2008 Alternator Output

[General GM]

Hi all -- just picked up a 2008 Silverado Crew to replace the 99' earlier this month. Very happy so far with my first all new truck in almost a decade. The front suspension, 4x4 steering and overall refinement are a nice improvement over a very solid previous generation.

 

I am concerned I may have a defective alternator / voltage regulator. When I first start the truck, it reads slightly past 14 volts...but as things warm up (think 2-3 mins into drive) the volt gauge tails off and ultimately drops well below 14. I have verified the alternator output at idle on a mutimeter between 12.6 and 12.3 volts typically. For the first 500 miles or so, I never once observed the gauge drop below 14 volts under any driving conditions.

 

There are no obvious drivability problems yet but this makes me nervous. I called service and scheduled an appointment, but when describing the problem I am already hearing the "may be normal", not sure, no TSB's on that, etc.

 

Could any other GMT 900 owners confirm where their alternator output shows on the gauge so I know how hard push service towards a proper resolution.

 

Appreciate all replies in advance.

[minibike132]

I owned an 03 and the gage was at 14 all the time, but when I traded it for an 05, I noticed the gage would do as you describe. I was told on the newer models that, rather than constantly charging at a low rate all the time, they are shutting off the alternator completely until the voltage drops down to a certain point. Reason being is to get a small decrease in drag and hopefully eek out a little more mpg.

 

 

Actually the real reason is to eek a few extra miles out of our crap batteries but this is normal. (ok fine they say for fuel economy too) But any way the newer system monitors battery state of charge. It is, in comparison to an older one, a smart system. Older models just supplied the alternator with a field and the rpms dictated how much charge was going on. With the state of charge being monitored the field can be pulse width modulated to turn on and off based on actual and desired state of charge and needed extra output to keep up with monitored loads (and nowadays every load is monitored some how). You don't usally talk to someone on the phone with any working knowledge other than "well we replace this alot or they tell me this is why it's normal".

[Rosedog]

So how do people then figure out that their alt./volt reg. is going bad?

[General GM]

Actually the real reason is to eek a few extra miles out of our crap batteries but this is normal.

 

Interestingly enough I notice the battery's been moved to a curious location near the firewall (presumably to keep onstar running after a front end crash?). Besides this, the battery appears to be top post rather than side post which is the first I've seen on a new GM vehicle since my days as a lil' tot (if then).

[minibike132]

So how do people then figure out that their alt./volt reg. is going bad?

 

warning message or light

 

Interestingly enough I notice the battery's been moved to a curious location near the firewall

 

I don't know the reason on that no one said and I have not asked.

 

the battery appears to be top post

 

Never asked about this either I just figured the number crunchers finally figured out the sideposts are more likely to leak.

[hubbycap]

I owned an 03 and the gage was at 14 all the time, but when I traded it for an 05, I noticed the gage would do as you describe. I was told on the newer models that, rather than constantly charging at a low rate all the time, they are shutting off the alternator completely until the voltage drops down to a certain point. Reason being is to get a small decrease in drag and hopefully eek out a little more mpg.

 

 

Actually the real reason is to eek a few extra miles out of our crap batteries but this is normal. (ok fine they say for fuel economy too) But any way the newer system monitors battery state of charge. It is, in comparison to an older one, a smart system. Older models just supplied the alternator with a field and the rpms dictated how much charge was going on. With the state of charge being monitored the field can be pulse width modulated to turn on and off based on actual and desired state of charge and needed extra output to keep up with monitored loads (and nowadays every load is monitored some how). You don't usally talk to someone on the phone with any working knowledge other than "well we replace this alot or they tell me this is why it's normal".

 

 

Additionally, I understand that the alternater output is dropped when in V4 mode and ECM will reinstate the V8 mode to charge if the battery charge state dictates it.

[Havasubum]

My new Yukon does the same thing. My 2006 GMC Sierra also did the same thing

[minibike132]

It doesn't turn off. It can be pulsed anywhere from 5% to 95% under normal conditions 0-5% and 95-100% is for self diagnostics.

[I-H8-4Ds]

I owned an 03 and the gage was at 14 all the time, but when I traded it for an 05, I noticed the gage would do as you describe. I was told on the newer models that, rather than constantly charging at a low rate all the time, they are shutting off the alternator completely until the voltage drops down to a certain point. Reason being is to get a small decrease in drag and hopefully eek out a little more mpg.

[General GM]

Thanks for the prompt responses. Based on the sentiments so far I'll probably put the service department on ice and continue to monitor how things progress for the next few hundred miles.