So if my alternator is reading 11V........

Let me preface this by saying the last time I had an alternator go bad was the fall of 1998. It's been a while. 

My truck, 2007 Silverado, 215k miles, flashed a dash message of SERVICE CHARGING SYSTEM this morning while driving kids to school. These trucks are a bit known for weird electrical stuff but I haven't personally had a problem with mine. Yet. The light went off after dropping off the kids, V at 14 on the gauge. Drove to Home Depot. When I started the truck again the service message was back and V were 11-12V. Got safely home and the battery reads 12V whether the truck is on or off and it reads 11.08 at the alternator when the truck is running. Classic case of bad alternator? Or is there something else to chase? 

Dead alternator or regulator - but on newer cars they tend to be one and the same.

It'd be either a bad ground or a bad alt.  I'd bet alternator.

There's always some tiny chance that the controls side of things is not running the alternator properly but usually the alternator itself is bad. If you're actually seeing a lower voltage at the charging post of the alternator then you are at the battery terminals, that probably means that one of the diodes in the alternator has failed and is actually sucking current OUT of the battery. If it's dropping over 1 volt on that charge wire between the battery and the alternator it's probably getting warm too. That would definitely be a bad alternator. 

minivan_racer said:

It'd be either a bad ground or a bad alt.  I'd bet alternator.

First thing to do is to clean all battery cable connections, at both ends of each cable.  Take them off and clean the insides and underneath each connection.  Even if that's not the root issue, it can't hurt.  Then if you have a handheld voltmeter, measure voltage at the battery terminals.  With the engine off it should read 12.5V or so, with the engine running it should be 13.5 to 14V or so.  If you don't get the higher voltage when the engine is running the charging system isn't working - whether it's just the voltage regulator or it's the alternator.

This is like:  What time is it when the clock strikes 13?  Time to get a new clock.  Or alternator.

buddy of mine posted same failure this morning on his 2007 Suburban, at 208k miles.  tinfoil hatters will call it a software issue.   the rest of us will say "holy E36 M3, that alternator's been spinning for over 200k miles.   it owes you nothing."

Ok, going to attack the grounds and then pull the alternator and have it tested. 

Thanks for looking over my shoulder.  You guys are the best. 

I would bet on alternator, especially at that age/mileage.  It is certainly possible that it could just be something loose or corroded, but my money is on alternator.

Someone here recently pointed me to research voltage drop testing. I'll pass that recommendation along to you. It can really help with troubleshooting in situ if you don't want to just pull the ale and have it tested off the truck.

Easy test to see if it's a case of worn out alternator brushes: with the truck running and the alternator not charging, take something metal and hit the alternator case firmly.  If the alternator suddenly starts working, it's the brushes getting to end of life and losing contact with the slip rings.  

Not sure if there is a correlation (and I suspect the alternator as well because it was throwing a fault) but our 08 Tahoe would vary the alternator’s output to match the need to help with fuel mileage. 

It was concerning to see it at ~12.5 and then later at 14 but the manual said that was normal.   Warnings on the dash, however, were not. 

In reply to jfryjfry :

Yeah, if it was just the bouncing around in the voltage it also might be a bad instrument panel, but the dash warnings mean it's something more!

I got 284,000 miles out of the alternator on my Chevy truck.  Yours must have been a bad batch.

Polished all of the terminals and grounds and got the voltage to read an even 11.54 both at the battery and the alternator. Cables seem good anyway. Alternator is off and I'm headed to the parts store. 

200k miles? That alt lived a good life. 

Kid at O'Reillys threw it on the tester and said it tested good. Threw it back on the truck and it's still sub 12V. 

Whoa what the hell? Maybe a huge power drain somewhere? I had a problem like that when a homemade main alternator wire started to cook itself from the inside...

How does the truck run with no alternator? If runs and stays above 10v, maybe the alternator isn't getting power (for the field with which to generate electricity?)

Wifrs van, similar issue last week.

Same thing. Alt bench tested good, but wasn't charging properly. 

Replaced the alternator anyway, and problem went away.

I dont trust the bench testers on new, pcm controlled alternators.

rslifkin said:

Easy test to see if it's a case of worn out alternator brushes: with the truck running and the alternator not charging, take something metal and hit the alternator case firmly.  If the alternator suddenly starts working, it's the brushes getting to end of life and losing contact with the slip rings.  

Truck was running fine at 11-11.5V. Turned ALL the power on. Dropped to 10.8V. Hit the alternator case with a hammer, gently, and we're now back up to 14+V though the charge light is still on. 

Soooooo.........alternator is bad anyway? Any suggestions on what are the "good" replacement ones? 

Yeah, I'd say the alternator brushes are worn, so the springs are probably right at the end of their travel.  When the parts store tested it, they were probably making contact so it put out good power.  At some point in use, they bounce slightly and lose contact because the springs can't push them back against the slip rings properly.  Hitting the alternator knocks the brushes around and is sometimes enough for them to start making contact again.  

I suppose another possibility could be a duff battery - if it has a dead cell it could be pulling the voltage down.  On the other hand, if it had a dead cell it probably wouldn't be able to start the truck...

stuart in mn said:

I suppose another possibility could be a duff battery - if it has a dead cell it could be pulling the voltage down.  On the other hand, if it had a dead cell it probably wouldn't be able to start the truck...

That was my thought too. I've had a bad battery in this truck before. I've started it at least a dozen times while trying to trouble shoot this and it always fired right up. 

Ive had good service from db electrical on Amazon, and orielleys lifetime warrenty parts.

You need an alternator, or my name ain't Fred. 

Internal regulator, right?  Nothing regulated by the ECU or anything, right?

O'Reilley's have been hit or miss for me.  Let's just say I'm glad they come with a lifetime warranty.  Napa has a better reputation.  If you don't mind potentially having to do it again in a year, try the O'Reilley's ones with a LT warranty.

SO there are only 3 wires that go to the alternator: The charge wire, the field control wire, and the voltage sense wire. You already know by deduction that the charge wire is fine because if there was no connection between the battery and the charge post you wouldnt see the 11.54 there.  The other two small wires on the connector could be issues, though. I would start by just checking that the terminals actually make a good connection to the alternator when you plug it in, and after that i would probably check continuity from the alternator end to the PCM end. There's also still a chance that its just worn brushes so i guess the first thing i would do is smack the back end of the alternator while the truck is running and see if the voltage moves.