Should I See AC Voltage at the Battery?

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PaulC

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An interesting issue developed on a 2001 Astro van where I can hear a faint whining or buzzing in the rear speakers with radio OFF that follows engine RPM. At night I can see dash lights faintly flickering. Armature shop says alternator is OK and voltmeter shows 13.6 DC AND 30 volts AC. Is this right or is that the source of the hissing in the speakers? Van is 100% stock and wiring has never been butchered.
 

jjm

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An automotive alternator should only be outputting DC power. If it's putting out AC power (which it produces natively), most likely a diode(s) is gone, or their is a problem with the field circuit. Either way, the problem needs to be corrected before the electronics start quitting in protest - permanently.

Hopefully, NickD - our resident electronics engineer - will chime in with his expertise.

Joe

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videobruce

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Try another DVM, it could be a issue with the one that was used. If you are on the opposite current, sometimes a meter will show a voltage when it shouldn't.
If there was 30 volts AC, I would like to think it would of caused serious damage by now worse than what was reported.
Is that voltage present when the engine isn't running?
 
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NickD

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An alternator is a three phase device where the stator generates an AC voltage with a peak to peak voltage of twice the battery voltage, in your case with an elevated temperature of 27.2 volts, but measuring the AC voltage with a typical average reading would only show 9.6 V across any stator pair, 30 VAC is impossible. The alternator has six main diodes to rectify this voltage and in good working order only a ripple voltage appears. Measuring this ripple with an oscilloscope would show about 0.6 peak to peak, but it certainly is not an alternating voltage with both positive and negative peaks and this is without the alternator connected to the battery. With the battery in the circuit, it filters out this ripple in the 0.1 to 0.2 volt range and this is peak voltage. True Root Mean Square voltage would be about 70% of this value.

An open alternator diode does not increase the peak ripple voltage, but leaves a gap where the diode should conduct reducing the TRMS voltage. A shorted diode does provide an AC voltage with both positive and negative peaks, but is not as powerful as the battery. The battery fights the alternator and you may get belt squeal as the alternator is really stalling with a shorted diode. Tell your shop to park the AC voltmeter and get a scope, that really tells the condition of the diodes.

I assume you get this buzz sound with the engine running, does it go away when the engine is turned off? Poor alternator performance like a leaky diode generates more of a buzz sound that increases in frequency or pitch as you rev the engine. Flickering lights is telling me you don't have an OE voltage regulator, the OE regulator is pulse width modulated at a 390 Hz frequency and generates a very constant voltage with load response control. The cheap replacements use an on off kind of switch whose frequency varies all over the place that will cause that flickering you are experiencing. I would check that first.
 
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NickD

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Should I See AC Voltage at the Battery?

A briefer answer is no. In regards to that small ripple voltage I was talking about, the configuration of the OE rotor pole pieces is as such as to produce a ripple that looks much more than a square wave than a sine wave. Again in the cheap rotor replacements, the output looks more like a sine wave where at it's peak voltage, a large current is produced several times greater than the average current. This not only causes noise, but increases the I^2R losses causing the stator to run much hotter and shortening the life of the diodes due to overheating.

Did your problems start with a Twilight Zone alternator? The top of the line NAPA replacements are the same as a Delco rebuilt and do cost a couple of bucks more.

Even the OE alternator is not without problems, the 12V circuit is easy to clean, but the ground circuit is through aluminum where even the bracket can corrode contributing to a higher resistant connection that increases ripple voltage and noise. All these have to be wire brushed and should be coated with silicone grease to retard corrosion and checking the battery ground connection at the engine block or wherever. Just cannot slap in either a new alternator or a starter and expect it to work, have to do some house cleaning first.
 

larche

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Someone correct me if I am wrong, but only higher quality meters can accurately read AC voltage in a DC system.
My Fluke can, but most meters I don't think can.
You would see a lot of problems at a lot less AC voltage. Check the specs of your meter and I think you will find it is not capable of reading AC over DV.
 
