Vehicle:

2000 4WD 2DR LS Blazer 84K miles

History:

My trusty steed of a truck has an RPM-based whine that sounds similar to a supercharger. I checked under the hood just to be sure no one installed one while I wasn't looking...no such luck. Some time ago I installed a set of electric air horns, nothing big. Small compressor with two plastic horns mounted behind the bumper. After a while I uninstalled them because I thought they were the culprit for the whine. After uninstalling, the whine remained. The whine was present before the horns as well.

I replaced the stock battery (AC Delco) with another AC Delco within the past year.

VIDEO

Problem:

Upon first starting the truck, it idles at about 1000 RPM. The voltage drops don't seem to be as severe at this idle. Once the truck idles down to about 600 RPM, the voltage drops are significantly more noticeable as seen in the video.

Questions:

Is this a bad ground issue? Alternator? Wiring? All of the above? It seems like the power windows and the rear defrost have the most draw, could it be singled out to these specific wires? My guess is alternator because the volts go back up with increased RPM. Does this mean my alt is worn and won't charge at low RPM's, only when it is under load?

Plan of attack:

My local Advance Auto's roll-out alternator/battery tester is broken and they don't know when they'll get a new one. I can't take the alternator out at this point, nor can I afford a new one. I'm having my truck looked at on Thursday (28th) for a continued whirring noise coming from the front end despite changing the hub bearings. Once I get the bill for repairs for that, I will take the alternator off that following weekend to get bench tested because I read that the roll-out ones aren't as reliable as a bench test.

Perhaps I'm making a bigger deal out of this than I need to, but I've seen a few threads with similar symptoms and figured this would be a good way to combine all the information. I will keep this thread updated as I go on and would appreciate any answers and feedback along the way.

 

I had the exact same problem, replaced the alternator and it's perfectly fine now. Everyone said I needed a new head unit; but ya turns out it was the alternator which took about half hour to change out

 

if you take a long screw driver or something along those lines, put one end of it to the body of the altenator and the other end ( the handle) to your ear with your ear resting up against the handle that is. you will be able to hear any wierd noises in the alt.. If you have a hearing problem then that test will not work LOLOLOL, but anyway that is the cheapest way to see if thats where the noise is coming from b4 replacing something that isnt the problem... I just replaced my alt yesterday cause it had whining and clicking noises coming from it, i opened it up b4 going to the parts store and the brushes in there were shot, there was nuthing left to them, I put it back togather brought it to the store got a reman alt for 100.00 I have AAA so i get a discount at any NAPA store.. hope this helps ya...

 

Looks like an alternator to me. I have seen many alternators start to loose their ability to charge when they get hot. Also that whine is the alternator whining with what sounds like bad diodes. And I personally DO NOT like bench testers, they can falsely test bad components as good, I have had this happen way to many times, so I don't use a bench tester, and in 11 plus years doing my job, I have never been wrong when the alternator or starter have been tested on the car. SO good luck with your adventure here!!

 

If it is the alternator and you have a place locally that rebuilds them, I would recommendhaving them do yoursrather than buying a rebuilt one. I've just had better luck going that route.

 

sounds like an alternator..easy to change..might want to look at your belt and your tensioner pulley while you are at it..both pretty inexpensive to change out!

 

Just received alternator from Jigg today (thanks again!) and was wondering what my options are as far as cleaning up the housing. I googled 'rebuild alternator' and didn't quite see what I was looking for. There were even mentions of the alt being a non-servicable part. I know this isn't true because people buy remanufactured alts all the time.

What I'm looking for is a guide to disassembly, what options I have to dress up the casing a bit (paint?), and how much trouble I'm going to get into if I open this bad boy up.

Any help?

 

To simply clean up the housing you shouldn't have any trouble... just pull it apart then put it back the same wayyou took it out. We had to rebuild alternators in my high school shop class so it really isn't too hard, just make sure you put it back togetherthe way it was.

 

Was able to take the ALT out today since my truck is out of commission anyway and had it tested. Actually did better than the one I bought to replace it. Stayed right between 14.5-5. Now since my gauge was higher when I first started and the ALT was cold, could it be possible that the earlier suggestion is true? Is it losing it's charging ability once hot?

EDIT: FINALLY able to put the replacement in and whine is gone. Just hope it charges the battery.