Lights Dimming Like Alternator Hit With Heavy Load...

[nootron69]

I have a 2008 Sierra 1500 ext cab. Had an 8 hour drive yesterday, and right around the 7th hour (at night) noticed occasionally (esp when coming to a stop) that headlights and dash dimmed, then recovered. In the final hour of driving, it started to happen more frequently. I am not putting any heavy demands on my electrical system (ie heavy hitting stereo, etc). Love this truck and have been totally worry and trouble free til last night. Anyone else have this problem or some insight? Thank you all in advance!

[Nytemare]

Pretty normal when coming to a stop. the engine idles down a little too far than bumps back up to correct. During that low idle your lights dim.

 

There is a lot of load on the system at night...head lights interior lights..and brake lights when stopping...plus you AC cycles to keep the air dry. Than the Voltage regulator keeps correcting its self when you idle down too...its not going to charge the battery wide open at highway speeds...but when its idling it will go to full charge mode...this takes time to kick in too. Basically the output of the alternator is governed by the VR...and its a function of engine rpm, so you don't cook your battery.

[Stevens03]

Mines done that since day 1... just when coming to a stop, they flicker dimmer than normal, then they are fine.

[DvlDogDmaxZ71]

mine too from day one....just get used to it i guess

[KMGZ400]

can someoone tell me why ever since i replaced the alternator on my 01 1500 with a Prostart alternator i now hear a faint whine through my speakers? it whines louder as the rpms get higher but its not real noticeable below 1500 rpm

 

sorry not trying to hijack the post

[TexasAl]

It may be as others have stated, or it might be that the battery is weak, and may have a bad cell. If you have time, and want more peace of mine, go to an auto parts store (or Walmar), and have them test your battery.

[FubarWS6]

Same here I have noticed that in my GMC Sierra, now I guess its normal seeing as you guys have the same problem.

[Nytemare]

If you pay enough attention to any vehicle...you'll notice the same thing.

 

Its the Auto Voltage Regulator doing its job.

[brims]

Mine has done it since day 1 as well, with both the GM battery and my Optima. Completely normal.

[Stex]

Yep normal, It bothered me on my 09 and I asked my service manager about it. He said the alternator runs as low as possible based on demand (notice your gauge and it will be well below 14 some times) to improve mpg. Then when demand hits, like compressor, it takes a second for the altnr to ramp up and you get the light dimming...

[Neutron]

can someoone tell me why ever since i replaced the alternator on my 01 1500 with a Prostart alternator i now hear a faint whine through my speakers? it whines louder as the rpms get higher but its not real noticeable below 1500 rpm

 

sorry not trying to hijack the post

 

 

Sounds like a loose ground somewhere.

[Pauldeere]

My truck does the same thing. The service writer at my local dealership told me that begining in 2004 that G.M. started useing a "Variable Output" alternator. These alternators will cause the condition that you guys describe. However I do not notice my lights dimming at idle as someone has described.

[jrrod2004]

I used to have the problem, but I had Justin change the Idle speed to 700 RPMs and it has really gone away...I guess a difference of 200 RPMs is just enough to cause a difference.

[Neutron]

I've noticed the same thing in my 2007 Silverado as well.

[Nytemare]

My truck does the same thing. The service writer at my local dealership told me that begining in 2004 that G.M. started useing a "Variable Output" alternator. These alternators will cause the condition that you guys describe. However I do not notice my lights dimming at idle as someone has described.

You wont notice it at idle...its when you come from speed to a stop where you idle at a light...its within the first 2 seconds of coming to a stop..where the idle dips for a second, the AVR (Auto Voltage Regulator) will step wide open so as to output the proper voltage to the battery at idle.

 

The delay of the AVR opening up is what causes the lights to dim.

 

Now if your idle in the driveway...turn on your AC...you'll see everything dim for a second....thats the increased load on the engine that drops the idle, but it will cause the same effect.