the mid 12 volt range quite often at highway speed, it is entirely normal. There is a temperature component also to the algorithm and during high ambient temperature the battery charge rate is kept lower so you are more likely to see system voltage in the 12 volt range during the summer while in winter it will be far less common when ambient temperature is very cold. AGM batteries still produce gas during charging but the catalyst inside attempts to recover the lost moisture that would be vented in a typical maintenance free however it can't keep up with a high charge rate coupled with high ambient temperature which is why our ATS batteries have a vent hose running outside of the compartment to vent hydrogen if the catalyst cannot keep up.

I am surprised the AC is not dropping out at high throttle, GM has done that for so many years so they either had a reason or simply felt it wasn't worth it. It would be interesting to see if your powertrain in Camaro trim drops the compressor at full throttle, perhaps Cadillac is biased towards luxury/keeping cool even under high throttle. Dropping the AC at full throttle definitely frees up a few ponies and the compressor probably will appreciate not being run at very high RPM. I remember back in the 90s Ford ran into an issue with some of their first SHO engines where the AC compressor had a short life at high RPM and the pulley diameter was changed to bias it towards survival with some drop in cooling effectiveness in low RPM city driving.

Rodger
 

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On my drive to work yesterday, I noticed my battery voltages drop to 12.4-12.5v at highway speeds (70mph). I monitored them for about 20mins and it continued to hold under 13v with the occasional spike to 13+v. Only accessory that was on was auto climate control.

Drive home last night, aprox. 30min at highway speeds, and voltage held above 14v the entire time.

Drive to work today and all is well until car had ran for about 15min and then hit highway speeds. Voltage dropped from 14+v to a steady 13.6-13.7v. Today no auto climate control, so maybe the reason voltage was above 13v instead of below.

i figured out that when I let off the gas and let it coast, it would go back to 14.4-14.5v. instant I hit the gas pedal, it would decline to a steady 13.6-13.7v.

Serpentine looks fine and is tight. No squeezing or funny sounds from belt or alternator.

Is this a common thing for these alternators when they start to go or is this more likely a belt slipping? Anything else this could be?
Totally agree with everything posted by rsingl. I have my battery voltage displayed all the time (makes me feel like I'm driving my dad's old '55 Ford which had the analog 'generator' meter as part of the instr. cluster, nice!) and my 2013 ATS 2.0T Auto/Performance will vary between 12.9-14.7, usually staying at 14.4 when cruising at highway speed. Normal.
 
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