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PaulC

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Nick, It's the original OE alternator with only 44K mi. I went ahead and replaced it anyway since the bearing was singing like a canary, I guess GM hasn't perfected the "greasless" bearing yet. Sourced from a local rebuilder, not chain store reman. It's sad what junk they're putting into these vehicles today, had a '68 Olds that nearly turned 200K miles with the ORIGINAL alternator and just a $3.00 set of brushes. Good point on the grounds though as I checked the body grounds at the battery and everything looked fine. Should have run a ground wire from the alternator to the block, too late now. Seem to recall the old 10-SI alternators with a ground terminal on the case.
 
N

NickD

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I'm cleaning and adding grease to any new bearing today, pop off the seal off any new bearing and the balls are barely coated, they last a lot longer when packed with grease the way they use to pack these limited lubrication bearings as I call them.

You will find the same thing in the AC pulley, tensioner, and idler bearings and they can seize in ten miles of driving from a quiet bearing to a broken single belt, leaving you stranded in some God forsaken place where no cellular towers are around.

The front bearing is rolled in the front housing and the lip breaks off when using a puller, a spring clip is available to hold a new bearing in place, but I prefer to drill and tap three counter sunk holes using #10 flat head screws with the flange of the screws firmly holding the new bearing in.

Absolutely no reason to epoxy the inner race of this bearing to the shaft of the rotor the clamping pressure of the pulley nut puts tons of pressure on that inner rack with the pole piece on the other side, but they epoxy it. Can't get a puller behind the bearing, but those using a wedged plate and a press can bend the shaft before popping that bearing off, they compromise by boring out the inside of the stator, very poor practice. On my own, pop the seal, break the plastic retainer and move all the balls to one side so the outer race can be removed, then use a Dremmel with a cut off wheel to put grooves on the inner race and finish it off with a chisel to pop off the inner race then wire brush off that epoxy so the new bearing slides on. Rear bearing can be removed with a small puller.

The pulley is easy to remove on the CS-130, but the CS-144 has a brass tapered collet that is really jammed and corroded in there, I just drill that out with a series of holes and take it out in pieces, but so easy to put in a new one.

I did change the bearing on my 82 Chevy P-30 10SI, grease was all dried up and had a brand new BMC laying around, but it was fun. But with previous GM vehicles, never had to change the bearings in the A-6 or alternator, were easy to change with real screws or retaining rings, miserable with the newer compressors as they are peened in. The cast iron hardens with age and can never get a decent peen, so resort to my #10 flat head screws, but most just press in a new bearing, I like screws. Bad enough the hub is pressed on, but if that falls off and it does come loose, can still drive.

Scope is still the best method to check for the ripple, especially if you are getting electrical noise, a voltmeter just does not show glitches or spikes.
 

jjm

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See Nick, now you know why I'm glad you're back... who the heck else could explain things so thoroughly technical yet in laymens terms?

Glad you're back safe, and hope you had a nice vacation; I was reading your comments about it on the <a href="http://www.ackits.com">ackits.com</a> forum.

Joe
 

0x6A7232

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@NickD Just measured the VDC on my brother's 2014 Dodge Ram pickup at 14.0 - 14.3 at idle, with 30 VAC. I read your replies but I'm not quite sure what you meant. Did you mean that an off the shelf voltmeter can't really tell if there's a problem and that an AC reading means nothing?
BTW, pretty sure bad battery - as soon as you shut the truck off, it has barely enough juice to crank over once or twice (but not start) before dying to just a click.
 

billr

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Sadly, NickD has passed away since that post in 2007.

There should be little-to-no AC on the system, under 1V at the very maximum. You probably have shorted diodes in the alternator that are discharging the battery rapidly when the engine isn't running (as well as causing the AC ripple voltage). The diodes I am referring to are not part of the voltage regulator and are not practical for the average DIYer to replace. You probably will have to replace the alternator.

Take the alternator to your FLAPS for testing, to confirm this diagnosis; but I suggest you buy an alternator from <dbelectrical.com>

At least, check the pricing from dbelectrical, so you can see how much money can be saved by waiting a couple of days for delivery.

