Alternator Question

[rrosen]

I just got a 2008 Yukon XL Denali and have a question about the voltmeter and alternator.

 

Usually when I start the car, the voltmeter is a little above the middle showing the alternator is charging the battery. But on a number of occasions, after driving as little as 2 or 3 minutes, it drops below the middle of the gauge to about 12.5 volts or so. If I stop and restart it goes right back up to above the middle.

 

Is this typical operation, i.e., alternator charging drops to battery voltage level? Or do I have some kind of intermittent problem?

 

Anyone seen this before? Like to know if this is typical before I go to the dealer (you know how hard it is to find intermittent problems).

 

(I tried searching but didn't find anything relevant)

[WayneNC]

I just got a 2008 Yukon XL Denali and have a question about the voltmeter and alternator.

 

Usually when I start the car, the voltmeter is a little above the middle showing the alternator is charging the battery. But on a number of occasions, after driving as little as 2 or 3 minutes, it drops below the middle of the gauge to about 12.5 volts or so. If I stop and restart it goes right back up to above the middle.

 

Is this typical operation, i.e., alternator charging drops to battery voltage level? Or do I have some kind of intermittent problem?

 

Anyone seen this before? Like to know if this is typical before I go to the dealer (you know how hard it is to find intermittent problems).

 

(I tried searching but didn't find anything relevant)

[WayneNC]

I believe this is normal, RVC, regulated volage control. At least that"s the way my 2005 Silverado does.

[Zembonez]

The OEM gauge cluster is not a very accurate representation of actual voltage. oil pressure, etc. The gauges there just give a visual indication of system operation. They are pretty close but the numbers are not exact.

 

What you describe does not sound abnormal. Starting zaps the battery for a short time because of the amperage needed to turn over the engine's high torque starter motor and the alternator kicks in to recharge it. Once the battery is charged, it's not uncommon for the gauge to drop a bit as the alternator "idles" a bit. It should do so depending on voltage use and requirements.

[HawkScot]

My 2008 Yukon XL Denali does the same thing. Sometimes it stays ~13volts for 20-30 minutes while driving and then goes up to ~14.2VDC or so... Never had a problem with not starting or anything and we have over 25000 miles now.

[68haywagon]

This should be in your owner's manual:

 

Electric Power Management

This vehicle has Electric Power Management (EPM) that estimates the battery’s temperature and state of charge. It then adjusts the voltage for best performance and extended life of the battery.

When the battery’s state of charge is low, the voltage is raised slightly to quickly put the charge back in. When the state of charge is high, the voltage is lowered slightly to prevent overcharging.

If the vehicle has a voltmeter gauge, you may see the voltage move up or down. This is normal.

If there is a problem, an alert will be displayed.

 

All GMT900 do this.

 

Mike

[ChrisNJ]

you can blame CAFE and over-regulation. Granted, I think it's pretty cool, but nothing is free.

[Spurshot]

Seems to me, when a high power consumption accessory comes on the voltage drops on my truck. When the accessory goes off, the voltage comes up.

 

Check to see what's going off at the time you see the voltage jump up. Could be the auto a/c blower, power windows, headlights going down to drl fuction from full headlight function.

[jrrod2004]

didnt know that...good info

[jf781]

I thought I read somewhere in an attempt to maximize mileage even more the alternator has a clutch in it. This way when the system is charged alternator pulley just free wheels and doesn't spin the armature (i think that is what it's called), or basically doesn't spin the mechanics of the alternator so there is less parasitic loses on the motor. Thus the voltage only shows around the 12 or so volts the battery provides. Once it drops below a certain level the clutch engages and the alternator starts to charge the battery again thus showing around 14 volts.

 

Correct me if I'm wrong on this but it would explain what you see perfectly. Mines does this too btw.

[rrosen]

This should be in your owner's manual:

 

Electric Power Management

This vehicle has Electric Power Management (EPM) that estimates the battery's temperature and state of charge. It then adjusts the voltage for best performance and extended life of the battery.

When the battery's state of charge is low, the voltage is raised slightly to quickly put the charge back in. When the state of charge is high, the voltage is lowered slightly to prevent overcharging.

If the vehicle has a voltmeter gauge, you may see the voltage move up or down. This is normal.

If there is a problem, an alert will be displayed.

 

All GMT900 do this.

 

Mike

 

Thanks Mike. I wondered if that might not be the case but must have missed it in my first pass through the owner's manual.

 

Thanks for saving me a trip to the dealer.

[68haywagon]

No problem. My 09 does the same thing and the dealer pointed it out to me. My 07 did not have the gauge so I never knew it did that.

 

Mike

[rrosen]

No problem. My 09 does the same thing and the dealer pointed it out to me. My 07 did not have the gauge so I never knew it did that.

 

Mike

 

That's one of the reasons manufacturers don't want to put gauges in or make them so damped they barely move (my sports car water temp gauge sits dead center whether the water is 180 degF or 220degF)-- too many customers worry about what they see on the gauges and bring the vehicles in and complain.

 

Again thanks to all for easing my concern.

[Spurshot]

I thought I read somewhere in an attempt to maximize mileage even more the alternator has a clutch in it. This way when the system is charged alternator pulley just free wheels and doesn't spin the armature (i think that is what it's called), or basically doesn't spin the mechanics of the alternator so there is less parasitic loses on the motor. Thus the voltage only shows around the 12 or so volts the battery provides. Once it drops below a certain level the clutch engages and the alternator starts to charge the battery again thus showing around 14 volts.

 

Correct me if I'm wrong on this but it would explain what you see perfectly. Mines does this too btw.

 

 

My 99 didn't have a clutched alternator. I've never seen one. But, I'll look see if my 2008 has a clutch. Frankly, I don't see it being necessary as an alternator by it's very design, free wheels with only bearing and brush drag when the field and armature are not energized. That drag is not much more than a idler pulley. The amount of drag an alternator produces while making electricity is directly related to the amount of output it is regulated to.