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The alternator can be considered a three phase generator with a rectifier to produce a smooth 12v DC. If one ore more of the diodes in the rectifier is failing, the output may no longer be a smooth 12v DC , which may appear as an AC voltage at the alternator terminals. You are, in effect, measuring noise on the signal which many systems on the vehicle will only tolerate to a certain level.There are two rules for success,
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Alternators produce an AC voltage, so to feed the vehicle with the DC voltage it requires they make use of a bridge-rectifier diode pack. When one or more of these diodes start to fail the voltage output becomes a mix of AC and DC. The total energy output of the alternator may still be good but the vehicle cannot make use of the AC component and some systems will not take kindly to it.
Sticking your DMM into AC mode checks for the presence of AC and, in effect, is a health-check of the diodes. The reason the check should be done at the alternator, rather than the battery, is to avoid the smoothing effect of the battery.
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3rd May 2016 6:05 pm
EstorilM
Member Since: 15 Oct 2009
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I JUST went through this, although it's with an NA spec 4.4 V8 but I'd imagine the alternators are similar or at least their PWM control is similar.
Copied and pasted from the other forum - just explained how mine died for another forum member.
I was lucky enough to notice some strange things in the month or two leading up to the failure, and even more lucky to notice the actual failure itself in traffic even though there were no actual warning lights or indicators. I guess I do kinda know her pretty well by now, which doesn't hurt lol.
Quote:
Well YMMV (I've seen them fail a few different ways) but personally, my progression was as follows.
-a few months ago (when it was still colder - harder starts, less amperage from battery, heated seats and windshield, more air compressor use, higher loads on alternator) the alternator started to whine after starting.
-whine after start got louder, and more recently I noticed that if you waited for the air compressor to shut off, you could hear most of the whine go away (ie it's load based and it's definitely struggling)
-maybe a month ago I made that thread about the volume lowering and enabling "low power mode" 20 minutes AFTER I started my car, while idling in traffic. That was the big red flag for me.
-about a week after that, while playing with a new scanner at work - I noticed that my charging voltage at idle was 14.4-14.8 - so that is NOT always a sure-fire way to check. These alternators vary their CURRENT output via PWM signal as you know, but the key is that it's current-related. You can have an open voltage of 14.4 with minimal loads and the alternator isn't working very hard. It's possible that when loads are applied AND the unit gets hotter, that's when voltage falls dramatically.
- Monday when I left work in evening it was 85 degrees for the first time in a while, I cranked the front and rear AC blowers to full and was listening to music in stop-go idling traffic (in hindsight, a LOT of load at a low RPM) - about 10 minutes later as the rpms dropped each time I stopped in traffic, I noticed a LOUD whine on rpm decel. Sounded like front right engine bay area.. as I left my windows down I smelled burning electronics smell (from the stories on here of people reconnecting new batteries after they die, everyone mentioned smoke/smell) so I connected the dots and turned everything off. As I said in other thread - pulled into indy lot about 45 min in (on same road I commute on) and decided to make the remaining 20 min run home.
That's when I checked and saw (engine off) 12.5, and 12.4 engine on.. 12 flat with engine on and all HVAC on.
***ENTIRE TIME the vehicle gave me no warnings, errors, codes, dash lights, whatever.. even when idling at 12.05v after I got home. Perhaps there's a time limit before it triggers, or I wasn't at the threshold yet? I'm shocked the OP's vehicle even ran at 10-something volts!
I think the heat and the high loads with low RPM was too much for it. If you're curious, you can get your car warm and throw on front/rear AC on max, front/rear heated windshield glass, and front/rear heated seats. Sit by your front passenger wheel well and listen - maybe with safety glasses.
Also FWIW I had a brand new H9 MTP interstate in my car, so that probably buffered a lot of the loads actually, and prevented major issues - if you've got an older or weaker battery I'd imagine things would be more dramatic.
edit: the delco/remy unit is fine so far, I did notice a slight bearing squeak down there (laying under car while running with that cover off) but I think it was coming from the larger fan belt tensioner pulley located next to it, hard to tell with your face a few inches from that huge fan.
edit2: i read a few people complaining of erratic fan behavior before alternator failures - probably some degree of AC voltage getting into the vehicle past battery and messing with PWM fan signals. I did notice a more defined fan sequence on startup. If you're having issues w/ fan maybe add that to the list above as well.
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