PS: I think NickD was wrong about 30 VAC being "impossible" from an auto alternator.
 

0x6A7232

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Sadly, NickD has passed away since that post in 2007.

There should be little-to-no AC on the system, under 1V at the very maximum. You probably have shorted diodes in the alternator that are discharging the battery rapidly when the engine isn't running (as well as causing the AC ripple voltage). The diodes I am referring to are not part of the voltage regulator and are not practical for the average DIYer to replace. You probably will have to replace the alternator.

Take the alternator to your FLAPS for testing, to confirm this diagnosis; but I suggest you buy an alternator from <dbelectrical.com>

At least, check the pricing from dbelectrical, so you can see how much money can be saved by waiting a couple of days for delivery.

PS: I think NickD was wrong about 30 VAC being "impossible" from an auto alternator.
RIP NickD. :/

I think you're right, but I think NickD was saying you can't get 30 VAC across a pair of stators (whatever those are, I assume part of the voltage generating stuff on the alternator) - so measuring at the battery, perhaps the stators combine to more VAC? IDK how everything interacts. There's some talk of how they work here I think but it's above my understanding without giving myself a headache for several hours (and probably still missing some things). https://electronics.stackexchange.com/questions/337254/3-phase-alternator-to-charge-batteries

EDIT: Did some testing. My 2009 Chrysler Town & Country was having problems as well so I measured that. Also 30 VAC. Hmm. Well maybe both have the same problem. So I measured my 2020 Honda Pilot (zero problems starting / charging since we bought it a year ago) and that's got 30 VAC as well so I can confidently say it's either normal or my multimeter has issues.
 
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billr

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It probably is your meter; the chances of having three identically-failing vehicles is pretty slim...

When the charging system is working properly the AC ripple should be less than 1V, probably even down in the 100-500mV range. That would be either at the battery or alternator output stud; both should be at the same voltage.

As far as getting 30VAC, that would depend on the alternator damage, design, and rpm. Some may not be capable of 30VAC no matter what; but others may be able to produce even more (again, when malfunctioning)

PS: I browsed through that article you linked to. One thing I didn't see mentioned is that alternators are typically 12-pole machines. That means output frequency is fairly high and AC ripple is easier to filter out. Also, I am not convinced power factor is important when discussing auto alternators.
 
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nickb2

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I found this thread interesting even though it is old. And yeah, RIP dear NickD. He was a hugely funny and very endearing guy once you got to know him through our rants and rage days together here. '

I wish his surviving family well. He spent alot of time here. You could see he took this suite to heart. @billr, if you may, when did he die? This year or last?

Ok, back to resurrecting this thread.

Yeah probably a meter thing. I used to use ac reading to see if a diode was shot. But not the best way to check.

Here is a link to some fundamentals. https://electronicsclub.info/acdc.htm

It will redirect, it is safe.

If you are on a MV scale in AC, sure, you will get 30mv, but as Billr said, if on all three cars same, highly doubt all three cars have a bad rectifier bridge. So you probably dont have a proper meter such as a fluke or such.

I have never owned a fluke nor do I haver the budget for one, I have scopes for precision now, but that is besides the point. I think using the ac scale in mv is a good way to check diodes fast. If the bridge is doing its job, a good normal meter will show close to zero, good to go to next step.

I found the above link by doing a google search and fell upon this thread from 2010 in a site called The Garage Journal. Here is a link to thread.

 

nickb2

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I for one love the link of the electronics club. I am still having issues with regards to properly adjusting my values, very helpful site for a non pro like me. Cuz I need to nail this down. I am going for full electric vehicle certification soon. So study is in order.

So to Ox6A newbie, thx for resurrecting this thread. It reminds me that Nickd is no longer with us, he would have jumped all over this one and jawed for a whole week.

And it also reminded me to get my head out my mustang project and back to studying. Thx.

Keep coming back, we are a cool community. ;)

:bat:
 
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